3
b) Rotating the table: The telescope is locked in
position so that it is not parallel to the collimator. The
prism table is rotated until the reflected ray enters
the telescope. The angle of the prism table is read
off the scale. The table is rotated further until the ray
again enters the telescope. The new angle is noted.
The difference between the two angles is called
θ
.
The refracting angle of the prism is then 180 -
θ
as
shown in Figure 3.
EXPERIMENT
Index of refraction of a prism at a specific wave-
length.
The path of a ray of light which passes through a
prism will be altered due to refraction. The magni -
tude of the deviation depends upon the index of
refraction of the prism material and the wavelength
(color) of the beam of light. For this reason mo -
nochromatic light should be used in the experiment.
First the refracting angle of the prism should be
found as described elsewhere. Then monochro matic
light e.g. from a sodium lamp or a laser should be
sent through the collimator slit. The prism is placed
on the prism table so that the light passes through
its refracting angle and the image of the slit can be
seen with the naked eye. If this image is observed as
the prism table is rotated, the image will first move in
the same direction, quickly at first then slowly. At a
certain point it will cease to move then reverse its
movement. The precise position where the image of
the slit stands still is called the primary position of
the prism.
For a given color the deviation of the ray depends
upon the angle of incidence. When the angle of
incidence increases the angle of refraction will first
decrease until it reaches a minimum before in
-
creasing. The telescope is rotated so that it receives
the light at the minimum deviation angle. The prism
is now in its primary position. The prism table and
the nonius scales are locked by means of locking
screws, and the fine adjustment screw is used to
determine the exact value of he angle of minimum
refraction. The position of the telescope is read off
the scale. Then the second least angle of refraction
is found, and the position of the telescope is again
read off the scale.
The average of these two readings is found. The
prism is now removed from the prism table, and the
telescope is rotated so that it receives light directly
from the collimator, and the position of the telesco-
pe is read off. The angle through which the telesco-
pe is rotated is the angle of minimum refraction,
θ
.
When
θ
is the angle of minimum deviation, the index
of refraction R is given by:
R = (sin((
θ
+
α
)/2))/sin(
α
/2)
where
α
is the refracting angle of the prism.
NOTE: If the light source is not monochromatic, a
numer of different colored images of the slit will be
seen. The size of the angle of minimum deviation de-
pends on the color of the light. This shows that the
index of refraction is as well.
EXPERIMENT
Determination of the wavelength of light from a
sodium lamp using an optical grating.
When the spectrometer has been set up, turn the
telescope so that the image of the collimator slit
coincides with the vertical cross hair. Now read off
the angle and rotate the telescope 90 degrees so
that it is normal to the collimator and lock it in place.
Now place the grating in the grating holder on the
prism table so that it is normal to the two adjustment
screws Y and Z (see Figure 4) and so that the light
from the collimator strikes the back side of the gra -
ting. The prism table is turned until the reflected lig-
ht from the prism is visible in the telescope. Now
make a fine adjustment so that the edge of the slit
coin cides with the vertical cross hair. Adjust the
screws X and Y so that the image of the slit lies
symme trically with respect to the horizontal cross
hair. The position of the prism table is read off the
scale then rotated 45 degrees so that the grating is
normal to the beam of light from the collimator, and
the grating faces the incident light. The prism table
is locked and the telescope is turned to the left until
the first order spectrum is visible. If the slit image is
not symmetrical around the horizontal cross hair,
then make adjustments using the screw Z. The tele-
scope is rotated to the right until the first order
spectrum appears. Adjust Z until the slit image is
symmetrical around the horizontal cross hair.
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