Describe wavefronts and rays in wave optics.

Wavefronts and rays in wave optics represent the propagation of light waves and their direction respectively.

In wave optics, a wavefront is a surface over which the phase of the wave is constant. In simpler terms, a wavefront is a surface that connects all the points in space that are being reached by a wave at the same time. The shape of the wavefront depends on the source of the wave. For instance, if the source of light is a point, the wavefronts are spherical. If the source is a large, distant object, like the sun, the wavefronts are effectively flat or planar.

Rays, on the other hand, are lines that are perpendicular to the wavefronts and indicate the direction of the wave's propagation. They are often used in diagrams to represent the path that light is travelling. The concept of rays is particularly useful in geometric optics, where light is considered to travel in straight lines. However, it's important to remember that rays are an abstract concept used to simplify the complex behaviour of light waves.

The relationship between wavefronts and rays is crucial in understanding the behaviour of light. When a wavefront encounters a new medium, its speed and direction may change, causing the wavefront to bend or refract. This change in direction is represented by a change in the direction of the rays. This is the basis of Snell's law, which describes how the angle of incidence and the angle of refraction are related when a wavefront encounters a boundary between two different media.

In conclusion, wavefronts and rays are fundamental concepts in wave optics. Wavefronts represent the phase of the wave, while rays indicate the direction of wave propagation. Together, they provide a comprehensive picture of how light behaves as a wave.