Huygen's principle of secondary waves | Wave optics | Physics | Khan Academy

Summary

The video explores the behavior of light as it passes through a small hole, expanding and spreading out, contrary to Newton's particle theory of light. The concept is explained using Huygens' Principle, which describes light as a wave. Each point on a wavefront acts as a secondary wave source, with the new wavefront formed by the common tangent of these secondary waves. The video helps to explain phenomena like diffraction and highlights how light's wave nature can explain behaviors such as spreading when passing through slits. The principle lays the foundation for understanding reflection and refraction.

Takeaways

• 😀 Light behaves as a wave, not just as a bullet, which is key to understanding phenomena like light bending through small holes.
• 😀 When light passes through a small hole, it spreads out and forms a larger spot, contrary to the bullet-like expectation.
• 😀 Huygens' principle is introduced as a way to explain how light waves evolve and spread after passing through a slit.
• 😀 Huygens assumed light propagates as a wave through an 'ether' medium, with wavefronts representing ripples in this medium.
• 😀 Light rays are always perpendicular to the wavefronts, and this relationship holds even when waves travel long distances.
• 😀 To predict how a wavefront evolves over time, we use Huygens' principle to reconstruct the new wavefront by considering secondary waves from each point on the original wavefront.
• 😀 Each point on a wavefront acts as a source for secondary spherical waves, which collectively form a new wavefront.
• 😀 A common tangent drawn to all the secondary waves represents the new wavefront.
• 😀 Huygens' principle works for any shape of the wavefront, whether spherical or plane, and explains various optical phenomena like reflection and refraction.
• 😀 By applying Huygens' principle, we understand why light spreads out after passing through a hole, transitioning from a plane wave to a curved wavefront.

Q & A

What happens when parallel rays of light pass through a small hole in a cardboard?

When parallel rays of light pass through a small hole in a cardboard, instead of traveling straight through, they spread out and form a larger spot on a screen placed behind the hole.

How does the particle theory of light explain this spreading behavior?

According to Newton's particle theory of light, light is seen as a series of 'bullets' or particles. These particles would travel straight through the hole and not spread out. However, experiments show light behaves differently, indicating the particle theory does not fully explain this phenomenon.

How does the wave theory of light explain the spreading of light through a small hole?

The wave theory explains that light behaves as a wave, and when light passes through the hole, the wavefronts bend and spread out. This spreading is explained using Huygens' principle, which suggests that each point on a wavefront can act as a secondary source of waves, causing the overall wavefront to expand.

What is Huygens' principle?

Huygens' principle states that every point on a wavefront can be considered a secondary source of spherical waves. The new wavefront at a later time is the common tangent to all of these secondary waves.

What are wavefronts in the context of light?

Wavefronts are the surfaces or fronts of the light wave where the wave's oscillations are in phase. For example, a spherical wavefront represents all points where the light waves are at the same stage of oscillation.

How does Huygens' principle help in predicting the future shape of a wavefront?

Huygens' principle allows us to predict the shape of a wavefront at a later time by considering every point on the current wavefront as a secondary source that emits its own waves. The new wavefront is then the common tangent to all these secondary waves.

Why are the rays of light always perpendicular to the wavefronts?

Rays of light are always perpendicular to wavefronts because the rays represent the direction of energy propagation, and the wavefronts represent the oscillations of the light wave. This relationship is inherent in the wave nature of light.

Why does light spread out after passing through a small hole?

When light passes through a small hole, each point on the wavefront acts as a secondary source of light, emitting its own spherical waves. The new wavefront is formed by the common tangent to all these secondary waves, causing the light to spread out.

What does Huygens' principle tell us about the direction in which light propagates?

Huygens' principle suggests that light propagates in the direction of the wavefront’s expansion. Since the wavefront spreads out from each point, light continues in the direction where the wavefronts evolve, which is determined by the secondary waves emitted from each point.

What is the significance of the 'common tangent' in Huygens' principle?

The common tangent, or envelope, to all the secondary waves represents the new wavefront. It shows the direction in which the light will propagate at a later time, helping to reconstruct the wavefront after the light interacts with obstacles or slits.