43
SAHRDAYA PH110 Engineering Physics Lab Manual Name of student………………………………… Semester/ Branch and Division………………. University Reg. No………….. S.R. No…………………………... 1
SAHRDAYA
PH110 Engineering Physics
Lab Manual
Name of student…………………………………
Semester/ Branch and Division……………….
University Reg. No…………..
S.R. No…………………………...
Class Roll No………………………
1
Expt.
No.
DateName of the Experiment Page No.
Marks Signature
1 AIR WEDGE
2SPECTROMETER: WAVELENGTH OF
MONOCHROMATIC LIGHT
3 MELDES STRING
4 NUMERICAL APERTURE OF AN OPTIC FIBRE
5WAVELENGTH OF LASER USING
GRATING
6 NEWTON’S RINGS
7 CHARACTERISTICS OF A SOLAR CELL
8 C R O
9 DISPERSIVE POWER OF GRATING
10 RESOLVING POWER OF GRATING
2
Air wedge expt. set up
Order Microscope reading Width of 20
MSR x
VSR y
Total reading=X+(Y x L.C)
bands
m
m+5
m+10
m+15
m+20
m+25
m+30
m+35
3
OBSERVATIONS
1MSD = cm/mm
No: of divisions on the vernier
(N) =
Least Count=1 MSD
N= cm/mm
m+40
Width of 20 fringes (mean) = cm /mm
Band width =20 β20 = cm
Wave length of Sodium light = 5893Å = 5893 x 10-8cm.
Length of the wedge ,l = cm
Diameter of the wire, d = λl
2β =
Date : EXPERIMENT 1
AIR WEDGE
AIM
To determine the diameter of a thin wire by observing the interference bands formed using an air wedge.
APPARATUS
Air wedge, sodium light, Plane glass plate, convex lens, Vernier microscope and reading lens. The air wedge consists of a wedge shaped air film formed between two thin, rectangular, optically plane glass plates; One of the plates is placed over the other at slight inclination keeping a thin straight wire near one edge, parallel to the edge. The plates are firmly tied together.
PRINCIPLE
The diameter (d) of the thin wire used in the air wedge is given by
d =λl/2β
Where,λ. = Wavelength of light used,
l= Distance of the wire from the line of contact of the two plates of the air wedge,
β= Bandwidth, the distance between two consecutive dark or bright bands.
PROCEDURE
The thin wire is placed between two optically plane glass plates and the plates are tightly tied together to form an air wedge Light from the sodium lamp S is rendered parallel by a convex lens L. This light is then made to
4
reflect from a glass plate P; inclined at 45 degree to the horizontal, so that it is incident normally on the air wedge. A verneir microscope M is arranged vertically above the glass plate. The microscope is properly focused Then a number of equidistant, alternately bright and dark interference bands are seen. The point of intersection of the cross wire is made to coincide with any dark band(mthband). The microscope reading is taken. Similarly readings are taken for (m+5), (m+ 10) etc bands. The bandwidth (β) is hence calculated The distance of the wire from the line of contact of the plates (l) is measured using a microscope. Assuming λ, the diameter of the wire is calculated
RESULT
Diameter of the given thin wire = cm
5
Spectrometer arrangement
OBSERVATIONS
1MSD = degree = min
No: of divisions on the vernier (N) =
Least Count =1 MSD
N = min
N =5000 lines/cm
order Verneir Diffracted Readings Difference Mean(2θ) θLeft Right λ =
sinθm N
MSR
VSR
Tota
l
(a)
MSR
VSR
Tota
l (b) 2θ
(a – b)
m=1 V1
V2
m=2
V1
V2
Mean λ = cm
6
= Å
Date: EXPERIMENT 2
SPECTROMETER: WAVELENGTH OF MONOCHROMATIC LIGHT
AIM
To determine the wavelength of monochromatic light using grating.
APPARATUS
A Plane transmission grating , sodium vapour lamp , spectrometer
PRINCIPLE
At normal incidence,
Sin θ = Nm λ
λ= sinθm N
Where θ =angle of diffraction,
λ=Wavelength of light used (Expressed in cm)
N=No of rulings on the grating per centimeter
m = Order of the spectrum
PROCEDURE
a) Preliminary adjustments of the spectrometer
1) Adjust the eye piece so that the cross wire is cleared seen .
2) Turn the telescope to the distant object and adjust the rack and pinion screw of the telescope to get a clear image of the distant object.
3) Open the slit of the collimator to a minimum width and turn it towards sodium vapour lamp.
4) Bring the telescope in a line with the collimator.
5) Looking through the telescope adjust the rack and pinion screw of the collimator so that a clear and well defined image of the slit is obtained at the telescope.
6) Clamp the vernier table and telescope.
7
Diffraction pattern
8
b) To set the grating for normal incidence
After the preliminary adjustments of the spectrometer the vernier table is clamped. The slit is made narrow and it is made to coincide with the vertical cross wire. Unclamp the vernier table and zero of the vernier 1 is made to coincide with the zero of the main scale and clamp it. Now the telescope is rotated through 90° and clamped. The grating is mounted on the prism table with its ruled surface facing the collimator and perpendicular to the line joining the two leveling screws of the prism table. The prism table alone is rotated until the reflected image of the slit is obtained at the cross wire of the telescope (There will be two images choose the brighter one). Prism table is clamped. The vernier table is unclamped and rotated through exactly 45 degree in the proper direction so that the surface of the grating becomes normal to the collimator. The vernier table is clamped. Now the grating is set for normal incidence.
c) To find the wavelength of sodium light
The telescope is unclamped. The direct image of the slit is obtained in the telescope. From this position, the telescope is rotated slowly to the left until the first order image of the slit is observed. The telescope is adjusted so that the vertical cross wire coincides with the yellow line. . Readings of both verniers are taken. The telescope is now moved to the right and the cross wire is made to coincide with the yellow line of the first order on the right side.. The vernier readings are again taken. The difference between the readings of the corresponding vernier on the left and right sides is determined. The mean value of this difference is 2θ . The angle of diffraction θ for the first order (m= 1) is thus determined. Knowing the value of N ,wavelength of sodium light is calculated from the formula
λ= sinΘm N
This is repeated for the second order (m=2) and then mean value of λ is calculated.
RESULT
The wavelength of monochromatic lightλ = Å
9
OBSERVATIONS
Linear density m = kg/m
Mass of the scale pan = gm = kg
Acceleration due to gravity, g = 9.8 ms -2
a) Transverse Mode
TrialNo
Massin the scale
pan in Kg
Total Mass including the mass of pan
M Kg
No of loops
X
Length of X loops
L metre
Length of one loop
l=L/X metre
Mean =
Frequency of the fork,
= Hertz
10
Ml2
Ml2
n=√ g4 m ( M
l2 )
Date : EXPERIMENT 3
MELDES STRING
AIM
To determine the frequency in the transverse and longitudinal mode of vibration using Meldes string.
APPARATUS
Electrically maintained tuning fork, fine thread, scale pan, weight box, balance
PRINCIPLE
a) Transverse mode of vibration
The frequency ‘n’ of the fork is calculated using the formula
b) Longitudinal mode of vibration
Where
m = linear density of string
M = total mass at the end of the string
l = average length of one loop
g = acceleration due to gravity
PROCEDURE
a) Transverse mode of vibration
The mass of the scale pan is determined correct to a milligram. 10 metres of the given string is weighed accurately. Hence its linear density (mass per unit lenth) ,’m’ is found.
The electrical connections are made as shown in the diagram. The string is arranged horizontally with its length parallel to the prong of the fork. Here , the fork vibrates in a direction perpendicular to the length of the string.
A mass of about 2 or 3 grams is placed in the scale pan. The circuit is closed. The fork vibrates. Transverse stationary waves are formed in the string. The length of the string between the prong and pulley is carefully adjusted by moving the fork, so that a no of well defined loops are formed in the string. Leaving the loops at the two ends , the lengths of a definite no of loops are measured. Then the average length of a loop is found(l).The total mass m at the end of the string (mass of scale pan+ mass placed in the pan) is noted . The value of is found.
11
n=√ g4 m ( M
l2 )
n=√ gm ( M
l2 )
Ml2
b) Longitudinal Mode
Meldes apparatus in longitudinal mode
TrialNo
Massin the scale
pan in Kg
Total Mass including the mass of pan
M Kg
No of loops
X
Length of X loops
L metre
Length of one loop
l=L/X metre
Mean =
Frequency of the fork,
= Hertz
12
Ml2
Ml2
n=√ gm ( M
l2 )
The experiment is repeated for different masses in the scale pan and mean value of is calculated. Then the frequency of the fork is calculated using the formula .
b) Longitudinal mode of vibration
Apparatus is arranged as shown in the diagram, with prongs perpendicular to the string.
Then the fork vibrates in a direction parallel to the string or string vibration in the longitudinal
mode.
The experiment is performed exactly as before for different masses and mean value of
is found. Then the frequency of the fork is calculated using the formula.
RESULT
The mean frequency of the fork is = Hz
13
Ml2
n=√ g4 m ( M
l2 )
Ml2
n=√ gm ( M
l2 )
Optic fibre expt. set up
OBSERVATIONS AND CALCULATIONS
D
(mm)
F
(mm) tan θ θ° sin θ
16
20
24
32
Average Acceptance Angle = °
Average NA =
Date: EXPERIMENT 4
14
NUMERICAL APERTURE OF AN OPTIC FIBRE
AIM
To determine the acceptance angle and numerical aperture of the given optic fibers using
diode lasers.
APPARATUS
Diode laser source,Travelling microscope. Optic fiber cable, bed carrying fixed screen and
movable chuck.
PRINCIPLE
Numerical aperture (NA) is the sin of the acceptance angle.
NA = sin θ
Where θ is acceptance angle.
If ‘D’ is the diameter of the spot and ‘f’ , the distance between the screen and OFC , then
PROCEDURE
Fig 2 Shows the experimental set up. A fixed screen is graduated with 2 mm pitch.The travelling microscope bed consists of an X-Y motion bench. A needle fixed above the exact scale indicates the distance ‘f’ between the screen and OFC.Y motion is used to adjust the spot to the centre of the screen.
To begin with the optical cable is coupled to the laser and the laser light coming through the other end is verified.The other end of the cable is coupled to the chuck fixed on bench.The chuck carrying the OFC is brought close to the graduated screen by using X scale and spot obtained is observed on the screen.The spot is moved to the centre of the screen by moving Y scale.By adjusting the fine motion screw ,the spot size is reduced to 8 mm(d= 8mm). The distance (f) between the screen of the OFC is noted in mm on the graduated scale along the X axis.
Tan θ is calculated .From this acceptance angle θ is found out. Numerical aperture is calculated using the formulae NA= sin θ .
The experiment is repeated by increasing the spot size 10mm , 12 mm….etc upto 22 mm and the corresponding values of ‘f’ measured and numerical aperture is calculated
The experiment is also repeated with another optic fiber cable and the corresponding readings obtained are tabulated. The NA of the second cable are also found out.
RESULT
NA of the OFC =
Acceptance angle of OFC = º
15
θ=tan−1 D2 ftanθ= D /2
f= D
2 f
Experimental arrangement
OBSERVATIONS: N=6000 lines/cm
Sl.No. Distance
(l cm)
order x (cm) Mean
X(cm)
λ¿ sinθmNtanθ= x
l
LHS RHS θ Sin θ
1 14cm
m= 1
m = 2
2 17cm
m = 1
m=2
3 20cm
m = 1
m=2
Mean λ = cm
= Å
16
Date : EXPERIMENT 5
WAVELENGTH OF LASER USING GRATING
AIM
To determine the wavelength of laser using grating
APPARATUS
2 m watt He –Ne laser , a diffraction grating, optical bench ,screen etc.
THEORY
A diffraction grating is an optical device which produces spectra due to diffraction. It has a large number of lines grooved on it. The spectra consisting of different orders is governed by the relation
Sinθ=mNλ
λ= sinθmN
PROCEDURE
He - Ne laser is mounted on its saddle.A plane transmission grating is mounted on an upright next to laser. A screen is mounted next to the grating. The laser is switched on.
The grating is placed at a distance of 14cm from the screen with its surface perpendicular to the beam of the laser. Ensure that five spots are clearly seen on the screen. The central maximum and other maxima are identified.
The distance x of the spots belonging to first order on either side of central maximum are measured and readings are entered in the tabular column.
The above procedure is repeated for second order also.
The distance between the grating and screen (l) is now changed to 17cm.
Again the distances of the first order and second are measured.
Repeat this for a third distances of 20cm also.
The wavelength of laser is calculated using the formula
RESULT
Wavelength of laser using grating is found to be =…………. Å
17
Expt. set up for newton’s ring Newton’s ring pattern
OBSERVATIONS
1MSD= cm/mm No: of divisions on the vernier table (N) =
Least Count=1M SD
N = cm/mm
K = 10
Order of the ring
Microscope Reading
Diameter
D
(a – b)
D2
Dn+k2 -Dn
2Left Right
MMSR
VVSR
TTotal(a)
vMSR
vVSR
TTotal (b)
20
18
16
14
12
10
8
6
4
2
λ= Dn+k2 −Dn
2
4 kR= cm = Å
18
Date : EXPERIMENT 6
NEWTON’S RINGS
AIM
To determine the wave length of sodium light using the reflected system of Newton's rings. Radius of curvature of the lens by Boys method
APPARATUS
Newton's rings apparatus, sodium vapour lamp, vernier microscope. The Newton's rings apparatus consists of an optically plane glass plate G2 on which is placed a convex lens L2 of large focal length. Above the lens, another glass plate G1 is arranged at 45° to the horizontal.
PRINCIPLE
Let Dn and Dn +k be the diameters of the nth and (n+k)th dark rings respectively. Then
Dn2= 4nRλ
D n+k2= 4(n+k)Rλ
D n+k 2-Dn2= 4kRλ
λ= Dn+k2 −Dn
2
4 kR where R=the radius of curvature of convex lens
PROCEDURE
a) To find D n+k 2-Dn2
Light from sodium vapour lamp is rendered parallel by a short focus convex lens L 1. The parallel rays fall on the glass plate G1, inclined at 45 degree to the horizontal, gets reflected, and then falls normally on the convex lens L2 placed over the glass plate G2. A pattern of bright and dark concentric circular rings are observed through a microscope arranged vertically above the glass plate G2. The microscope is properly focused so that the rings are seen most clearly .As a trial move the microscope towards left and right over about 30 fringe on both sides to make sure that we can measurement on the necessary fringes .
Starting from the center of the fringe system, the microscope is moved towards the left so that the crosswire (one of the wires) is tangential to the 20 th dark ring on the left side. The main scale and vernier scale reading on the horizontal scale is taken. Looking through the microscope the cross wire is carefully moved toward right and it is made to coincide tangentially on the 18 th dark ring and the reading are taken again. Repeat this process on 16 th,14th etc dark ring in succession up to the second dark ring on the left and the corresponding readings are taken. Then the cross wire is made tangential to the second dark ring on the right side. Readings are taken corresponding to the 2nd ,4th etc….. 20th dark rings on the right side as before. The difference between the readings on the left and right of each ring gives the diameter D of the respective ring. Then D2 is found out .Hence (Dn+k
2 – Dn2) is calculated.
19
To find ‘d’
1 2 3 Mean(d) cm
To find ‘f’
1 2 3 Mean(f) cm
R= f df −d=
Measurement of Boy’s distance - d
Measurement of Focal length - f
20
b)To find R of the convex lens
The radius of curvature R of the lower surface of the lens which is in contact with the glass plate is to be found by Boy's method. For this ,Boy’s distance (d)is to be measured first. The convex lens L2 is placed in front of an illuminated wire gauze, with the surface ,whose radius of curvature is to be found out, is kept away from the wire-gauze. With a black paper held behind the lens the position of the lens is adjusted so that a clear but faint image of the wire-gauze is formed side by side with it. The distance ’d’ between the lens and the wire-gauze is measured . This is called the Boy’s distance. This is repeated 3 times and the mean value of’d’ is found out.
Then the focal length 'f ' of the convex lens L2 is determined by plane mirror method. For this plane mirrir is held behind the lens and the position of the lens is adjusted so that a clear image of the wire is formed side by side with .The distance between the wire gauze and the convex lens is the focal length(f).Repeat the measurement three times and the average value of ' f ' is found out .
Then the radius of curvature of the lens is found out using the formula
R= f df −d
The wave length of sodium light is hence calculated using the formula,
λ= Dn+k2 −Dn
2
4 kRRESULT
Wavelength of sodium light = Å
21
OBSERVATION
Connection diagram model graph
Distance =27cm
Open circuit voltage (VOC) = Short circuit current (ISC ) =
Open circuit current (IOC) = Short circuit Voltage (VSC ) =
22
Resista
nce
Ω
Distance =27cm Resistan
ce
Ω
Distance=27cm
Voltage
(mV)
Current
(A)
Power=
VxI (mW)
Voltage
(mV)
Current
(A)
Power=
VxI (mW)
00.1 2
00.2 3
00.3 4
00.4 5
00.5 6
00.6 7
00.7 8
00.8 9
0.9 10
1 20
Fill factor,FF= PmpV OC × I SC
=
Data : EXPERIMENT 7
CHARACTERISTICS OF A SOLAR CELL
AIM
To draw the I-V Characteristics of a solar cell
APPARATUS
Solar cell, digital d.c ammeter, digital d.c voltmeter, light source, a.c power supply and resistance box
PRINCIPLE
Solar cell is a p-n junction device, which produce electric voltage across its terminals by absorbing sunlight due to photovoltaic effect .Solar cells are cascaded in series or parallel to obtain solar cell module for commercial use.
Fill factor is the ratio of maximum obtainable power to the product of the open-circuit voltage and short-circuit current.
FF=Pmp
V OC × I SC
PROCEDURE
The circuit connections are made as shown in figure.. The halogen lamp and cooling fan
is switched on. The solar cell is placed under the halogen lamp at distance of 27cm from lamp.
The resistance box is shorted (R=0) and short circuited current is noted. This current is I sc
Both the terminals of the resistance box are disconnected and open circuit voltage is noted
in the mill voltmeter. This is Voc. The resistance box is reconnected and a resistance of 0.1 Ω is
introduced in that. The corresponding voltage and current is noted and recorded. Trial is repeated
by setting the resistance to 0.2 Ω , 0.3 Ω etc up to maximum of 20 Ω . The corresponding voltage
and current noted is recorded and power is calculated. Maximum out power is noted (Pmp).
Fill factor is calculated by using the formula FF=Pmp
V OC × I SC.
Draw a graph between V (x-axis) and I (y axis). A parabolic graph is obtained.
RESULTS
I V characteristics of the solar cell has plotted
23
Fill factor for distance 27cm =
RC oscillator circuit
OBSERVATIONS
signal
Amplitude measurement
Amplitude V
Frequency measurement
No of vertical divisions
Volt/Div scale
Amplitude (Peak to peak)
No of horizontal divisions
Time/ Div scale
Time period
Frequency
Sine wave
Mean amplitude cm Mean frequency Hz
To measure amplitude and frequency of a signal
24
Date: EXPERIMENT 8
C R O
AIM
To measure the amplitude and frequency of signal.
APPARTUS
CRO, function generator, unknown frequency source.
PRINCIPLE
Cathode ray oscilloscope is an instrument which gives the visual representation of electric
signals. Cathode Ray Tube is the heart of CRO. It is a vaccum tube which generates a narrow
electron beam and made to fall on a fluorescent screen.The beam is deflected horizontally and
vertically by horizontal and vertical deflection plates respectively. The waveform to be observed is
fed across the vertical depletion plates while the horizontal depletion plates elongates the wave
form in the time axis.
PROCEDURE
To measure the amplitude of the signal.
1. Switch on the CRO. Obtain a sharply defined trace of a horizontal line on the screen by adjusting INTENS and FOCUS knobs.
2. Adjust the y position knob to coincide the center line on the screen by keeping the AC –DC switches in ground position.
3. Connect the unknown frequency source with the CRO using a probe and connect the power
supply to the unknown frequency source 4. Switch on the power supply .5. Count the no of divisions occupied by the signal from peak to peak.6. Multiply this by the scale indicated by AMP/DIV knob .This gives the peak to peak
amplitude of the signal. Half of this gives maximum (peak) value of voltage.7. Repeat the above steps for various settings of AMP/DIV knob.
25
To measure the frequency by forming Lissajou’s figurs.
Unknown
source
Known frequency
from signal
generator
Lissajous figures. Frequency of unknown
source
Sine
wave
26
Mean frequency = Hz
.
27
To measure the frequency of the signal
1. Obtain a sharply defined trace of horizontal line on the screen by adjusting INTENS and FOCUS knob . Feed the signal (sine wave) whose frequency is to be measured to either of the channels using probe and observe signal on CRO .
2. Adjust the TIME/DIV knob so as to see the three cycles of waveform.
3. Count the no of horizontal division in one cycle of the waveform . Multiply this by the time base setting. This is the time period of the signal.
4. Reciprocal of time period will give the frequency of the signal.
5. Repeat the above steps for various settings of TIME/DIV knob
To measure the frequency of an unknown signal by comparing with known frequency forming
lissajous figures.
The unknown frequency is connected to one terminal and the signal from the signal generator
is connected to the other terminal of the CRO. The unknown oscillator is energised by 12 V battery
eliminator and the mode is switched in the X-Y position. The frequency of signal generator is varied
so that a circle is obtained on the CRO screen. Now the frequency of unknown signal is equal to the
frequency shown on the signal generator. Again vary the frequency of signal generator until we get a
‘8’ shape on the screen.The frequency of unknown signal is half of the frequency shown on the
signal generator.
RESULT Amplitude and frequency of the given signal is measured.
The frequency of unknown signal found out by forming lissajous figures.
28
Spectrometer arrangement
Diffraction pattern
29
Data: EXPERIMENT 9
DISPERSIVE POWER OF GRATING
AIM
To determine the dispersive power of grating by the normal incidence method.
APPARATUS
Spectrometer , grating ,mercury vapour lamp, Reading lens .
PRINCIPLE
At normal incidence, grating equation is
Sin θ = Nm λ
λ= sin θ/Nm
dispersive power dθ/dλ = mN/ cos θ
Where θ =angle of diffraction,
λ=Wavelength of light used (Expressed in cm)
N=No of rulings on the grating per centimeter
m = Order of the spectrum
PROCEDURE
a) To set the grating for normal incidence
The preliminary adjustments of the spectrometer are done . The slit is made narrow. The telescope is brought in a line with the collimator and the direct image of the slit is made to coincide with the vertical cross wire. The vernier table is unclamped The zero of the vernier I is made to coincide with the zero of the main scale. Now the telescope is rotated through 90° and clamped; The grating is mounted on the prism table with its ruled surface facing the collimator and perpendicular to the line joining the two leveling screw of the prism table. The grating table alone is rotated until the reflected image of the slit (white slit) is obtained at the cross wire of the telescope (There will be two images ,choose the brighter one). The vernier table is unclamped and rotated through exactly 45 degree in the proper direction so that the surface of the grating becomes normal to the collimator. The vernier table is now clamped.
b) To determine the wavelength
The telescope is unclamped. The direct image of the slit is obtained in the telescope. From this position, the telescope is rotated slowly to the left until the first order spectrum is observed. The telescope is adjusted so that the vertical cross wire coincides with the yellow I line on the left. Readings of both verniers are taken. The telescope is rotated to the right side of the image and adjusted so that the cross wire coincides with the yellow I line on the right side of the first order spectrum. The vernier readings are again taken. The difference between the readings or the corresponding vernier on the left and right sides is determined.
30
OBSERVATIONS
1MSD = degree = min
No: of divisions on the vernier (N) =
Least Count =1 MSD
N = min
N =5000 lines/cm
Vernie
r
Diffracted Readings Difference Mean(2θ
)
θ
colour Left Right 2θ λ=sinθmN
MSR
VSR
Tota
l
MSR
VSR
Tota
lYellow I
V1
V2
Yellow II
V1
V2
To find the dispersive power
Spectral lines θ Cos θ dθ/dλ = mN/ cos θ
Mean dθ/dλ =
31
The mean value of this difference is 2θ . The angle of diffraction θ for the first order (m= 1) violet is thus determined. This is repeated for the yellow II lines of the first order. Angles of diffraction for different lines are found out. Knowing the values of N and m, λ is calculated using the above formula.
The dispersive power of grating is calculated using the formula, dθ/dλ = mN/ cos θ
RESULT
Dispersive power of the grating =
32
Spectrometer arrangement
Diffraction pattern
33
Date : EXPERIMENT 10
RESOLVING POWER OF GRATING
AIM
To determine the resolving power of grating.
APPARATUS
Spectrometer , grating ,mercury vapour lamp, Reading lens, A rectangular aperture of adjustable width .
PRINCIPLE
Resolving power =n N1
Where dλ is the difference in wavelength between two spectral lines λ1 and λ2
N1 - Total no of lines in the exposed width of the grating in just resolution position a n - order of the spectrum
PROCEDURE a) To set the grating for normal incidence
The preliminary adjustments of the spectrometer are done . The slit is made narrow. The telescope is brought in a line with the collimator and the direct image of the slit is made to coincide with the vertical cross wire. The vernier table is unclamped The zero of the vernier I is made to coincide with the zero of the main scale. Now the telescope is rotated through 90° and clamped; The grating is mounted on the prism table with its ruled surface facing the collimator and perpendicular to the line joining the two leveling screw of the prism table. The grating table alone is rotated until the reflected image of the slit (white slit) is obtained at the cross wire of the telescope (There will be two images ,choose the brighter one). The vernier table is unclamped and rotated through exactly 45 degree in the proper direction so that the surface of the grating becomes normal to the collimator. The vernier table is now clamped.
b) To determine Resolving power
Mount the rectangular aperture of adjustable width on the collimating lens of the collimator such that its axis is parallel to the slit. Turn the collimator to the mercury source. Now keep the aperture of the adjustable slit fully opened and turn the telescope to the left and bring 2 yellow lines Y1 and Y2 in its field of view. Gradually reduce the width of the aperture till the two spectral lines just cease to separate. Measure this width of the aperture.
Again put the aperture in the same position and open the aperture fully. Turn the telescope towards right and bring the 2 yellow lines of 1 st order in the field of view. As before reduce the width of the aperture till the two lines cease to appear as separate. Again measure the width of the aperture. Take the mean of the width.
34
λdλ
OBSERVATIONSMeasurement of rectangular aperture for just resolutionNo of lines /cm of the grating , N =L.C =
n =1
Sl No
Side
READINGS ONWidth of the aperture for just resolution
X – Y cm
No of lines of the grating in this width N1=(X–Y)N
One end of the aperture x mm
Other end of the aperture y mm
M S R V S R TOTAL (X)
M S R V S R TOTAL (Y)
1 Left side
2 Right side
The difference in wavelength of two yellow lines =5790 Å -5770 Å =20 Å
Mean wavelength λ= =
=5780 Å
Theoretical resolving power = = =289
No of lines in the mean width N1
n N1 Difference
35
5770+57902
λ1+ λ2
2
578020
λdλ
λdλ
RESULT Comparison of the theoretical and practical values of resolving power is shown in the table
36
37
