Answer to Question #118052 in Optics for Priya Mehra

Question

Why do we place the grating surface normal to incident day in diffraction grating experiment?

Answer 1

Diffraction gratings are paramount when light needs to be separated into varied wavelengths for instance in spectroscopy.

The angle at which a maximum occurs is affected if the incident ray is not normal to the grating.

Light needs to be incident to the grating surface. In fact, that keeps the mathematics simpler. The equation for normally incident light must be modified if the light is not normal to the grating. I describe the mathematics below.

Let  theta be the angle needed for constructive interference, “e” be the distance between  the two adjacent slits and lambda  to be the wavelength of  light. The path variance amidst light from the two slits is e multiplied by sine theta. Or path difference = e (sin theta).

For a constructive interference maximum, e (sin theta) = m lambda (where “m” is a whole number).

However, if the light is incident at an angle “i” to the normal then the equation becomes: e (sin i + sin theta) = m lambda. The path difference for constructive interference must be the same, of course. So, the value of theta must be different for different angles of incidence.