1. What a wavefront is
A wavefront is an imaginary surface connecting all points of a wave that vibrate in the same phase. That means every point on a wavefront reaches its crest, trough, or zero displacement at the same moment.
Wavefronts help visualise how light spreads and how it interacts with objects and boundaries.
2. Why wavefronts are useful
Instead of thinking of light as thin rays, imagining wavefronts gives a clearer picture of the overall wave behaviour. Wavefronts show:
- the direction in which the wave travels,
- how the shape of the wave changes,
- how bending (refraction), spreading (diffraction), and reflection occur.
3. Types of wavefronts
The shape of a wavefront depends on the nature of the light source.
3.1. Spherical wavefront
Formed by a point source. All points are at the same distance from the origin of light. The wavefront expands outward like a growing sphere.
3.2. Plane wavefront
Produced when the source is very far away. The curvature becomes negligible and the wavefront appears straight (like parallel planes).
3.3. Cylindrical wavefront
Produced by a long, narrow source such as a slit. The wavefront expands in one direction and remains unchanged in the perpendicular direction.
4. What rays are
A ray is a line drawn perpendicular to a wavefront. Rays represent the direction in which light moves. Even though wavefronts give a full picture, rays are easier to use for geometric constructions in ray optics.
5. Relationship between wavefronts and rays
Wavefronts and rays are always connected:
- Rays are perpendicular to wavefronts.
- Wavefronts show surfaces of constant phase.
- Rays show the path of energy flow.
5.1. Example
For a plane wavefront, rays are parallel straight lines. For a spherical wavefront, rays radiate outward from a point.
6. Using wavefronts to explain reflection
When a wavefront reaches a reflecting surface, each point along the boundary produces secondary wavelets. The new reflected wavefront follows the law:
\( \angle i = \angle r \)
The reflected rays remain perpendicular to the new wavefront.
7. Using wavefronts to explain refraction
When a wavefront enters a medium where the speed of light changes, different parts of it move at different speeds. The wavefront bends, producing refraction. Rays, being perpendicular to the bent wavefront, show the bending clearly.
8. Using wavefronts to explain diffraction
If only part of a wavefront passes through a narrow opening, the edges behave like new point sources. The resulting spread of the wavefront is diffraction — easier to visualise using wavefronts than rays.
9. Why the wavefront–ray picture is powerful
Wavefronts help me understand physical wave behaviour, while rays provide simple geometric paths. Using both together makes it easier to connect ray optics with wave optics.