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Reflection, Part 1

When light reflects off a surface, it follows some rather basic rules which have been gradually determined by observation. Consider the animation to the left. A ray of light approaches a reflecting horizontal surface at an angle of 45°, bounces from the surface, and leaves at an angle of 45°.

So that we can agree fully on what we are talking about, we need to define a few terms:

Incident Light

Light approaching a surface is known as incident light. This is the incoming light before it has reached the surface.

Reflected Light

After light has struck a surface and bounced off, it is known as reflected light. This is the light that is now departing from the surface.

Angle of Incidence

The angle at which a ray of light approaches a surface, reflective or not, is called the angle of incidence. It is measured from an imaginary line perpendicular to the plane of the surface in question to the incoming ray of light.

Angle of Reflection

Once the light has reflected from a reflective surface, the angle at which the light departs from the surface is called the angle of reflection. This angle is also measured from a perpendicular to the reflecting surface to the departing ray of light.

When light reflects from a surface, the angle of reflection is always equal to the angle of incidence.

 When multiple rays of light approach a reflecting surface, each individual ray behaves independently of all the others. Thus, in the figure to the left, each of the three incident rays depicted has its own individual angle of incidence, and each reflected ray has its corresponding angle of reflection. If all three angles of incidence are the same and the surface of reflection is perfectly flat as shown, all three angles of reflection will also be the same.

 When the surface is irregular instead of flat, each ray of light still has its angle of incidence and its angle of reflection. However, the angle is measured at the point at which the light strikes the surface. Thus, as shown to the right, light striking an irregular surface gets scattered in all directions upon reflection. This is the case with ordinary walls and surfaces. Actually, this is all to the good, because it is this scattered reflected light by which we can see such walls and surfaces.

What is far more interesting and useful is what happens when the surface is smooth but curved. We'll see that situation on the next page.

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