/
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OBJECT: To determine the wavelength of prominent lines of mercury by
plane diffraction grating.
APPARATUS REQUIRED: A diffraction grating, spectrometer, mercury lamp,
reading lens.
FORMULA USED: The wavelength of any spectral line can be calculated by
the formula · : .f .·Ot'~ . .
@~fsm0 ,:;:: nA-
Where --
. (g,+J = grating element,
-e = angle of diffraction
n = order of the spectrum
DESCRIPTION OF THE APP~').}(µ$: .
Spectrometer consists of the foriow.1:hg<:three parts:
l. The collimator: The collimator consists of two hollow concentric metal
tubes, one being longer than ·.the other. The longer tube carries an
achromatic lens at one end and the smaller tube on the other end. The
smaller 'tube is provided with ::i.: s}it at tp:e outer end (width of the slit
can be adjusted with the help of a scr~w attached to it) and can be
moved in or out of the longer tube with the help of rack and pinion
arrangement.. The slit is adjusted in the focal plane of the lens to obtain
a pencil of parallel rays from the collimator when light is allowed to be
incident upon slit. The collimator is also provided with two screws for
adjusting the inclination of the axis of the collimator. This is rigidly
fixed to the main part of the .apparatus.
2. The prism table: It is a circula:r table supported horizontally in the
center of the instrument an<l the position can he read with the help of
two verniers attached to it and moving _over graduated circular scale
cartied by telescope. The leveling of the prism table io made with the
help of three screws provided at· t'lie lower surface. The fa.bre can be
raised or lowered a,nd clamped-i n any desired position with the help of a
{. screw. The prism table is als.o provided with a tangent screw for a slow
motion. There are concentric circles and straight lines parallel to the
line joining two of the leveling screws on the prism table.
3. Tiie telescope: The telescope consists of similar tubes as in case of
collimator carrying achromati_c objective lens at one end and Ramsdon
eye piece on the another side end. The eyepiece tube can be taken in or
out with the help of rack and pinion arrangement. The cross wires are
focused on the focus of the eyepiece . The telescope can be cJamped to the
main body of the instrument and can be moved slightly by tangent
screw. The telescope is attached to the main scale and when it rotates,
_i~fN~Ji
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the graduated scale rotates with it. Two screws provided at the lower
surface adjust the inclination of the telescope.
L
PROCEDURE:
• Before using the spectrometer, the foUowfog adjustments are made:
1. The axis of the telescope and that of the collimator must intersect the
principal vertical axis of rotation of telescope.
2. Prism table should be leveled. ·
3. Telescope and collimator are adjusted for parallel light.
• Grating should be normal to the axis of the collimator:
1. Collimator and telescope are arrangement in a line and the image of the
slit is focused on the vertical cross wire. The reading noted on both the
verniers.
2. Telescope is now rotated through 900.
3. Mount the grating on the prism taBle ·and rotate the prism table so that
reflected image is seen on the vertical cross wire. Take the readings of
the verniers.
4. Turn the prism table from this position through 450 or 1350_ In this
position grating is normal to th~incident beam.
The slit should be adjusted parallel to the lines of grating. For this setting, the
slit is rotated in its own plane till the spectral line becomes very sharp and
bright.
• Procedui-e for the determination of angles of diffraction:
1. Rotate the telescope to the left .side of direct image and adjust the
different spectral line (violet, green and yellow) turn by turn on the
vertical cross wire for the first order. Note down the reading of both
verniers in each setting.
2. Now rotate the telescope to the right of the direct image and repeat the
above procedure for the first order as well as for the second order.
3. Find out the difference of the same kind of verniers (V1 from V1 and V2
from V0 for each spectral line in the first order. The angle is twice the
angle of diffraction for that particular colour. ijalf of it will be the angle
of diffraction. ·
4. Find out the angle of diffraction for each colour in first order.
OBSERVATIONS:
No. of lines per inch on the grating, N = \5oo 0
\
e, Least count of the spectrometer =
Colour Vernier Spectrum on the left Spectrum on the right 20 ~ ~
of
light
side
Reading of telescope
M.S. V.S.
side
Reading of telescope
Total M.S. v.s. Total
a~b
-eo
. ,., {ao) . . · .. ·.
•. :,: (bO)
.
,,
Violet V1
~
Green V1
~ .
Yellow V1
~
CALCULATIONS~ ~ t-J)
ia .
1• · Grating element ~ +~ = 2.54/N = ................ cm· 1.
The wavelength of various spectral lines in the first oi-der (n=l) can be
calculated by
'A = (d+dt) sine
RESULT:
The wavelength of violet colour is = )
The wavelength of green colour is = A
The wavelength of y~llow colour is = /
PRECAUTIONS:
1. The light _should be incident on the side of the grating. on which there
are no rulmgs.
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2. Before performing the experime1it, the spectrometer should be
adjusted.
3. Grating should be set normal to the incident light.
4. Grating should not be touched by fingers.
5. While taking observations, telescope and prism table should be kept
fixed.
TRY TO ANSWER:
1. What is a plane transmission grating?·
2. Why should the grating be kept normal to the incident parallel beam of
light?
3. If the number of lines on your grating were doubled, what will happen
to the grating element?
4. You can't get third order spectrum with grating in any case. Why?
5. What is the main difference between the spectrum obtained by grating
1. e and that due to prism?
6. Which gives more intense spectrum and why- a prism or a grating?
7. Why do you make the light incident on the side of grating on which
there are no rulings?
8. Why violet colour is diffracted most in case of grating?
