Short Answer

Huygens’ principle explains refraction by stating that when a wavefront enters a new medium, every point on that wavefront produces secondary wavelets. These wavelets travel with a different speed in the new medium. Because one part of the wavefront enters the second medium first, it slows down or speeds up earlier than the rest.

This change in speed causes the wavefront to bend, which we see as refraction. Thus, refraction happens because different parts of the wavefront travel at different speeds when moving from one medium to another.

Detailed Explanation :

Huygens’ Principle Explain Refraction

Refraction is the bending of a wave when it travels from one medium to another, such as from air to water or from air to glass. Huygens’ principle provides a simple and clear explanation for why refraction occurs and how the direction of the wave changes.

According to Huygens’ principle, every point on the wavefront acts as a source of secondary wavelets. These wavelets spread out in all directions with a speed that depends on the medium. When a wavefront moves from one medium into another, the speed of these wavelets changes. This speed change causes the wavefront to tilt or bend, resulting in refraction.

This explanation is widely used in wave optics because it shows refraction as a natural result of changes in wave speed. It also helps us understand Snell’s law, which relates the angles of incidence and refraction.

1. Change of Medium and Change of Speed

Waves move at different speeds in different materials. For example:

• Light travels faster in air than in water.
• Light slows down when it enters glass.
• Light speeds up when it moves from water back to air.

When the wavefront reaches the boundary between two media, the part that enters the new medium first changes speed first. This difference in speed between different parts of the wavefront is what makes refraction occur.

Huygens’ principle helps visualize this process clearly.

2. Secondary Wavelets in the New Medium

When a wavefront crosses into a new medium, each point on the boundary creates secondary wavelets. In the second medium:

• These wavelets move with a different velocity.
• The direction of spreading also changes because wavelets advance slower or faster.

If the wave slows down, the new wavefront bends toward the normal.
If the wave speeds up, the wavefront bends away from the normal.

This bending of the wavefront creates the bending of the ray of light.

3. Geometry of Wavefront Bending

To understand refraction using Huygens’ principle, imagine the wavefront approaching a boundary at an angle.

• One edge of the wavefront enters the second medium first.
• This edge begins generating secondary wavelets that move at the new speed.
• The other edge is still in the first medium and continues to move at the old speed.

Because one side moves slower or faster than the other, the wavefront changes its angle. This turning of the wavefront is exactly what we call refraction.

Thus, wavefronts do not remain straight; they rotate depending on the speed change.

4. Refraction Into a Denser Medium

When light goes from a rarer medium (like air) to a denser medium (like water):

• The speed of the wave decreases.
• The wavefront slows down on the side that enters first.
• The opposite side continues moving faster in air.
• The wavefront rotates toward the normal.

This matches everyday observations—light bends toward the normal when moving into water or glass.

5. Refraction Into a Rarer Medium

When light moves from a denser medium to a rarer medium:

• The speed of the wave increases.
• The part entering the rarer medium moves faster.
• The rest of the wavefront still moves slowly in the denser medium.
• The wavefront rotates away from the normal.

This is why objects inside water appear raised or displaced when viewed from above.

6. Connection With Snell’s Law

Huygens’ principle can derive Snell’s law, which is the mathematical rule for refraction:

n₁ sin i = n₂ sin r

This relationship holds because wavefronts change angle exactly according to the change in speed between media.

Thus, Huygens’ principle not only explains why refraction happens but also supports the mathematical law that describes it.

7. Examples of Refraction Explained by Huygens’ Principle
8. a) Bending of a stick in water

The light from the underwater part bends because the wavefront slows down when entering air.

1. b) Lens focusing light

Convex lenses bend wavefronts toward the axis using the change of speed inside the glass.

1. c) Mirage effect

Light refracts through layers of air at different temperatures and speeds.

1. d) Rainbows

Light refracts inside water droplets because of changing speeds.

All these behaviours arise from wavefront bending due to unequal speed changes.

8. Why Huygens’ Principle Is Useful

Huygens’ principle is useful because:

• It provides a visual and conceptual explanation.
• It explains the direction of bending clearly.
• It works for all electromagnetic waves (light, radio waves, microwaves, etc.).
• It connects directly to Snell’s law.
• It shows refraction as a natural wave behaviour.

This makes refraction easier to understand for both students and scientists.

Conclusion

Huygens’ principle explains refraction by showing that when a wavefront enters a new medium, secondary wavelets in the new medium move with a different speed. This speed change makes the wavefront tilt or bend, producing refraction. Waves bend toward the normal when they slow down and bend away when they speed up. This simple and powerful idea helps explain all refraction phenomena and supports Snell’s law.