Next we have to figure out what a light ray does when it hits a mirror. Light travels very fast. So fast that it looks virtually instantaneous to us. In order to figure things out we have to pretend or imagine what happens if the light beam travels slower. If we pretend that light travels slow enough then we can watch the light beam bounce off a mirror like this

[WARNING: NEVER look at a light like this especially if the light source is a laser. It could damage your eyes. Always check with your mom or dad before doing these experiments.]

[WARNING: NEVER look at a light like this especially if the light source is a laser. It could damage your eyes. Always check with your mom or dad before doing these experiments.]

The red line is the light ray as it travels from the light source to the mirror, bounces off the mirror and then travels to your eye. I’ve indicated a few important angles which I’ll explain now.

For a flat mirror when the light bounces off it, the light bounces off at exactly the same angle as it had when it hit the mirror as illustrated below. That is, the “pie” shape indicated by the angles labeled and are exactly the same.
This is an important result and has been known for thousands of years. For example, we know that the ancient Greeks and probably other civilizations understood this fact. While this result is true for flat (also called plane) mirrors it is an inconvenient way of describing the situation especially if the mirror is curved instead of being flat.

Instead of using these angles what physicists use is the angle with respect to a special line called the “normal”. The normal to a curve is an imaginary line (see you have to constantly use your imagination in science) which is perpendular to the mirror’s surface like this and because it is perpendular then the interior angles (that is, the angles formed by the light ray and the normal line) are equal.These angles are illustrated below

and are called the “angle of incidence” which is the angle formed by the light ray before it strikes the mirror and the normal, and the “angle of reflection” which is the angle formed by the light ray after it strikes the mirror and the normal. The “law of reflection” states that these two angles are equal. This an important result and we will use it to explain reflection.

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Tags: Physics, Science

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