Refraction of Waves

Refraction occurs when a wave passes from one medium to another and changes speed. If the wave enters at an angle to the boundary, it also changes direction.

Why refraction happens

Different media transmit waves at different speeds. When a wavefront enters a new medium at an angle, one side of the wavefront changes speed before the other, causing the wave to bend.

Direction of bending

Transition	Speed change	Bending direction
Less dense to more dense (e.g. air to glass)	Speed decreases	Bends toward the normal
More dense to less dense (e.g. glass to air)	Speed increases	Bends away from the normal
Along the normal (perpendicular entry)	Speed changes	No bending (goes straight through)

What happens to wavelength and frequency

• Frequency stays the same. The source determines the frequency.
• Wavelength changes. In a slower medium, wavelength decreases. In a faster medium, wavelength increases.
• Speed changes. This is the cause of refraction.

Worked example

A light ray passes from air into a glass block at an angle of incidence of 40$^\circ$. Describe what happens to the ray.

The ray slows down as it enters the glass (glass is optically denser). It bends toward the normal, so the angle of refraction is less than 40$^\circ$. The frequency stays the same but the wavelength decreases.

Common errors and how to correct them

Saying “the wave speeds up in a denser medium.” For light, the opposite is true: light slows down in a denser medium (glass, water). The wave bends toward the normal when it slows down.

Stating that frequency changes during refraction. Frequency is determined by the source and does not change when the wave crosses a boundary.

How to apply this in an exam

Draw the normal at the boundary. Show the incident and refracted rays on opposite sides of the normal. If the wave slows down, bend it toward the normal (smaller angle). If it speeds up, bend it away (larger angle). Always state that frequency is unchanged.