Explain the theory of formation of spectra by a concave grating and describe the Eagle's mounting.
Answers
Imagine a reflection grating of a shape ….Concave…so here the diffraction maxima will lie on a circle of radius of curvature R.
and the source of light is also kept on the same circle.
concave grating of 600 lines/mm and radius R = 1075 mm. It is placed in a Rowland mounting.
In such a mount, the grating is constrained to be on a circle of diameter equal to the radius of curvature of the grating (the Rowland circle) on which the entrance slit (the slit through which light is incident on the grating) is also located.
In this experiment, strong lines from the visible part of the spectrum of the elements mercury and cadmium are used to calibrate the spectrograph.
The spectrograph is then used to measure the wavelengths of the lines in the visible spectrum of hydrogen (Balmer series) from which a value of the Rydberg constant for hydrogen can be obtained.
Several concave grating mountings can be used. Descriptions of the Rowland type with spherical or toric gratings, Wadsworth with spherical gratings, tandem Wadsworth gratings in additive or subtractive dispersion and some other types may be used.