2009 Question 7
(i) When light shines on a compact disc it acts as a diffraction grating causing diffraction
and dispersion of the light. Explain the underlined terms.
(ii) Derive the diffraction grating formula.
(iii)An interference pattern is formed on a screen when green light from a laser passes normally
through a diffraction grating. The grating has 80 lines per mm and the distance from the
grating to the screen is 90 cm. The distance between the third order images is 23.8 cm.
Calculate the wavelength of the green light.
(iv) Calculate the maximum number of images that are formed on the screen.
(v) The laser is replaced with a source of white light and a series of spectra are formed on the
screen.
Explain how the diffraction grating produces a spectrum.
(vi) Explain why a spectrum is not formed at the central (zero order) image.
