`EXPERIMENT NO…………..
ObjectiveTo determine the wavelength of prominent lines of mercury by plane transmission grating.
Apparatus usedDiffraction grating, spectrometer, mercury lamp, sprit level and reading lens.
Formula usedThe wavelength  of any spectral line can be calculated by the formula
(a+b) sin θ
𝝀=
𝟐.𝟓𝟒
= nλ
𝐬𝐢𝐧 𝜽
𝟏𝟓𝟎𝟎𝟎
Where, θ
= Angle of diffraction,
n
= order of the spectrum
(a+b) = grating element
Adjustments(A) Before using the spectrometer, the following adjustments are made.
1. The slit is illuminated by a light source and the slit and the collimator are suitably
adjusted to receive a narrow, vertical image of the slit.
2. The telescope is turned to receive the direct ray, so that the vertical slit coincides with the
vertical crosswire.
(B) Grating should be made normal to the axis of collimator.
1. Collimator and telescope are arranged in a line and the image of the slit is focused on
the vertical cross wire. The reading is noted on one of the verniers.
2. The telescope is now rotated by 900. And note the reading.
3. Mount the grating on the prism table and rotate the prism table so that the reflected
image of slit is seen on the vertical cross wire. In this case scale will not move.
4. Turn the prism table from the position through 450 or 1350, as appropriate. In this
position grating is normal to the incident beam.
Procedure -The spectrum obtained in a grating is shown in figure.
1. Now move the telescope to the left hand side, spectrum appears (violet to red) in
telescope (Ist order).
2. Set the cross-wire of telescope on violet line and note the readings of both the scaleV1 &
V2.
3. Move the telescope now to green & red line and note the readings of both the scale.
4. Now move the telescope to the right hand side, same spectrum appears (violet to red) in
telescope (Ist order).
5. Similarly take the readings for violet, green & red line.
6. Place all the readings in the observation table. Difference of each scale for particular
color is twice the angle of diffraction θ.
7. You will find II order too but you have to perform only for first order spectrum
Observations:
Number of lines over grating per inch(N)= ……….
Least count of the spectrometer………
Reading of the spectrometer for the direct image ....……..
Reading of the circular scale when reflected image is obtained on the cross wire……………
Reading after rotating the prism table through 45o or 135o …………………………
Table
hhah fi
suatahpn
fofh
fi ogRra
gaia remh sgha haehgmR
)e(
SV
SV
lfaeo
gmh
fi
rahmgah
ggRra remh sgha haehgmR
) b(
SV
SV
lfaeo
2θ=
(a~b)
naemθ
Sgfoaa
ghsa
fhhah
nhaam
gah
Calculations:
Grating element (a+b) = 2.54/N = ……per cm.Where N is the number of lines on grating
The wavelength of various spectral lines in the first order (n=1) can be calculated by
 = (a+b) Sin / 1 = (a+b) Sin =
=
2.54
15000
sin 𝜃
tluseR:
eel t lelnrReu gl rg ln gn Rel R eel
rersg rs uolrRg e
egnl
tgrelR
nglln
tls
 observed
uR ns gs  In Å
% lggrg
4385
4916
rg 69r8 6234
sglr sRgrnu1. Image should be sharp and properly focused.
2. Grating should be placed for normal incidence, and should not alter during measurements.