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Thermodynamics

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Rowland Mounting

The principle of Rowland mounting is illustrated in fig. G is the concave grating and P is the plate holder. The grating and the plate holder are mounted at the ends of a beam of length R equal to the radius of curvature of the grating surface. This beam GP can slide along two rails SX and SY. G, P and G’, P’ represent two positions of the beam. The slit S is set at the point of intersection of the rails SX and SY. With an arrangement of this type, the region of the spectrum imaged at P can be altered by sliding the beam. Sliding the beam alters the angle of incidence i. The spectrum obtained with this arrangement is nearly normal because the angle θ is nearly zero. For any position of the beam the spectrum is imaged at P.

From the equation,

(a + b)(sin i – sin θ) = nλ

If, θ = 0; sin θ = 0

∴ (a + b) sin i = nλ

Here (a + b) is a constant. For a given order

sin i ∝ λ

But, sin i ∝ SP

∴ SP ∝ λ

Thus with a mounting of this type, which is mostly of historical interest it is possible to calibrate the rail SP for the wavelength of spectral lines.