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CSEC Physics: Reflection of Light


Although you might think about reflection as only happening when you look into a mirror, reflection is actually responsible for most of what we see. For anything that doesn’t give off its own light, we see it from the light that reflects off of it and reaches our eyes. Objects which reflect little light appear as dark and dull black colours, while objects reflecting all light appear the same colour as the light they reflect (so when sunlight shines on them, they will appear white).

But consider this: a sheet of white paper and a mirror both reflect all of the light that hits them; however, only a mirror will show you your face when you look into it (whether that’s a good thing or not is up to you). So, there must be a difference in the reflection that is occurring on either surface.

The difference in reflection is due to the nature of the surfaces of these materials. On a mirror or a polished sheet of metal, the surface is very smooth and uniform. Thus, parallel rays of light from a particular source will be reflected in only one direction. This type of reflection is called regular or specular reflection.

On irregular surfaces like sheets of paper or concrete walls, light rays will be scattered in various different directions, resulting in diffuse reflection.

At a plane mirror, it is very simple to identify what is happening during reflection.

  • The normal is a line drawn perpendicular to the mirror where the incident ray meets it.

  • The incident ray is the ray of light which meets the mirror.

  • The reflected ray is the ray of light reflected from the plane mirror.

  • The angle of incidence is the angle between the incident ray and the normal.

  • The angle of reflection is the angle between the normal and the reflected ray.

The Laws of Reflection

  1. The angle of incidence equals the angle of reflection

  2. The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane. (This really just means that all the rays involved can be drawn on a flat sheet of paper)

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