Waves and the particle nature of light

Question 1

Define diffraction.

Answer 1

The spreading out of a wave as it passes through an aperture or around an obstacle.

Question 2

Light from a lamp is reflected from the surface of a CD, which consists of a spiral track of elongated bumps. Although the light from the lamp is white, different colours are seen in the reflected light. Explain this observation in terms of diffraction and interference.

Answer 2

When the light is reflected from the CD surface, diffraction occurs and light is scattered in all directions
Interference/ superposition occurs due to light diffracting from different points on the CD
Constructive interference occurs in directions where there is a path difference of a whole number of wavelengths
White light is a mixture of wavelengths
Hence constructive interference occurs in a different direction for each wavelength so a spectrum of colours is seen