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Specular Reflection: Definition & Examples

Specular Reflection: Definition & Examples
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  • 0:01 Definition
  • 0:38 Law of Reflection
  • 1:07 Mirrors vs. Non-Mirrors
  • 2:07 Reflections on a Lake
  • 2:59 Eyeglasses and Glare
  • 4:49 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

This lesson will define specular reflection, explain how it differs from diffuse reflection, and give some examples of specular reflection: when it is useful and when it is problematic. A short quiz will follow.

Definition

How is a mirror different than other surfaces? In our everyday lives, we see a mixture of two different types of reflection. With most objects the light bounces off at all kinds of seemingly random angles due to the imperfections in the surface - this is called diffuse reflection.

But mirrors are special. Mirrors are super smooth, and allow us to see ourselves. And this is all thanks to specular reflection, the bouncing of light from the surface of a mirror, or something similarly shiny and smooth, where parallel rays of light all bounce off at the same angle.

Law of Reflection

When light hits a surface that is opaque, it will mostly reflect off that surface. The law of reflection describes the way it bounces off that surface. It tells us that the angle at which light hits the surface (the incident angle) is equal to the angle at which it bounces away from it (the reflected angle).

These two angles are measured from an imaginary line called the normal, which is a line at 90 degrees to the surface.

Mirrors vs. Non-Mirrors

When you look in a flat mirror, what do you see? Perhaps the answer says something about your psychology. But from a scientific perspective, you see a nice, clear picture of yourself. Everything is the right size, and all the parts of you are in the right place. This is because of the law of reflection and the fact that the light rays bounce off in nice, predictable ways.

But it turns out that the law of reflection applies to everything, not just mirrors. While your kitchen table might feel smooth to your hands, the surface contains millions of tiny imperfections. Because of those imperfections, a light wave doesn't hit the flat surface we see. Most of the time it hits an imperfection and bounces off at an angle you might not expect. The light ray is still following the law of reflection, it's just that you can't see which way the tiny bumps are pointing. What makes a mirror special is how flat and polished the surface is, allowing most of the light rays to bounce off in the same way.

Reflections on a Lake

An example of specular reflection is the crystal clear image that appears on a still lake. But why do some bodies of water reflect better than others? To see a perfect reflection, the lake must be truly still so that the surface is nice and smooth. This is what turns diffuse reflection into specular reflection.

But, the color and content of the water is important, too. Due to the suspended particles in dirty or salty water, the reflections may not be as strong. Light rays hitting these particles are more inclined to bounce off at inconsistent angles.

Last of all, the angle of the sun is important. For example, if the sun is behind you, there might not be a way for the sun's light to bounce off the lake and into your eyes. If a photographer wants to take a stunning reflection shot, he or she must take all these things into account.

Eyeglasses and Glare

Let's say you're watching television, but it's a sunny day and the light from the window is creating a reflection on your screen. You can't see the TV properly! In that example and many others, specular reflection isn't useful at all.

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