FIRST SEMESTER B E-I PHYSICS PRACTICALS MANUAL

APPLIED PAHSICS DEPARTMENT

FACULTY OF TECHNOLOGY AND ENGINEERING

THE M S UNIVERSITY OF BARODA

VADODARA, GUJARAT, INDIA- 390001

APPLIED PHYSICS PRACTICALS LIST

LIST OF EXPERIMENTS for BE- 1 (July – 2007)

LIST OF EXPERIMENTS for BE- 1 (July – 2007)

1. To study the characteristics of ultrasonic waves.

2. Measurement of temperature using mercury thermometer, thermistor (PTC, NTC),

thermocouple and platinum resistance (PT-100) sensors.

3. To study p-n junction solar cell characteristics and determine its efficiency.

4. Wavelength and energy gap determination using Newton’s rings in case of Light emitting diode (LED).

5. He-Ne laser beam parameters: laser beam power, spot size and divergence angle.

6. To find the wavelength of light emitted by sodium vapor lamp using Fresnel’s

biprism.

7. To study half-wave, full-wave and bridge rectifiers. To determine the DC output voltage and the ripple factor of the bridge rectifier using a capacitor filter.

8. To determine the resolving power of a telescope.

9. L-I-V characteristics of a semiconductor laser (Laser Diode) and to determine its

parameters.

10. To determine the value of the Rydberg’s constant with the help of a spectrometer using a hydrogen gas discharge tube.

EXPERIMENT 1

AIM: To study the properties of the ultrasonic waves. APPARATUS: Cathode Ray Oscilloscope (CRO), Signal/Frequency generator, Ultrasonic

Transducers (One Transmitter and Two Receivers), connecting wires, etc. THEORY:

Interference: λ = d sin ∆θ ; where λ → Wavelength of the ultrasonic wave; d → Separation between two transmitters; D → Distance between the midpoint of the two transmitters and the receiver; ∆x → Fringe separation (i.e., ∆x = distance between two consecutive maxima); ∆θ ( ≈ ∆x/D) → ‘Angular fringe separation’.

PROCEDURE: This experiment has the following four parts:

1) Finding the resonance frequency of the ultrasonic transducer. 2) Reflection of the ultrasonic waves by a flat surface. 3) Diffraction of the ultrasonic waves through a single slit. 4) Interference between two ultrasonic waves.

Part 1: To find the resonance frequency of the ultrasonic transducer.

Figure 1. Measurement of frequency using CRO Part A:

• Connect the circuit as shown in Fig. 1.

• The signal generator is set for a ‘sine’ wave, with the voltage range set to 30V (or, depending on the type of your signal generator, select the appropriate range for the input voltage), and the frequency range is set for 100 kHz. Adjust the appropriate knobs of the signal generator to have 6.0 Vpp and 40 kHz.

• Now connect the output of the function/signal generator to the channel input of the CRO. Determine the voltage and the time period (and, hence, the frequency, f = 1/T) of the signal from the function generator using the trace obtained on the CRO.

NOTE: The “CAL” knob on the CRO should NOT be disturbed. Part B:

Figure 2. Determination of the resonance frequency of the transducer. (The transmitter and

the receiver are represented by different symbols but they are identical in structure.)

• Disconnect the signal generator from the CRO. • Now connect the signal generator to the transmitter (T) as shown in Fig. 2. • The receiver (R) is kept facing the transmitter about 50 cm away; the receiver is connected with

the CRO at some convenient settings of volt/div and time-base knob (e.g., 0.5V/cm and 10µs/cm). • Resistor/potentiometer of 10kΩ is connected as shown in the Fig. 2. • To find the resonance frequency adjust the time-base knob of the CRO to obtain a sinusoidal

waveform on the screen and measure its period, T, and hence calculate the frequency, f ( = 1/T). • Draw the trace on the graph paper with appropriate scales.

Part 2: Reflection by a flat surface.

• The connections are same as that of the previous part. • Now, as shown in Fig. 3, keep the receiver (R) and the transmitter (T) side by side facing a wall (or

any other surface that can act as a reflector of ultrasonic waves). • One should get the sinusoidal waveform. Here the variation in the amplitude can be studied by

varying the position of the receiver in an arc.

Figure 3. Reflection by a flat surface.

• When both the transducers are aligned properly, i.e., i = r, one gets a maximum amplitude. • Measure the period and thus find the frequency. • Observe what happens for other positions of T and R. Part 3: Diffraction through a single slit.

Figure 4. Diffraction through a single slit.

• To observe the diffraction of the ultrasonic waves through a “slit.” • Here the transmitter (T) is placed about 50 cm behind the slit and the receiver (R) is placed about

25 cm in front of the slit as in Fig. 4. Observe the variation in the amplitude of the trace on the CRO by keeping the transmitter at the fixed position and moving the receiver in the transverse direction (Y-axis in Fig. 4.) The central maximum will be found near the axis of symmetry with secondary maxima and minima in the two shaded regions.

• Observe what happens for different distances of T and R and also for different width of the slits. Record qualitatively your observations.

Part 4: Interference property of the Ultrasonic Waves

Figure 5. Set up to study Ultrasonic Interference.

• In order to study the interference property of the ultrasonic waves we require two transmitters and

one receiver as shown in Fig. 5. (Both the transmitters are connected with the signal generator whereas the receiver is connected to the CRO.)

• Observe the variation in the amplitude of the trace on the CRO by keeping the transmitters at the fixed positions and moving the receiver in the transverse direction (Y-axis in Fig. 5.). Note the positions for consecutive maxima. Record your observations.

OBSERVATIONS: Part 4:

f = _____ Hz; D = ____ cm; d = _____ cm.

OBSERVATION TABLE:

x [cm]

∆x [cm]

λ = (∆x d/D) [cm]

v = fλ [m/s]

CALCULATIONS: Show sample calculations for each equation used. RESULTS: DISCUSSION OF RESULTS:

CONCLUSIONS:

D

EXPERIMENT 2 Aim: Measurement of temperature using mercury thermometer, thermistor (PTC, NTC),

thermocouple, platinum resistance (PT-100) sensors and compare the different temperature sensors

Apparatus: Thermistor (NTC and PTC); Thermocouple (Copper Constantan); Pt 100; flask, stand,

Digital Multimeter (DMM), Micro Ammeter/Voltmeter, test tube, stand, beaker, burner etc. Diagram:

1-Pt 100 2-PTC 3-NTC 4-Thermocouple (Copper-constantan) 5-Test Tube 6-Beaker 7-Parafin 8-Water 9-Mercury Thermometer 10- Rubber Cork DMM – Digital Multimeter DV-Digital DC micro volt ammeter K- Four point contact key

6

2194

8 7

5

3

10

DMM

3 2 1

K

4

D V

Gas Burner

Tripod Stand

Procedure:

• Connect the circuit as per given figure. • Note reading at room temperature (RT). • Light the gas burner and keep below the beaker kept on the tripod. • Note reading at every increase (Heating) of 5 [oC] up to 75 [oC]. • Repeat the same for cooling. • Plot four graphs for individual sensors as shown below.

Precaution:

• Remove the burner 1 [oC] before the required temperature (as in column-1) is reached. Wait for required temperature and note down your readings.

Observation Table

NTC [kΩ]

PTC [Ω]

Pt 100 [Ω]

Thermocouple [mV] Temperature

[°C] Heating Cooling Heating Cooling Heating Cooling Heating Cooling

Room Temp. 35 40 45 50 55 60 65 70 75

Note: Temperature measurement using Mercury Thermometer. Graph:

Conclusion:

NTC/ PTC Thermocouple

R [Ω]

T [oC]

Pt 100 a) b) c)

T [oC] T [oC]

emf [mV] R [Ω]

EXPERIMENT 3 AIM: To determine the current-voltage (I-V) characteristics of a p-n junction Solar

cell for a given constant light intensity and to determine

i) Open circuit voltage (Voc) ii) Short circuit current (Isc)

iii) Fill Factor (FF)

iv) Maximum Output power (Pmax)

v) Efficiency (η) APPARATUS: Solar Cell, Light Bulb, Digital Multimeter (DMM) and 100Ω ten turn potentiometer

Formula: Fill Factor (FF) =scoc

mpmp

scoc IVIV

IVP

=max

Efficiency (η) = 1001max

APP

in

A- Area of Solar Cell

Circuit Diagram:

PROCEDURE:

• A 60 watt lamp is placed above the solar cell at a distance ‘D’ from it. • The load resistance (100 Ohm ten turn potentiometer) is not connected into the circuit, open circuit

voltage is noted. • Connect the circuit as shown in the figure. • The load resistance is varied with the ten turn potentiometer. Note down current value at every

change of 50mV voltage. • Repeat this for different intensity of light. • Plot the I-V characteristics for each intensity of light and determine the efficiency.

PRECAUTIONS:

• The terminals of the solar cell should be connected properly (Red Terminal = + and Black Terminal = -)

-+ I

100 Ohm

-

++

-

Solar Cell V

Input power (Pin) to the solar cell:

Distance between lamp and solar cell D [cm]

Intensity of light bulb [mW/cm2]

13 15 10 25 07 28

(Measured using Central Electronics Limited make ‘Suryamapi-SM201’)

OBSERVATIONS:

Area of the solar cell = ________cm2 OBSERVATION TABLE:

For Pin = 15 [mW/cm2] For Pin = 25 [mW/cm2] For Pin = 28 [mW/cm2]

V [mV] I [mA] V [mV] I [mA] V [mV] I [mA]

Note: Plot your V, I data for each input power on the same graph paper

From Graph 1. I sc =______ µA and I max =_______ µA 2. Voc =______mV and Vmax =______mV

CALCULATIONS: 1. P max = Imp x V mp = _______ Watt 2. Fill Factor (FF) = ________ 3. Efficiency (η) =________%

GRAPHS:

RESULTS: CONCLUSIONS:

V [mV]

I [mA]

Pmax = Vmax x Imax

Vmax=Vmp Voc

Isc

Imax=Imp

EXPERIMENT 4

AIM: 1. To determine the radius of curvature of a given plano-convex lens using a source of

known wavelength and the phenomenon of interference of light viz. Newton’s rings.

2. To determine the unknown wavelength of the given LED by obtaining Newton’s

rings using the plano-convex lens of known radius of curvature and hence determining the energy band gap of the given LED

APPARATUS: Traveling microscope, Plano-convex lens, Glass plates, LEDs with power supply, Magnifying glass, etc.

THEORY: n) - (m 4

D - D R

2n

2m

λ= (1)

where R → Radius of curvature of a given plano-convex lens; Dm→ Diameter of the mth ring; Dn→ Diameter of the nth ring; m, n → numbers of the rings as measured from the center of the ring; λ → wavelength of the light source.

λ

=hc E (2)

where h → Planck’s constant = 6.626 x 10-34 J-s; 1 eV = 1.6 x 10-19J; c → speed of light = 3 x 108 m/s.

Diagram:

Division of amplitude

Newton’s Rings

PROCEDURE: • Please ensure that all the glass plates and lenses are clean. • Familiarize yourself with the Traveling Microscope (TM); in particular, with the fine and coarse

motion of the TM. • Recall how to read the scales on the TM: identify the Main Scale and the Vernier Scale. Find the

Least Count of the TM and make sure that you know how to read the scale (see below). • Align the source of light of the given wavelength such that the light is incident at the center of the

inclined glass plate as shown in the diagram. (Note: The success of optic experiments depends largely on good alignment.)

L

M

L- LED M- Microscope G- Glass Plate P-Plano Convex Lens

G

P

to LED power supply

• Focusing of the TM: First of all, move the eyepiece back and forth until the cross-wires are distinctly observed. Now take a small piece of paper and mark on it a ‘cross’ and put it on the top of the horizontal glass plate. Focus the TM on the marked ‘cross’. Do not disturb this focusing arrangement throughout the experiment. Do not forget to remove the paper after focusing is done.

• Identify the flat and the curved surfaces of the plano-convex lens. Take the plano-convex lens and place it on the horizontal glass plate as shown in the diagram.

• If your adjustments are OK then you should be able to see the Newton’s Rings. If not, then go back and repeat the previous steps carefully.

• Once the rings are obtained ensure that the center of the pattern of the rings and the intersection of the cross-wires coincide. The horizontal cross-wire should be along the diameter of the rings.

• Count the rings from the center, which is taken as the zeroth ring, and go to the left-hand side until you reach the 18th ring. Arrange the transverse cross-wire along the tangent line to the bright/dark 18th ring. Note the readings on the horizontal scale of the TM. Take the readings for the other numbered rings as mentioned in the observation table below. In particular, move the TM in one direction only (say, left to right) while taking the readings. This will prevent errors due to what is known as the “back-lash” error. Using the value of the wavelength, find the radius of curvature of the given plano-convex lens.

• Now without disturbing the TM (in particular, do NOT change the lens) replace the source with the source of unknown wavelength. Repeat the procedure as in the last step. Enter your observations in a new observation table.

• Find the wavelength of the second source and hence find the energy band gap for the LED source.

PRECAUTIONS: OBSERVATIONS: Smallest Division on the Main Scale (SDMS) = ______ ; Total Number of Divisions on the Vernier Scale (TNDVS) = _______;

Least Count (L.C.) = TNDVSSDMS = ---------- = _______ .

How to read the scales? Total Reading (T.R.) = MSR + VSDC x L.C. ; MSR = mark on the Main Scale which comes JUST before the zeroth mark of the Vernier scale. VSDC = mark on the Vernier Scale which coincides with some mark of the Main Scale (one should read the Vernier Scale Division)

OBSERVATION TABLE:

1) Least count of the traveling microscope = cm.

2) Wavelength of the light source λ = ___________ nm.

Microscope Readings Obs. No.

Number of Ring

LHS [cm]

RHS [cm]

Diameter of Ring D

(LHS~RHS) [cm]

D2 [cm2]

1 18 2 16 3 14 4 12 5 10 6 8 7 6 8 4

Note: You need to make another table for another source. CALCULATIONS: GRAPHS: Graph 1 (No. of rings vs diameter squre) for the known wavelength source.

λ=

=

4Slope R

BCABSlope

Note: You need to draw another graph for the second source and find the wavelength of that source using the value of R found from the first graph.

No. of Ring

D2 cm2

C B

A

0

RESULTS: Part I: Radius of curvature of the given plano-convex lens using the source of a known wavelength:

1) From calculation: R = cm; From graph: R = cm.

Part II: Wavelength of the unknown source:

2) From calculation: =λ cm; From graph: =λ ____ cm;

3) Energy band gap of the LED: E = cm.

DISCUSSION OF RESULTS: CONCLUSIONS:

EXPERIMENT 5

AIM: To determine the Helium-Neon (He-Ne) laser beam parameters viz. the spot size, and the divergence angle.

APPARATUS: Helium-Neon Laser, BPW 34 Photodetector, DMM, ruler, etc.

Figure:

Figure 1: Measuring Beam Divergence.

Figure 2. Measurement of photocurrent PROCEDURE:

• Put the He-Ne laser on the table in such a way that the area where the laser beam is pointing is clear (particularly of others working in the laboratory).

• Place the photodetector, which is attached to the traveling microscope, directly in front of the He-Ne laser such that the light from the laser is incident normally on the photodetector.

VR = 9V

mV

R = 1kΩ

+Photodetector

D

d

• Place the photodetector at a distance of one meter from the He-Ne laser. Move the photodetector along the transverse direction and record the photocurrent (Iph) and the corresponding transverse distance X.

• Repeat for a few different distances (above one meter) D. • For each D draw a separate graph of Iph vs. X. • Determine He-Ne laser spot size d from each of the above graphs as shown in figure 1. • Draw a graph of d vs D. • Calculate the slope of the graph and thus obtain the angle of divergence of the laser.

Distance (X) cm

2.85 2.90 2.95 3.00 3.05 3.10 3.15

Pho

tocu

rren

t x10

-6 (

I ph)

0

1

2

3

4

5

Figure 1. Iph vs X Note: Find Iph[max] and calculate 1/8 x Iph[max] to determine d as

shown in figure. OBSERVATION TABLE Profile of the Laser beam: D = cm

Transverse position of the photodetector X

[cm]

Photocurrent of the detector Iph

[µA]

Note: For each D make separate observation table as above.

Iph[max]

d

Divergence:

Distance between the laser and the screen

D [cm]

Diameter of laser beam spot on the screen

d [cm]

CALCULATIONS:

Draw the graph of the diameter of the spot on the screen d[mm] as a function of the distance between laser and screen D[mm] for the He-Ne Laser.

Determine the Slope of the graph = __________________

Angle of divergence = Slope = __________________ [radians].

= _______________x (180/π ) [degrees]. GRAPHS: RESULTS: DISCUSSION OF RESULTS: CONCLUSIONS:

EXPERIMENT 6 AIM: To determine the wavelength of light emitted by sodium vapor lamp using Fresnel’s

biprism APPARATUS: Optic bench with required accessories, sodium lamp, convex lenses, micrometer

eyepiece, slit, biprism’s mount Formula:

Ddβλ =

λ = Wavelength of light [nm] β = Fringe width d = Separation between the two virtual images of the slit D = distance between the slit and the eyepiece

21ddd = d1 = distance between the magnified image of the two virtual images of the slit

(Lens near the biprism) d2 = distance between the diminished image of the two virtual images of the slit

(Lens near the eye piece) Diagram:

Part-I

S- Source of light (Sodium lamp) as seen from the slit (not shown in the figure) S1, S2- Two virtual images of the source S separated by distance d B- Fresnel’s Biprism; F- fringes as seen with the help of eyepiece

d

S1 S S2

B

F

Part-II

E-eye piece; L- convex Lens

Figure a) shows an arrangement to obtain a magnified image of the virtual images, S1 and S2 of the

source S and measure the distance d1 between the real images as seen through the eyepiece.

Figure b) shows an arrangement to obtain diminished image of the virtual images of the source and measure the distance d2

PROCEDURE:

• Preliminary Adjustments: o Familiarize yourself with the optical bench and the various stands. Handle the

Biprism carefully and mount it in the appropriate stand. Also, mount the slit and the “micrometer-eyepiece” on the optical bench.

o Align the source and the mounted slit. Now place the Biprism a little behind the slit. Looking thru the eyepiece, you should be able to see the interference fringes (a vertical band of bright and dark lines).

o Lateral Shift: On moving the eyepiece further from the biprism the fringes should remain in view; if it shifts in the transverse direction then one says that there is a ‘lateral shift’. Try to remove this lateral shift before proceeding further.

• Part-I: Measurement of fringe width β:

• Adjust the vertical wire of the eyepiece on one ‘end’ of the interference pattern and note the

reading of the micrometer scale attached to the eyepiece. Take the next reading for the nth fringe, then 2nth fringe, etc., and record it. Thus find the fringe width β for a particular distance of the eye- piece from the biprism, D.

• Part-II: Measurement of the distance between the two virtual sources created by the

biprism, d:

v u

B

S1 S S2 L E

u v

B

S1 S S2 L E

a)

b)

• Without disturbing the arrangement, now introduce a convex lens between the biprism and the eyepiece.

• On moving the convex lens along the optical bench from the biprism towards the eyepiece, one should be able to see a magnified and a diminished separation between the images of the virtual line sources.*

• Now keep the lens near the biprism and obtain the image. Measure the distance between the two real line images of the slit, d1.

• Then bring the lens near the eyepiece and again obtain the image. Measure the distance between the two real line images of the slit, d2.

• Calculate the distance between the two virtual sources, d, from the measurements of d1 and d2. (* Before doing part I, one should check this step.)

PRECAUTIONS: OBSERVATIONS:

• Least count of the micrometer screw =______ cm OBSERVATION TABLE:

• Measurement of Fringe width β: n =______ and D =______

Fringe No. M. S. R. [cm]

C. S. R. [cm]

T. R. [cm]

nβ [cm]

β [cm]

M. S. R. = Main scale reading, C. S. R. = Circular scale reading and

T. R. = Total reading = M. S. R + (C. S. R. x L. C.) • Measurement of d: Convex lens near the biprism (magnified image):

Sr. No. M. S. R. [cm]

C. S. R. [cm]

T. R.=r1 [cm]

M. S. R. [cm]

C. S. R. [cm]

T. R.=r2 [cm]

d1 = r1~ r2 [cm]

Mean [cm]

Convex lens near the eye piece (diminished image)

CALCULATIONS: RESULTS:

DISCUSSION OF RESULTS: CONCLUSIONS:

Sr. No. M. S. R [cm]

C. S. R [cm]

T. R.=r1 [cm]

M. S. R. [cm]

C. S. R. [cm]

T. R.=r2 [cm]

d2 =r1~ r2 [cm]

Mean [cm]

EXPERIMENT 7 Aim: To study half-wave, full-wave and bridge rectifiers. To determine the DC output voltage

and the ripple factor of a bridge rectifier using a capacitor filter.

Apparatus: Transformer, Diodes, capacitors, CRO and Digital multimeters (DMM).

Formula:

(a) Half-wave rectifier

1. DC output Voltage = PVπ

(Sine wave input)

2. Diode Peak Inverse Voltage (PIV) = VP

3. Output frequency = Input frequency = f

(b) Center-tap Full-wave rectifier

1. DC output Voltage = 2 PVπ

(Sine wave input)

2. Diode Peak Inverse Voltage (PIV) = 2 VP

3. Output frequency = 2 × Input frequency = f

(c) Full -wave Bridge rectifier

1. DC output Voltage = 2 PVπ

(Sine wave input)

2. Diode Peak Inverse Voltage (PIV) = VP

3. Output frequency = 2 × Input frequency = f

(d) Full-wave Bridge rectifier with filter

1. DC output Voltage ( )recVCfR

V PL

DC ⎟⎟⎠

⎞⎜⎜⎝

⎛−=

211

2. Ripple Voltage (Peak to peak) ( ) ( )recVCfR

ppV pL

r1

=−

3. Ripple Factor 100)(

×−

=DC

r

VppV

r %

Procedure:

Part-I

• Connect the circuit for the half-wave rectifier, as shown in the figure and obtain the output

waveform on the CRO.

• Measure the voltages at the secondary of the transformer Vs, peak voltage across the load

resistor Vp(rect) using a CRO and DMM (AC voltage).

• Use DMM (DC voltage) to measure VDC. Calculate VDC using given formula.

• Repeat the procedure for centered tapped full wave rectifier and bridge rectifier without

filter.

Part-II

• Connect the bridge rectifier circuit along with the capacitor filter as shown in the figure.

• Measure the values of Vp(rect), VDC, Vr (p-p) with the help of CRO and DMM. Also

calculate the values of VDC, Vr (p-p) by using the given formulas.

• Calculate the ripple factor both theoretically and observed. Compare the values.

• Repeat this for a given set of capacitors.

Observations Table: Without capacitor filter

Half wave rectifier Center tap full wave rectifier

Full wave bridge rectifier Measured

Parameters DMM CRO DMM CRO DMM CRO Vs

Vp (rect) VDC

Note: Calculate VDC using the formula for all the parts.

Circuit diagram

1N4007

AC Mains Ω

Half wave rectifier

CT230 V VP(rect)

RL

1N4007

A

1 k

D1 B

VsNeon

AC Mains

Ω

Full wave bridge rectifier

CT230 V

V P(rect) RL

4 x 1N4007

1 k

D1 D 2

D4 D 3

VsNeon

AC Mains RL

1 kΩ

Full wave bridge rectifier with Filter

CT230 V 4 x 1N4007

D1 D 2

D4 D 3

Vs+–

C

Neon

Center tapped full wave rectifier

AC Mains Ω

Vs

CT230 VRL

1N4007

1 k

D1

D2

A B

Vs

Neon Vp(rect)

Figure 2. Output waveform of full wave bridge rectifier with capacitor filter

Observation Table: Full wave bridge rectifier with capacitor filter

Capacitors Measured Parameters 22µF 100µF 470µF 1000µF 2200µF 4700µF

Vp(rect) VDC

Vr(p-p) r %

Calculated Parameters

22µF 100µF 470µF 1000µF 2200µF 4700µF

VDC Vr(p-p) r %

Note: Measured parameters are taken with the help of CRO and DMM

Vdc(measured)=Vp(rect)-0.5 Vr(p-p)

Conclusion:

Vp(rect)

EXPERIMENT 8

AIM: To determine the Resolving Power (R. P.) of a Telescope. APPARATUS: Sodium lamp, Traveling Microscope, Telescope, Retort stands, Wire-gauze, Adjustable slit with a holder and a scale, meter scale, etc. THEORY:

(R. P.)th = a/λ; (R. P.)exp = D/d ; where a = average width of the slit for resolution of the image;

λ = Wavelength of the source of light used; D = distance between the wire gauze and the objective of the telescope; d = distance between two consecutive wires of the wire gauze.

DIAGRAM:

PROCEDURE: • Determine the average distance, d, between two consecutive wires of the wire gauze with the help of a

traveling microscope. • Now place the wire gauze in a retort stand at a known distance, D, from the objective of the telescope,

which is fitted to another retort stand ( m 3 to2 D ≈ ). • Place the Sodium lamp source on the other side of the wire gauze. • Focus the telescope so that the wire gauze is seen very clearly. • Now mount an auxiliary (adjustable) slit as close as possible to the objective of the telescope. • Diminish the width of the slit gradually and adjust for the minimum width of the slit for which the two

consecutive vertical wires of the wire gauze ceases to be resolved. Measure this minimum width (a1) and then close the slit completely.

• Similarly, go on opening the slit until the vertical wires are just resolved. Measure this width (a2). • Repeat the experiment for different distances (D). (Reminder: Focus the telescope for every new

distance, D).

Arrangement for the measurement of a1, a2

Sodium Lamp

Wire gauze

Telescope

Auxiliary slit

Wire gauze

Sodium Lamp

Microscope

Arrangement for the measurement of distance between two consecutive wires

OBSERVATIONS: Smallest Division on the Main Scale (SDMS) = ______ ; Total Number of Divisions on the Vernier Scale (TNDVS) = _______;

Least Count (L.C.) = TNDVSSDMS = ---------- = _______ .

How to read the scale? Total Reading (T.R.) = MSR + VSDC x L.C. ; MSR = mark on the Main Scale which comes JUST before the zeroth mark of the Vernier scale. VSDC = mark on the Vernier Scale which coincides with some mark of the Main Scale (one should read the Vernier Scale Division). OBSERVATION TABLES: Least Count of the Microscope = ____ cm. No. of wire Microscope

reading [cm]

No. of wire Microscope reading

[cm]

Distance between five consecutive wires (L = a~b)

[cm]

Distance between two consecutive

wires (d) [cm]

1 6 2 7 3 8 4 9 5 10

Mean d = cm L. C. of the auxiliary slit = cm Wavelength of Sodium source λ = 589.3 nm

Minimum width of Resolution While

S. No. Distance between objective of

telescope and wire mesh (D) (cm) Closing

a1 cm Opening

a2 cm

Mean

a = a1 + a2

2

(R.P.)th = a/λ

(R.P.)exp = D/d

(R. P.)th / (R. P.)exp

1. 2. 3. 4.

CALCULATIONS: RESULTS: DISCUSSION OF RESULTS: CONCLUSIONS:

EXPERIMENT 9

AIM: To study the shape of the L-I-V Curve of a Laser Diode and determine the wavelength of the Laser Diode.

APPARATUS: Red Laser Diode with output power less than 1 mW, 1.2 Volts to 3.5 Volts Variable DC power supply, connecting wires, 3 DMM (or 2 Voltmeters and 1 Ammeter), BPW 34 Photodetector with battery for optical power measurement.

THEORY: How to find the output optical power (L) of laser diodes?

Specifications of the Silicon Photodiode: Responsivity (S) = Photocurrent generated per each Watt of Incident Light of a given wavelength

Power (L) = IPH / L; where Responsivity at 650nm of the given photodiode ≡ S = 0.59A/W and VR = 9V; Wavelength range: 350-1100 nm. Photocurrent = IPH / L IPH = VPH / R , R ≡ Series resistance = 47 Ω, and VPH ≡ voltage across the series resistance (R). Responsivity: S = IPH / L ; The Light Output Power of Laser Diode: L = IPH /S.

Laser diode’s wavelength:

From the theory of the operation of a semiconductor p-n junction, we know that:

on-turnon-turn V e

ch hence and V e ch h E =≅== λλ

υ ;

where:

• h → Planck’s constant = 6.625 x 10 -34 [J-s]; • λ → Wavelength of the light emitted by the laser diode; • e → Electronic Charge = + 1.602 x 10 -19 [C]; • c → Velocity of light = 3 x10 +8 [m/sec]; • Vturn-on → “Turn-on” voltage of the Laser Diode.

PROCEDURE:

• Connect the DC electrical circuit as shown in Fig.1 by connecting various components (Power Supply, Laser Diode, DMM, Photodetector, etc.) using wires with alligator clips. Make sure the polarities of the battery and the laser diode are correct. Also, select the appropriate range for the DMM to measure the voltage or the current.

Figure1: Experimental Set-up.

• Now connect the third DMM across the resistor R, which is shown in the diagram. Set the photodetector as close as possible to the laser diode and align it so that the light from the laser diode falls directly on the photodetector. Do not move any part of the set-up once you have aligned it or else the readings will be affected by the background glare. (The experiment should be performed in a “dark-room” type environment.)

• Switch “ON” the DMMs that are connected in the circuit. “Slowly” change the voltage applied to the laser diode by using the “pot” (i.e., potentiometer) attached to the Laser Diode power supply and record the current I (mA) and voltage V (volt) as displayed in the two Multimeters, m1 and m2, respectively; furthermore, for each I and V record the voltage, Vph, of the third Multimeter, m3, (in mV).

• Note the value of V and I at which you can just/barely identify a visible output of the Laser Diode; this voltage is known as the “turn-on” voltage of the laser diode.

PRECAUTIONS:

Do NOT stare into the laser beam! It will burn your retina without your becoming aware of it.

In order not to damage the Laser Diode, the supply voltage should not exceed 3.5 volts.

VR = 9V

1.2 to 3.5 Volts DC

Power supply

Laser Diodes

mV

R = 47Ω

+ +Photodetector

A

V

OBSERVATIONS: *Responsivity of the photodiode at 650nm ≡ S = 0.59A/W and VR = 9V; *Wavelength range: 350-1100 nm. OBSERVATION TABLE:

V [volt]

I

[mA]

Photodetector output

VPH [mV]

Photodetector current

IPH [mA]

Output Optical power of Laser Diode

L [mW]

CALCULATIONS: GRAPHS:

From the recorded data, draw a graph of the current vs. voltage (I-V curve). Draw a graph of the L vs. I. From the graphs find: The first point where the slope changes abruptly and the laser barely begins to glow (“Turn on” voltage). The second point where the slope changes abruptly and the laser begins to glow very brightly (“Lasing Threshold” voltage).

Figure 2: I-V Graph of the Laser diode showing the lasing and turn on voltages.

Turn on voltage of Laser Diode measured form the I-V graph = ______________[Volts]. Lasing voltage of Laser Diode measured form the I-V graph = ______________[Volts]. RESULTS:

DISCUSSION OF RESULTS:

CONCLUSIONS:

EXPERIMENT 10

AIM: To determine the Rydberg constant “RH” using Hydrogen gas Discharge tube and a

Diffraction Grating. APPARATUS: Hydrogen gas Discharge tube, Diffraction Grating, Spectrometer, spirit-level, etc. THEORY:

, n n 2, n ;

n1-

n1 R 1

ifi2f

2i

>=⎟⎟⎠

⎞⎜⎜⎝

⎛= ∞λ

where n. transitioin the involved ly,respective states, initial theand final the n , n

nucleus);hydrogen theof mass infinite (assuminghydrogen ofconstant Rydberg heRnm;in light ofh wavelengt the

if →→

→λ

∞

PROCEDURE: • Familiarize yourself with the spectrometer. There are many ‘knobs’, identify the function of the knobs

for the ‘coarse’ and ‘fine’ motions of the ‘grating table’ and the ‘telescope’. Identify the Main Scale

and the Vernier Scale and find the least count as well as make sure you know how to read these scales.

Preliminary adjustments of the spectrometer:

o Focusing of the Telescope of the Spectrometer: First, focus the eyepiece of the telescope on a

‘nearby’ object so that the cross-wires are distinctly visible. Then focus the telescope on a

‘distant’ object. Do not disturb this focusing throughout the experiment.

o Leveling of the ‘grating’ table: Take the spirit-level and place it on the ‘grating’ table along

any two screws of the table and adjust the screws to get the air bubble in the center of the spirit

level. Now place the spirit level along the third screw and adjust the third screw to get the air

bubble in the center. Repeat this alternately until in both positions of the spirit-level the air

bubble remains in the center.

o Alignment of the spectrometer with the source:

Align the collimator’s slit with the source and focus the collimator to obtain a fine image of the

slit. Now look into the collimator thru the telescope and get the image of the slit on the vertical

cross-wire.

Do not disturb the adjustments throughout the experiment!

DIAGRAM:

Hydrogen Discharge Tube

• Adjustment of the Diffraction Grating at 90° to the incident light:

Mount the diffraction grating on the grating table such that it is transverse to the incident beam.

Observe the image of the slit directly thru the telescope and note the reading of any one window* of the

spectrometer scale. Keep the grating table locked. Rotate the telescope by 90° and lock it. Now

unlock the grating table and rotate it so as to obtain the reflected image of the slit on the vertical cross-

wire. Note this reading and rotate the grating by 45° so that the grating is now at right angles to the

incident beam. Keep the grating table locked for the rest of the experiment.

• Now unlock the telescope and see whether the spectrum is visible thru it on both sides. Take data

from one side in proper order using the fine motion of the telescope. Also obtain the data for the other

side of the spectrum.

(* We read here only one window just for convenience-not advisable, in general!)

PRECAUTIONS: Do not touch power supply: 5000 Volts high voltage Do not touch discharge tube: fragile. OBSERVATIONS: Smallest Division on the Main Scale (SDMS) = ______ ; Total Number of Divisions on the Vernier Scale (TNDVS) = _______;

Least Count (L.C.) = TNDVSSDMS = ---------- = _______ .

How to read the spectrometer scale? Total Reading (T.R.) = MSR + VSDC x L.C.; MSR = mark on the Main Scale which comes JUST before the zeroth mark of the Vernier scale. VSDC = mark on the Vernier Scale which coincides with some mark of the Main Scale (one should read the Vernier Scale Division)

OBSERVATION TABLE:

Spectrometer Readings Color

Position of the

Telescope W1 W2 2θ = A ~ B* Mean

2θ θ

Violet A: LHS Violet B: RHS

Aqua A: LHS Aqua B: RHS

Red A: LHS Red B:RHS

• Note: For a given color, and for a given window, say W1, 2θ is the difference in the readings of the same window (W1).

nf 1/nf2 λ = d Sin θ

(nm) 1/λ RH (cm-1)

3 0.110 4 0.063 5 0.040

CALCULATIONS:

GRAPHS: RESULTS: DISCUSSION OF RESULTS: CONCLUSIONS:

2fn

1

λ1

Slope = Intercept =