1
Name of the Experiment: Determination of the Angle of Prism
using a Spectrometer.
Theory:
Sodium
Monochromatic
Light Source
A
θ
B
C
T2
T1
Figure 1: Arrangement of the angle of prism
When a parallel beam of light strikes on the surface of transparent
material (Glass, Water, Quartz, Crystal, etc.), a portion is reflected. When
the beam of light strikes on the plane of surface, the angle of reflection
will be the same as the angle of incidence. So the angle made by the two
refracting surfaces of the prism is called the angle of prism.
If the angle between two reflected rays is measured as θ, then the angle
of the prism is,
𝐴=
𝛉
𝟐
(i)
Apparatus:
Spectrometer, prism,magnifying glass, sodium monochromatic light
source, sprit level, etc.
Procedure:
2
1) First the prism table, spectrometer and the collimator were leveled
using a sprit level till the the air bubble came at the center of the
sprit level very carefully.
2) The light source was turned on and was waited till the light fully on.
3) The telescope was turned towards the white wall or screen and
looking through eye piece, and it was adjusted its position till the
cross wires were sharply seen.
4) Then the telescope was placed to the collimator.
5) The collimator was placed directed towards the light source and a
thin sharp ray of light was passed through the collimator still.
6) The collimator slit was then focused using the collimator adjusting
screw.
7) The Prism table was clamped in which the sharp edge of the prism,
not opaque edge, was faced towards then collimator. Also the base
of the table was clamped.
8) The telescope was roated in one direction, (say, left direction), up
to which the reflected ray of light from the sharp edge of the prism
was seen through the telescope.
9) Noted down the corresponding main scale and Vernier scale reading
in both Verniers, i.e Vernier-I and Vernier-II, for different times.
10)
The telescope was rotated in the opposite direction, (i.e. right
direction), to view the reflected ray of light from the sharp edge of
the prism. And the step (9) was repated.
11)
Then the difference between the readings was found θ.
12)
Finally, the angle of the prism, A =
equation (i).
𝛉
𝟐
was calculated from the
3
Experimental Data:
Table: Data for the angle of prism. Vernier’s constant, V.C = 30´ =
a
b
No. of Observation
Vernier Name
0.5°
1
Spectrometer reading for
the refracted images at
the left side in degree
Spectrometer reading
for the refracted images
at the right side in
degree
M.S.R
(M)
V.S.
R
(V)
T.R.
X
=M+
V
Mean
x
x̅ =
3
M.S.
R
(M)
V.S.
R
(V)
T.R.Y
=M+
V
100
0.66
100.6
6
101.6
6
101.1
6
282.3
5
282.8
5
282.2
222.
5
222
0.68
101.1
6
222.
5
42
0.5
223.1
8
223.7
3
223
0.41
42.41
41.5
0.70
42.2
41.5
0.5
42
2
101
0.66
3
100.5
0.66
1
281.5
0.85
2
282
0.85
3
281.5
0.7
282.4
7
0.73
Mean
y
y̅ =
3
222.9
7
42.20
Differenc
e
θ=
X̅~Y̅
in
degree
Mea
n
θ
1
2
°=
Angl
e of
Pris
m
θ
A=
2
in
degr
ee
In
degre
e
120.
77
60.3
9
121.81
119.73
4
Calculations:
Angle of the prism,
θ a = X̅a ~ Y̅a = 121.81o
θ b = X̅b ~ Y̅b = 119.73o
Mean angle,
θa+θb
2
= 60.39 ≈ 60o
Result:
The angle of the given prism, A = 60o in Degree
5
Precautions and Discussion:
1) All the table, collimator and telescope should be leveled with the
help of sprit level accurately.
2) The telescope and the collimator should be set for parallel
rays
individually.
3) Slit should be as narrow as possible.
4) While taking observations, the telescope and the prism table should
clamped with the help of clamping screws.
5) The prism should be placed properly on the prism table very
carefully.