According to wave theory of light, a source of light sends out disturbances in all directions. In a homogeneous medium, the disturbance travels with equal velocity in all directions from the source. So, the disturbance reaches all those points of the medium in phase, which are located at the same distance from the source and all such particles must vibrate in phase with each other. Evidently, at any instant, the wave motion lies upon the surface of a sphere whose center is the generating point and the radius is equal to the product of the velocity and time. The surface of the sphere is called wave front. 

Wave front: A wave front is defined as the envelope or trace drawn through all the points on a wave that is exactly in the same phase of vibration. 

Depending upon the shape of the source of light, wave front can be of the following three types:

1) Spherical wave front: If the light is given by a point source of light, and light travels with equal velocity in all directions, then a spherical wave front is produced.

2) Cylindrical wave front: When the source of light is linear in shape (e.g. a slit) a cylindrical wave front is produced. It is because, all the points which are equidistant from the linear source, lie on the surface of a cylinder. 

3) Plane wave front: A small part of a spherical or a cylindrical wave front originating from a distant source, will appear plane and hence it is called a plane wave front.