When an object is dropped in still water, the circular wave fronts that are produced move out from the contact point over the two‐dimensional surface. A light source emits light uniformly in all directions of the three‐dimensional world. The wavefronts are spherical, and the direction of motion of the wave is perpendicular to the wavefront, as depicted in the figure below.  This straight line path shown by the arrow is called a ray. Depicting light as rays in ray diagrams provides a method to explain the images formed by mirrors and lenses.

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Far from the source, the curvature of the wavefront is small, so the wavefront appears to be a plane. Then, the light rays will be nearly parallel. Rays from the sun are considered to be parallel when reaching the earth.

The law of reflection

Most visible objects are seen by reflected light. There are few natural sources of light, such as the sun, stars, and a flame; other sources are man‐made, such as electric lights. For an object to be visible, light from a source is reflected off the object into our eyes (except in the special case of phosphors).

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NOTE: Vision is the result of light reflected from the object.

As shown above, light strikes a mirror and is reflected. The original ray is called the incident ray, and after reflection, it is called the reflected ray. The angles of the incident and reflected rays are always measured from the normal. The normal is a line perpendicular to the surface at the point where the incident ray reflects. The incident ray reflected ray, and normal all lie in the same plane perpendicular to the reflecting surface, known as the plane of incidence. The angle measured from the incoming ray to the normal is termed the incident angle. The angle measured from the outgoing ray to the normal is called the reflected angle. The law of reflection states that the angle of incidence equals the angle of reflection. This law applies to all reflecting surfaces.


Light undergoes either diffuse or regular reflection. 

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Diffuse reflection occurs when light reflects from a rough surface.Regular reflection is reflected from a smooth surface, such as a mirror. The reflected rays are scattered in diffuse reflection. This scattering is because the local direction of the normal to the surface is different for the different rays. By contrast, in regular reflection, the reflected light rays are orderly because each local region of the surface has a normal in the same direction.