Question

In what way diffraction of light waves differs from diffraction of sound waves?

Hint:

Compare the wavelength of sound (cm to m) with that of light (hundreds of nm).

Answer 1

Step 1: Recall when diffraction is noticeable:
A wave bends around an obstacle or opening clearly only when the size of the obstacle or opening is about the same as the wavelength, or smaller. So the wavelength decides how easily a wave diffracts.

Step 2: Compare the wavelengths:
Sound that we can hear has wavelength from about 2 cm to 20 m. Ordinary doors, windows, walls and corners are of this size. So sound bends round them easily, and we can hear someone talking from behind a wall or round a corner.
Visible light has wavelength only about 400 nm to 700 nm, which is thousands of times smaller than everyday objects. So light travels almost in straight lines past them and gives sharp shadows.

Step 3: State the difference:
Diffraction of sound is easy to observe in daily life because its wavelength is comparable to the size of common obstacles. Diffraction of light is observed only with very narrow slits or very small obstacles (of the order of a micrometre or less), because the wavelength of light is very small.
Also, sound is a longitudinal mechanical wave that needs a medium, while light is a transverse electromagnetic wave. Both diffract, but the effect for light is hard to see without special arrangements.

Final Answer:
Sound waves have large wavelengths, so they diffract around everyday objects and are easily noticed. Light waves have very small wavelengths, so their diffraction is noticeable only for very small apertures or obstacles.