Reflection of Light by 2E4 Zhang Ling

What is reflection?
1．Reflection is the change in direction of a wave front at an interface between two different media so that the wave front returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves.
Reflection of light may be specular (that is, mirror-like) or diffuse (that is, not retaining the image, only the energy) depending on the nature of the interface. Furthermore, if the interface is between dielectric-conductor or dielectric-dielectric media, the phase of the reflected wave may or may not be inverted, respectively.

2．（1）Reflection of light
In the diagram, a light ray PO strikes a vertical mirror at point O, and the reflected ray is OQ. By projecting an imaginary line through point O perpendicular to the mirror, known as the normal, we can measure the angle of incidence, θi and the angle of reflection, θr. The law of reflection states that θi = θr, or in other words, the angle of incidence equals the angle of reflection.
In fact, reflection of light may occur whenever light travels from a medium of a given refractive index into a medium with a different refractive index.

（2）Laws of regular reflection
If the reflecting surface is very smooth, the reflection of light that occurs is called specular or regular reflection. The laws of reflection are as follows:
The incident ray, the reflected ray and the normal to the reflection surface at the point of the incidence lie in the same plane.
The angle which the incident ray makes with the normal is equal to the angle which the reflected ray makes to the same normal.
（3）Diffuse reflection
When light strikes a rough or granular surface, it bounces off in all directions due to the microscopic irregularities of the interface. Thus, an 'image' is not formed. This is called diffuse reflection.

3．Light bounces exactly back in the direction from which it came due to a nonlinear optical process. In this type of reflection, not only the direction of the light is reversed, but the actual wavefronts are reversed as well. A conjugate reflector can be used to remove aberrations from a beam by reflecting it and then passing the reflection through the aberrating optics a second time.

4．http://en.wikipedia.org/wiki/Reflection_%28physics%29#Reflections