Physics Practical File

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EXP-1> TO DETERMINE THE RESISTANCE PER CM OF A GIVEN WIRE BY PLOTTING A GRAPH OF POTENTIAL DIFFERENCE VERSUS CURRENT 

Materials Required: A Resistance wire, voltmeter, ammeter, battery, rheostat, meter scale, one way key, connecting wires etc.

Theory: If I be the current flowing through a conductor & V be the potential difference across its ends, then according to Ohm’s law

         V∝I or V = RI

Where, R is the constant of proportionality. It is known as resistance of the conductor.

                                                      V/I=R 

R depends upon the nature of material, temperature & dimensions of the conductor.

Procedure: 

 1. Clean the ends of the connecting wires with the help of sand paper in order to remove any insulating coating on them.

2. Connect various components - resistance, rheostat, battery, key,  voltmeter and ammeter

3. Note whether pointers in milli-ammeter and voltmeter coincide with the zero mark on the measuring scale. If it is not so,  adjust the pointer to coincide with the zero mark by adjusting the screw provided near the base of the needle using a screw driver.

 4. Note the range and least count of the given voltmeter and milliammeter.

 5. Insert the key K and slide the rheostat contact to one of its extreme ends, so that current passing through the resistance wire is minimum.

6. Note the milli-ammeter and voltmeter readings.

7. Remove the key K and allow the wire to cool, if heated. Again insert the key. Shift the rheostat contact slightly to increase the applied voltage. Note the milli-ammeter and voltmeter reading.

8. Repeat step 7 for four different settings of the rheostat. Record your observations in a tabular form.

OBSERVATIONS:

    1. Range of ammeter = 0 ... mA to ...mA
    2. Least count of ammeter = ... mA
    3. Range of voltmeter = 0 ... V to ...V
    4. Least count of voltmeter = ...V
    5. Least count of meter scale = ... m
    6. Length of the given wire, l = ...m
RESULT: 
1. The potential difference across the given wire varies linearly with
the current.
2. The resistance per unit length of the wire is (R ± ΔR) = (... ... Ωm-1).
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EXP-2> TO DETERMINE RESISTANCE OF GALVANOMETER BY HALF DEFLECTION METHOD AND TO FIND ITS FIGURE OF MERIT.

MATERIAL REQUIRED: A moving coil galvanometer, a battery or a battery eliminator (0 - 6 V), one resistance box (R-BOX 1) of range 0-10kΩ, one resistance box(R-BOX 2) of range 0-200Ω, 2 one way keys, voltmeter, connecting wires and a piece of sand paper.

 Theory: Galvanometer is a sensitive device used to detect very low current. Its working is based on the principle that a coil placed in a uniform magnetic field experiences a torque when an electric current is set up in it. The deflection of the coil is determined by a pointer attached to it, moving on the scale. When a coil carrying current I is placed in a radial  magnetic field, the coil experiences a deflection θ which is related to I as                                           

                                                                 I=kθ 

where k is a constant of proportionality and is termed as figure of merit of the galvanometer. 

Procedure:  

1. Clean the connecting wires with sand paper and make neat and tight connections as per the circuit diagram
 2. From the high resistance box (R-BOX 1) (1-10 kΩ), remove 5 kΩ key and then close the key K1. Adjust the resistance R from this resistance box to get full scale deflection on the galvanometer dial. Record the values of resistance, R and deflection θ.
 3. Insert the key K2 and keep R fixed. Adjust the value of shunt resistance S to get the deflection in the galvanometer which is exactly half of θ. Note down S. Remove plug K2 after noting down the value of shunt resistance, S.
 4. Take five sets of observations by repeating steps 2 and 3 so that θ is even number of divisions and record the observations for R, S, θ and 2 in tabular form.
 5. Calculate the galvanometer resistance G and figure of merit k of
galvanometer using Equations.

OBSERVATIONS:

 Emf of the battery E =.....V
 Number of divisions on full scale of galvanometer = ....
RESULT:
 1. Resistance of galvanometer by half deflection method, G = ... Ω
2. Figure of merit of galvanometer, k = ...ampere/division

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EXP-3> TO VERIFY LAWS OF COMBINATION OF RESISTOR USING A METER BRIDGE.

Materials Required: Meter bridge, galvanometer, one way key, a resistance box, battery, jockey, two unknown resistance wires, and connecting wires.

Theory: The effective resistance Xs of the combination of two resistors when connected in series can be calculated by :

                                     Xs = X1+ X2-----------(i) 

The resistance (X ) of a resistor is given by:

                                      -----------(ii)

Where ‘l’ is the balancing length and R, the known resistance.

  

Procedure:

 (i) Measure the unknown resistances X1and X2 separately using the procedure explained in experiment 2, after making the suitable connections as done in experiment 2. Here you can find the value of unknown resistances without interchanging the R. B. and unknown wire in the gaps.
 (ii) After finding the value of X1 and X2, connect the two in series as shown in figure.
 (iii) Proceed in the same way as in experiment 2. Take at least three sets of observations for balance point lying between 30 cm and 70 cm.
 (iv) Calculate the effective resistances using equation (ii).
 (vi) Compare the measured values of effective resistances using Meter Bridge with the calculated values.

OBSERVATIONS:

 1. Range of ammeter = 0 ... mA to ...mA

2. Least count of ammeter = ... mA

3. Range of voltmeter = 0 ... V to ...V

4. Least count of voltmeter = ...V

5. Length of the wire of unknown resistance, L = ...cm

RESULT: 

The value of unknown resistances X1 = ... and X2 = ....
The value of effective resistance in series:
                                                       By calculation = ...

                                                       By experiment, = ....
Hence the law of combination of resistances in series is verified.

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EXP-4> To find resistance of a given wire using meter bridge and hence determine the specific resistance of its material.

Materials Required: A meter bridge, galvanometer, one way key, a resistance box, a battery jockey, unknown resistance wire about 1 meter long, screw gauge and connecting wires.

 Theory: A meter bridge is the practical application of Wheatstone bridge arrangement as shown in figure below. The four resistances are connected to each other as shown and if the bridge is inbalanced state, i.e., there is no deflection in the galvanometer (G),

                                                                         P/Q=R/S

We can use this relation to find the unknown resistance of the given material of wire. The unknown resistance 'X' can be found by Meter bridge which uses the principle of Wheatstone bridge.

The unknown resistance 'X' of the given wire is obtained by relation:

                                                                      

 And specific resistance of the material of a given a wire is obtained by:

                                                                      

where d = diameter of the wire and L = length of wire.

Procedure:

1. Find the average diameter of the wire with a screw gauge. From this, obtain the value of its radius r.
 2. Clean the insulation at the ends of connecting wires with a piece of sand paper. Tighten all plugs of the resistance box (RBOX) by pressing each plug.
 3. Set up the circuit as shown in Fig. E 2.1 with unknown resistance wire of known length in gap E.
 4. Next, introduce some resistance R in the circuit from the resistance box. Bring the jockey J in contact with terminal A first and then with terminal C. Note the direction in which pointer of the galvanometer gets deflected in each case. Make sure that jockey remains in contact with the wire for a fraction of a second. If the galvanometer shows deflection on both sides of its zero mark for these two points of contact of the jockey, null point will be somewhere on the wire AC. If it is not so, adjust resistance R so that the null point
is somewhere in the middle of the wire AC, say, between 30 cm and 70 cm.
 5. If there is one-sided deflection, check the circuit again, especially junctions, for their continuity.
 6. Repeat step 4 for four different values of resistance R.
 7. Interchange the position of the resistances S and R and repeat steps 4 to 6 for the same five values of R. While interchanging S and R, ensure that the same length of wire of resistance S is now in the gap F. The interchange takes care of unaccounted resistance offered by terminals. 

OBSERVATIONS:

1. Length of the wire of unknown resistance, L = ...cm
2. Measurement of diameter of wire of unknown resistance
         Least count of the screw gauge (L.C.) = ... mm
         Zero error of the screw gauge = ...mm
        Zero correction of the screw gauge = ...mm

RESULT: 

1. The unknown resistance of the given wire is found to be S + ΔS = ... ... Ω

2. The resistivity of the material of the wire is ρ ± Δρ = ... ... Ω m

      Here S and ρ are mean values. ΔS and Δρ are maximum of the

      five values of error.

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EXP-5> To find frequency of AC mains with help of a sonometer.

Materials Required: Sonometer with non-magnetic wire (brass wire) stretched over it, step-down transformer of 6-8 volts, horse shoe magnet, meter scale, screw gauge and half kg weights.

 Theory: The frequency of A.C mains is given by the following formula:

              n = 1/2l√ (T/m)

Where,

             l = length of the sonometer wire between the two bridges when it is thrown into resonant vibrations.

             T = tension applied to the wire.

             m = mass per unit length of the wire

        

Procedure:

(i) Connect the primary of the step down transformer to A.C mains, while the secondary to the two ends of the sonometer wire.

(ii) The horse shoe magnet is placed in the middle of the wire such that the magnetic field is applied in a horizontal plane and at right angles to the length of the wire.
 (iii) Hang a mass M ( say ½ kg) from one end of the wire and adjust the distance l between two bridges C and D symmetrically with respect to magnet till the wire appears to be vibrating with the maximum amplitude. Note the distance l between the two bridges.
 (iv) By increasing the tension on the wire, repeat the experiment for three or more different tensions.
 (v) Take readings with decreasing weight.
 (vi) Measure the diameter of the wire in mutually perpendicular directions at various points.
 (vii) Note the density of the material from the table of constants

OBSERVATIONS:

 1. Length of the wire = ...cm = ... m
2. Mass of the wire = ...g = ...kg
3. Mass per unit length, m = ... g/cm = ...kg/m
4. Acceleration due to gravity, g = ...ms-2

RESULT:

 1. Frequency of ac supply f = n/2
          (i) from calculation ...Hz
          (ii) from graph ...Hz 

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EXP-6> To draw I-V characteristics curve of a P-N junction in forward bias and reverse bias.

Materials Required: Diode Characteristics Kit , Power Supply, Ammeter (0-20mA), Voltmeter (0-20V), Connecting Leads.

 Theory: A P-N junction is known as Semiconductor diode or Crystal diode. It is the combination of P-type & N-type Semiconductor. Which offers Nearly zero resistance to current on forward biasing & nearly infinite Resistance to the flow of current when in reverse biased.

Forward biasing : When P-type semiconductor is connected to the +ve terminal and N-type to –ve terminal of voltage source. Nearly zero resistance is offered to the flow of current.

 Reverse biasing : When P-type semiconductor is connected to the –ve terminal and N-type to +ve terminal. Nearly zero current flow in this condition. 

Procedure:

(1) Connect the ckt. as shown in fig.

(2)Switch on the power supply.

(3)Vary the value of input dc supply in steps.

(4)Note down the ammeter & voltmeter readings for each step.

(5)Plot the graph of Voltage Vs Current

OBSERVATIONS:

1. Range of ammeter = 0 ... mA to ...mA

2. Least count of ammeter = ... mA

3. Range of voltmeter = 0 ... V to ...V

4. Least count of voltmeter = ...V

RESULT:

The graph has been plotted between voltage and current. 

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EXP-7> To draw characteristics of a zener diode and to determine its reverse breakdown voltage.

Theory: A Zener Diode is constructed for operation in the reverse breakdown re-gion.The relation between I-V is almost linear in this case

                                                     Vz =Vz0+Iz Rz,

where Rz is the dynamic resistance of the zener at the operating point.

Vz0 is the voltage at which the straight-line approximation of the I-V characteristic intersects the horizontal axis. After reaching a certain voltage, called the breakdown voltage, the current increases widely even for a small change in voltage. However, there is no appreciable change in voltage. So, when we plot the graph, we should get a curve very near to x-axis and almost parallel to it for quite sometime. After the Zener potential

Vz there will be a sudden change and the graph will become exponential.

Procedure:

 1.Draw a neat circuit diagram as shown in figure. Connect all the components by leads and ensure that the Zener diode is reverse biased. Also ensure that milliammeter is connected in series with Zener diode having protective resistance and voltmeter is connected in parallel with Zener diode. Now adjust the slider of rheostat so that the power supply shows minimum potential. 

2.Switch on the power supply and gradually increase the potential difference applied across the Zener diode and note the reading of potential difference from voltmeter also note corresponding value of reverse current in milliammeter.
 3.Increase the value of applied potential difference in the steps of 0.5 V and read the corresponding current in milliammeter to each applied potential.
 4.Continue increasing the potential difference till you get a sudden increase in the reverse current in microammeter.
 5.The reverse potential corresponding to this value of reverse. current is the breakdown or Zener-voltage of the Zener diode. Take the observations near the breakdown voltage by varying applied potential difference in the steps of 0.1 V.
 6.Record all the observations in the table given.
 7.Plot the graph between V and I as shown.
 8.Mark on the graph the value of Breakdown Voltage or Zener Voltage Vz as shown. Take the value of V, corresponding to I, where it suddenly increases. This value of V, is called Zener voltage or Breakdown voltage Vz.

OBSERVATIONS:

 Least count of milli-ammeter =...................mA
Least count of voltmeter =.......................V

RESULT:

The characteristic curve of the given Zener is plotted as shown in the graph.

The reverse breakdown voltage of the given Zener Diode is ……………V 

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EXP-8> To study characteristics of a common emitter N-P-N or p-n-p transistor and to find out the value of current and voltage gain.

Theory: An NPN transistor can be considered as two diodes with a shared anode region.In typical operation, the emitterbase junction is forward biased and the base collector junction is reverse biased. The electrons in the base are called minority carriers because the base is doped p-type which would make holes the majority carrier in the base.The base region of the transistor must be made thin, so that carriers can diffuse across it in much less time than the semiconductor’s minority carrier life-time, to minimize the percentage of carriers that recombine before reaching the collector base junction.

                                                                 β=∆Ic/∆Ib

Procedure: 

Connect -5V and -12V DC power supplies at their indicated position from external source or

ST2612 Analog Lab. 

 To plot input characteristics proceed as follows :
 1. Rotate both the potentiometer P1 and P2 fully in CCW (counter clockwise direction).
 2. Connect Ammeter between test point 2 and 3 to measure input base current IB (mA).
 3. Short or connect a 2mm patch cord between test point 4 and 5
 4. Connect one voltmeter between test point 1 and ground to measure input voltage VEB and

another voltmeter between test point 6 and ground to measure output voltage VEC.
 5. Switch ‘On’ the power supply.
 6. Vary potentiometer P2 and set a value of output voltage VEC at some constant value (1V, 3V,)
 7. Vary the potentiometer P1 so as to increase the value of input voltage VEB from zero to 0.8V in
step and measure the corresponding values of input current IB for different constant value of output voltage VEC in an observation Table 1.
 8. Rotate potentiometer P1 fully in CCW direction.
 9. Repeat the procedure from step 6 for different sets of output voltage VEC.
 10. Plot a curve between input voltage VEB and input current IB as shown in figure 1 using suitable
scale with the help of observation Table l. This curve is the required input characteristic.

OBSERVATIONS:

 1. Range of ammeter = 0 ... mA to ...mA

2. Least count of ammeter = ... mA

3. Range of voltmeter = 0 ... V to ...V

4. Least count of voltmeter = ...V

RESULT:

Input resistance Rin = ______________

Output resistance Rout = ______________

Current Gain bac = ______________

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EXP-9> a) To determine angle of minimum deviation for a given prism by plotting graph between angle of incidence and angle of deviation.

b) To determine refractive index of material of prism.

Requirements: Drawing board, a white sheet of paper, prism, drawing pins, pencil, half meter scale, office pins, graph paper and a protractor.

Theory:  For angle of deviation

                                  δ = (i – r) + (e – r′) = i + e – A

At the position of the prism for minimum deviation δm, the light ray

passes through the prism symmetrically, i.e. parallel to the base so

that when

δ = δm, i = e which implies r = r′.

The refractive index (n) of the material of the prism is given by

                                           

where Dm angle of minimum deviation and

And A angle of prism.

Procedure:

1. Fix a white sheet of paper on a drawing board with the help of cello tape or drawing pins.
 2. Draw a straight line XY, using a sharp pencil nearly in the middle and parallel to the length of the paper.
 3. Mark points O1, O2, O3. ., . . . . on the straight line XY at suitable distances of about 8 to 10 cm and draw normals N1 O1, N2 O2, N3 O3. . . . on these points (Fig. E 13.2). 

4. Draw straight lines, P1 O1, P2 O2, P3 O3, . . . corresponding to the incident rays making angles of incidence at 35°, 40°, 45°, 50°, ... 60° respectively with the normals, using a protractor. Write the values
of the angles ∠P1 O1 N1, ∠P2 O2 N2, ∠ P3 O3 N3,... on the white paper sheet ( Fig. E 13.2).
 5. Place the prism with its refracting face AB on the line XY with point O1 in the middle of AB as shown in the figure. Draw the boundary of the prism with a sharp pencil.

6. Fix two alpins Pl and Q1 with sharp tips vertically about 10 cm apart, on the incident ray line Pl Ql such that pin Q1 is close to point O1. Close one eye (say left) and looking through the prism, bring your right eye in line with the images of the pins Pl and Ql. Fix alpins Rl and Sl about 10 cm apart vertically on the white paper sheet with their tips in line with the tips of the images of pins Pl and Ql. In this way pins R1 and S1 will become collinear, with the images of pins P1 and Q1.
 7. Remove the pins Rl and Sl and encircle their pin pricks on the white paper sheet with the help of a sharp pencil. Remove the pins P1 and Q1 and encircle their pin pricks also.
 8. Join the points ( or pin pricks) Rl and Sl with the help of a sharp pencil and scale, to obtain the emergent ray Rl Sl. Produce it backwards to meet the incident ray Pl Ql (produced forward) at T1. Draw arrowheads on Pl Ql and R1 S1 to show the direction of the rays.
 9. Measure the angle of deviation δ l and the angle BAC (angle A) of the prism (Fig. E 13.1) with a protractor and write the values of these angles indicated in the diagram.
 10. Repeat steps 5 to 9 for different values of angle of incidence (40°, 45°, 50°...) and measure the corresponding angles of deviation δ2, δ 3... with the protractor, and indicate them in the respective
diagrams.
 11. Record observations in tabular form with proper units and significant figures.

  OBSERVATIONS:

Least count of the protractor = ...............(degree)

Angle of the prism, A = ..............(degree)

RESULT:

Angle of minimum deviation, δm = ... ± ... degree

Refractive index of the material n =...........

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EXP-10> To compare EMF's of  2 given primary cells using potentiometer.

Requirements: Potentiometer, battery, two one way key, rheostat of low resistance, galvanometer, high resistance box, fractional resistance box, ammeter, voltmeter, a cell, jockey, connecting wires etc.

Theory: The internal resistance of a cell is given by

                                                                       

E1 emf of primary cell 1 (Lechlanche cell),  l1 is the balancing length for cell 1

E2 emf of primary cell 2 (Daniel cell),  l2 is the balancing length for cell 2

Procedure:

1. The connections are made as shown in the circuit diagram. The circuit is checked for opposite side deflections.
 2. Using DPDT switch the Leclanche cell is included in the secondary circuit. The jockey is pressed on the potentiometer wire.
 3. The point (J) where the galvanometer wire shows full scale deflection is noted.
 4. The balancing length AJ = l1 is measured.
 5. Using DPDT switch the Daniel cell is included in the secondary circuit.
 6. The above steps are repeated and the balancing length l2 is measured.
 7. By varying the rheostat values l1, l2 are measured and the readings are tabulated.
 8. The ratio of emf of the given two primary calls are calculated using the formula 

OBSERVATIONS:

Balancing length for Lechlanche cell,

                                                        l1=..................cm.

Balancing length for Daniel cell,

                                                l2 =......................cm.

RESULT:

EMF of 2 given cells are:

                                           E1 =....................V

                                           E2 =...................V 

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EXP-11> To determine the internal resistance of a given primary cell using potentiometer.

Requirements: Potentiometer, battery, two one way key, rheostat of low resistance, galvanometer, high resistance box, fractional resistance box, ammeter, voltmeter, a cell, jockey, connecting wires etc.

Theory: The internal resistance of a cell is given by

                                                                        

Where length 1 and 2 and are the balancing lengths without shunt & with shunt, respectively & R is 

the shunt resistance in parallel with the given cell.

Procedure: 

1. Connect different electrical components as shown in the circuit (Fig. E 5.1). After checking the circuit connections, close key K1.
 2. With keys K2 and K3 open and a protective high resistance P from the R-BOX 2, find the position of the balance point. For final reading, short circuit the resistance P by closing the key K3 and find the balance length lo.
 3. Take R = 10 Ω (from RBOX 1), close the key K2 and quickly measure the new balance length l . Open K2 as soon as this has been done.
 4. Keep the readings in the ammeter constant throughout the above observation.

5. Reduce the value of R in equal steps of 1 Ω and for each value of R obtain the balance length l.
 6. At the end of the experiment, open key K2 and repeat step 2 to find lo again.

OBSERVATIONS:

lo=.....cm(in the beginning of the experiment)

lo =......cm(in the beginning of the experiment)

Mean lo = ... cm.  

RESULT:

The internal resistance of the given cell r
(i) by calculation ... Ω
(ii) by graph ... Ω

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EXP-12> To find focal length of a concave lens using a convex lens.

Requirements: An optical bench with uprights for holding lens, mirror and two needles, two needles (pins), a thin convex lens, a convex mirror,  index needle (may be a knitting needle or a pencil sharply pointed at both ends), a meter scale and a spirit level.

Theory: A convex lens L1 converges the light rays starting from the object AB to form a real and inverted image A′B′ at position I1 [Fig. E 12.2(a)]. If a concave diverging lens L2 is inserted between the lens L1 and point I1 as shown in Fig. E 12.2 (b), for concave lens L2 image A′ B′ behaves as virtual object. A real and inverted image A′′ B′′ is formed at point I2 by the diverging lens L2. Thus, for the concave lens L2 the distances O′ I1 and O′ I2 would be the distances u and v, respectively. It is important to note that the focal length of convex lens L1 must be smaller than the focal length of the concave lens L2. The second image A′′ B′′ is formed only when the distance between lens L2 and first image A′B′ is less than the focal length of L2.
The focal length of the concave lens L2 can be calculated from the relation

  

                                                             

Here for the concave lens both distances u and v are positive and since u will be found to be less than v, f will always be negative.

Procedure: 

1. In case, if the focal length of the given thin convex lens is not known then rough value of its focal length (fL) should be estimated first to ensure that its focal length is less than that of the concave lens.

2. Place the optical bench on a rigid platform and using the spirit level, make it horizontal with the help of levelling screws provided at the base of the bench.

3. Place the uprights mounted with pin P1 (object pin), convex lens L1, and another pin P2 (image pin) on the optical bench. You may put a small piece of paper on image pin P2 to differentiate it from the image of object pin P1 [Fig. E 12.2(a)].

4. Check the col-linearity of the tip of pin P1, optical center O of convex lens L1, and the tip of image pin P2 along a horizontal straight line  which is parallel to the length of the optical bench. In this condition the planes of lens and both the pins would be perpendicular to the axis of the lens.

5. For the determination of the index correction, bring a mounted pin close to the concave lens L2. Adjust the index needle (a sharp edged knitting needle would also serve the purpose) horizontally such that its one end touches one of the curved surfaces of the lens and the other end touches the tip of the pin. Note the positions of the two uprights on the scale provided on the optical bench. The difference of the two would give the observed length of the index needle. The actual length between the tip of the pin and optical

center O′ of the lens L2 would be length of the index needle (as measured by a scale) plus half of the thickness of the lens at its

optical center. The difference of the two lengths is the index correction. (If the concave lens is thin at the center, its thickness at the center can be ignored).

6. Separate the object pin P1 from the convex lens by a distance slightly greater than the focal length fL of the lens.

7. Locate its real and inverted image at point I1 on the other side of the lens by removing the parallax between the image pin P2 and image of the object pin P1 [Fig. E 12.3(a)].

8. Read the positions of the uprights holding the object pin P1, convex lens L1, and image pin P2 (i.e. point I1). Record these observations in Table E 12.1.

9. From now on, do not change the position of the convex lens L1 and the position of the object pin P1. Insert the concave lens L2 in between the convex lens L1 and image pin P2. Now the image of object pin will shift further from the convex lens L1 to a point I2(say). Adjust the position of the concave lens so that the point I2 is sufficiently away from the point I1.

10. In case the image formed by the combination of convex and concave lenses is not distinctly visible, try to see it on moving the concave lens nearer to the point I1 and to locate the image by using a pencil held in hand, and keeping the image pin P2 at point I1 as a guide to decide which way to shift the concave lens L2. After having seen the clear image at point I2 and ensured that it lies within the range of the optical bench, move image pin P2 to locate the image (or point I2) more accurately using the method of parallax [Fig. E 12.3(b)]. Since the image forming at I2 is quite enlarged, it can be blurred.

11. Note the position of uprights holding the concave lens and image pin P2, i.e., point I2. Note the readings in the Observation Table.

12. Change the position of upright holding the object pin P1 and repeat the steps 6 to 10. Take five sets of observations.

OBSERVATIONS:

1. Focal length of the convex lens, fL = ... cm

2. Length of the index needle as measured by the scale, s = ... cm

3. Thickness of the thin concave lens (given) at its optical centre,

                                                                                                    t = ... cm

4. Actual length between the optical centre O of the lens and tip of the pin,

                                                                                                   l = s + t/2 = ... cm

5. Observed length of the index needle, l′

                              = Distance between the pole of the lens and tip of the pin

                              = Position of lens upright - position of pin upright on the scale

=......cm.

RESULT:

The focal length of the given concave lens is (f ± Δf) = ... ± ...cm.

Here f is mean value of the focal length.

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EXP-14> To determine refractive index of a glass slab or transparent liquid using a traveling microscope.

Requirements: Given liquid or glass slab, glass beaker, traveling microscope, lycopodium powder, pin, etc.

Theory: A traveling  microscope is an ordinary microscope fixed on a stand in such a way that it may be made to travel in vertical as well as horizontal direction without disturbing its adjusted focus.The readings are recorded by means of main scale and vernier scale of high accuracy (0.001 cm) attached to the instrument. 

The refractive index of glass slab or liquid water:

                                                                           

Procedure:

1.The least count of the microscope is determined as in the case of vernier calipers.

2.A small pin is fixed horizontally with wax or cello tape at the bottom of the empty beaker. The tip of pin is focussed clearly on the microscope and the corresponding main scale reading (MSR) and vernier scale

coincidence (VSC) in the vertical scale are noted in the tabular column. (Reading 1)

3.The given liquid is taken in the beaker. Now the pin is apparently raised through a height. Therefore, it will be no longer in focus. The microscope is moved up without changing the adjusted focus, so that the image of the pin is clearly seen through the microscope. The corresponding main scale reading (MSR) and vernier scale coincidence (VSC) in the vertical scale are noted in the tabular column.(Reading 2)

4.Finally, a little lycopodium powder or saw dust which can float on liquid is scattered on the surface of the liquid. The microscope is further moved up without changing the adjusted focus, so that the clear image of

lycopodium powder is seen through the microscope. The corresponding main scale reading (MSR) and vernier scale coincidence (VSC) in the vertical scale are noted in the tabular column. (Reading 3)

5.The difference between this Reading 3 and Reading 1 gives the real depth of the liquid, whereas the difference between Reading 3 and Reading 2 gives the apparent depth of the liquid.

6.By substituting the readings in the formula, the refractive index of the liquid (water) is determined.

OBSERVATIONS:

To find Least count of microscope:
                                   Least count: Value of 20 MSD = .......cm
                                   Value of 1 MSD =........ cm
                                   No. of divisions on the vernier scale(n) = ......... 

Least Count (LC) = 1/n x value of 1 MSD

                                                                  =..............cm

RESULT:

 The refractive index of glass slab or liquid water =........................

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EXP-15> To find focal length of a convex lens by plotting a graph between "u and v" or 1/u and 1/v.

Requirements: An optical bench with three uprights (central upright fixed, two outer uprights with lateral movement), a convex lens with lens holder, two optical needles, (one thin, one thick) a knitting needle and a half meter scale.

Theory: The relation between u, v and f for a convex lens is

                                                               

Where,

             f= focal length of convex lens

            u= distance of object needle form optical center of the lens

            v= distance of image needle from optical center of the lens.

TERMS AND DEFINITIONS: 

1. Principal axis of a lens is the line joining center of curvature of the two surfaces.

2. Optical center is the point, through which a ray passes undeviated through the lens.

3. Principal focus is the point where rays parallel to the principal axis focus after passing through the lens (convex) or appear to come from after passing through the lens (concave).

4. Focal length is the distance between optical center of lens and focus.

5. Intercepts of a graph: If a graph cuts x-axis and y-axis, then lengths between origin and points of interception are intercepts

Procedure: 

1.Obtain approximate value of the focal length of the thin convex lens by focusing the image of a distant object. It can be found by obtaining a sharp image of the Sun or a distant tree on a screen, say a plane wall, or a sheet of paper placed on the other side of the lens and measuring the distance between the lens and the image with a scale. This distance is a rough estimate of the focal length, f of the convex lens.

2. Place the optical bench on a rigid table or on a platform, and using the spirit level, make it horizontal with the help of leveling screws provided at the base of the bench.

3. Clamp the convex lens on an upright and mount it vertically almost near to the middle of the optical bench such that its principal axis is parallel to the optical bench. In this position, the lens would lie in a plane perpendicular to the optical bench.

4. For the determination of the index correction, bring a mounted pin close to the lens. Adjust the index needle (a sharp-edged knitting needle would also serve the purpose) horizontally such that its one end touches one of the curved surfaces of the lens and the other end touches the tip of the pin. Note the positions of the two uprights on the scale provided on the optical bench. The difference of the two would give the observed length of the index needle. The actual length between the tip of the pin and optical center O would be length of the index needle (as measured by a scale) plus half of the thickness of the lens because optical center of a double convex lens with surfaces of equal curvature is at its geometrical center. The difference of the two lengths is the index correction. Find index correction for both the pins.

5. Place the vertically mounted sharp pins P and P′ (Fig. E 10.3) on left and right hand sides of the lens

respectively. Adjust the pins P and P′ so that the heights of the tips of these pins become equal to the height of the optical center O of the lens from the base of the optical bench. Let the pin P (placed on left hand side of the lens ) be the object pin and the pin P′ (lying on right hand side) be the image pin. Put a small piece of paper on one of the pins (say on image pin P′) to differentiate it from the object pin P′.

6. Displace the object pin P (on left side of the lens) to a distance slightly less than 2f from the optical centre O of the lens (Fig. E 10.3). Locate the position of the real and inverted image on the other side of the lens above the image pin P′.

7. Using the method of parallax, adjust the position of the image pin P′ such that the image of the object pin P coincides with the image pin P′.

8. Note the upright position of the object pin, convex lens and image pin on the optical bench and record the readings in an observation table.

9. Move the object pin P closer to the optical centre O of the lens (say by 2 cm or 3 cm). Repeat the experiment and record at least six sets of readings for various distances of object pin between f and 2f from the lens.

OBSERVATIONS:

1. Approximate focal length of the convex lens = ... cm
2. Length of the index needle as measured by the meter scale,
                                                                          Lo =... cm
3. Thickness of the thin convex lens (given), t = ... cm
4. Actual length between the optical center O of the lens and tip of the pin, lo = Lo + t/2 = ... cm
 5. Observed length of the index needle, l′o = Distance between the center of convex lens and tip of the object pin  = Position of lens upright – position of object pin upright on the scale.
                                                                                                               = ... cm – ... cm =...cm

6. Index correction for object distance, eo = lo – l′o = cm; similarly for image pin

                 

                                   ei = li-l'i   = ......cm  

RESULT:

 The focal length of the given converging thin convex lens:
 (i)  f ± f = ...cm (here f is mean value of the focal length)
(ii) from u – v graph =...cm, and
(iii) from 1/u-1/v graph=....cm

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