How do wavefronts change during reflection and refraction?

During reflection, wavefronts maintain their shape but change direction, while during refraction, they change both shape and direction.

In the case of reflection, the wavefronts hit a surface and bounce back, maintaining their original shape but changing their direction. This is based on the law of reflection which states that the angle of incidence is equal to the angle of reflection. For instance, if a plane wavefront (a straight line) hits a flat mirror, it will reflect back as a plane wavefront. However, if it hits a curved mirror, the reflected wavefront will take on the shape of the mirror, becoming either concave or convex.

Refraction, on the other hand, involves a change in both the direction and the shape of the wavefronts. This occurs when waves pass from one medium to another with a different refractive index, causing them to bend and change speed. The law of refraction, also known as Snell's law, governs this process. It states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the ratio of the velocities of light in the two media, or equivalently, to the inverse ratio of the refractive indices.

For example, when a plane wavefront passes from air into water, it slows down and bends towards the normal (the line perpendicular to the surface at the point of incidence). This causes the wavefront to change shape, becoming curved. The extent of this curvature depends on the angle at which the wavefront hits the surface and the difference in refractive indices between the two media.

In summary, the behaviour of wavefronts during reflection and refraction is governed by the laws of reflection and refraction respectively. Reflection involves a change in direction while maintaining the shape of the wavefront, whereas refraction involves a change in both direction and shape due to the change in speed of the wave as it passes from one medium to another.