Refraction of Waves

Refraction happens when a wave enters a new medium and changes its speed. Because the speed changes, the wave usually changes direction too. The bending of the wave at the boundary is what we call refraction.

I like to picture refraction as a wave ‘adjusting’ to a new environment — slowing down or speeding up — and turning slightly as a result.

Definition: Refraction is the change in direction of a wave as it passes from one medium to another due to a change in its speed.

This happens to all kinds of waves — sound waves, water waves, and light waves.

When a wave enters a medium where it moves faster or slower, the parts of the wavefront reach the new medium at different times. This uneven entry causes the wave to bend.

Depending on whether the wave slows down or speeds up, the direction of bending changes.

Refraction can be seen easily in water waves. When waves move from deep water to shallow water:

• They slow down.
• The wavelength decreases.
• The direction changes.

This bending makes water waves nearly parallel to the shore as they approach it.

Light also refracts when entering a different medium such as glass, water, or air.

This explains many everyday observations:

• A pencil appearing ‘bent’ in a glass of water.
• Objects underwater looking closer than they really are.
• A straw in a cup appearing shifted at the surface.

Sound waves change direction if they move through layers of air at different temperatures. Since sound travels faster in warm air and slower in cold air, the wave bends toward the slower region.

When waves refract:

• Frequency stays the same (the source doesn’t change).
• Speed changes depending on the medium.
• Wavelength adjusts because of the change in speed.

The relationship still holds:

Since \( f \) is constant, any change in \( v \) changes \( \lambda \).

Refraction creates many familiar effects:

• A coin at the bottom of a glass appearing raised.
• The twinkling of stars caused partly by refraction in the atmosphere.
• Mirages formed on hot roads due to hot and cold air layers.
• Spearfishing difficulty because fish appear at shifted positions under water.

Refraction is essential for understanding lenses, prisms, optical instruments, underwater visibility, and atmospheric effects. Once the idea of waves bending because of speed change becomes clear, many optical and wave behaviours become easy to understand.