Lab Manual
B.Tech – I
Physics Laboratory
“Determining Wavelength of Spectral
Lines with Plane Transmission Grating
by Minimum Deviation Method”
, Objective: To determine the wavelength of different spectral lines of mercury light with a plane
transmission grating by minimum deviation method.
Apparatus required: Spectrometer, diffraction grating of known grating element, mercury
lamp, a reading lens and spirit level
Description of apparatus
Spectrophotometer:
This is an arrangement for producing pure spectrum. The essential parts of a spectrometer include
collimator, grating table, and a telescope (See figure 1).
Figure 1: Spectrometer arrangement
(a) Collimator: The collimator provides a narrow parallel beam of light. It consists of a horizontal,
cylindrical, metallic tube fitted with an achromatic convergent lens at one end and a short coaxial
tube at the other end. The short coaxial tube is provided with a vertical slit of adjustable width at the
outer end, can be moved inside the main tube with the help of a rack and pinion arrangement. The
slit is illuminated by the source of light, whose spectrum is to be examined and the distance
between the slit and the convergent lens is so adjusted that the slit lies in the first focal plane of the
lens. Under this condition, the rays of light emerging from the collimator are parallel. Usually in a
spectrometer, the Collimator is rigidly fixed with its axis horizontal, but in same instrument it can be
rotated about the vertical axis passing through the center of instrument.
(b) Grating Table
It is a circular table supported horizontally on a vertical rod at the center of the spectrometer. It can
be rotated independently of the collimator and telescope about the vertical axis passing through the
instrument’s center of a circular scale graduated in half degrees carried by the telescope. The
rotation of the grating table can be read with the help of two diametrically opposite verniers
attached to it and sliding over the circular scale. The grating table can be clamped with the help of a
clamping screw and then a fine rotation can be given to it with the help of a tangent screw provided
at the base. The grating table can be raised or lowered and maybe clamped at any desired height
with the help of a clamping screw provided for it. It is also provided with the three levelling screws
B.Tech – I
Physics Laboratory
“Determining Wavelength of Spectral
Lines with Plane Transmission Grating
by Minimum Deviation Method”
, Objective: To determine the wavelength of different spectral lines of mercury light with a plane
transmission grating by minimum deviation method.
Apparatus required: Spectrometer, diffraction grating of known grating element, mercury
lamp, a reading lens and spirit level
Description of apparatus
Spectrophotometer:
This is an arrangement for producing pure spectrum. The essential parts of a spectrometer include
collimator, grating table, and a telescope (See figure 1).
Figure 1: Spectrometer arrangement
(a) Collimator: The collimator provides a narrow parallel beam of light. It consists of a horizontal,
cylindrical, metallic tube fitted with an achromatic convergent lens at one end and a short coaxial
tube at the other end. The short coaxial tube is provided with a vertical slit of adjustable width at the
outer end, can be moved inside the main tube with the help of a rack and pinion arrangement. The
slit is illuminated by the source of light, whose spectrum is to be examined and the distance
between the slit and the convergent lens is so adjusted that the slit lies in the first focal plane of the
lens. Under this condition, the rays of light emerging from the collimator are parallel. Usually in a
spectrometer, the Collimator is rigidly fixed with its axis horizontal, but in same instrument it can be
rotated about the vertical axis passing through the center of instrument.
(b) Grating Table
It is a circular table supported horizontally on a vertical rod at the center of the spectrometer. It can
be rotated independently of the collimator and telescope about the vertical axis passing through the
instrument’s center of a circular scale graduated in half degrees carried by the telescope. The
rotation of the grating table can be read with the help of two diametrically opposite verniers
attached to it and sliding over the circular scale. The grating table can be clamped with the help of a
clamping screw and then a fine rotation can be given to it with the help of a tangent screw provided
at the base. The grating table can be raised or lowered and maybe clamped at any desired height
with the help of a clamping screw provided for it. It is also provided with the three levelling screws
