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Ray Tracing with Mirrors: Reflected Images Video

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  • 0:00 What Is a Mirror?
  • 1:07 What Is Ray Tracing?
  • 1:42 The Law of Reflection
  • 2:27 Example
  • 3:02 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.

After watching this lesson, you will be able to explain what a mirror is and use ray tracing diagrams, along with the law of reflection, to explain what you see when you look in a mirror. A short quiz will follow.

What is a Mirror?

We all know what a mirror is. But why can we see ourselves in them? What's special about a mirror compared to, say, a regular wall? Or the surface of a dining room table?

A mirror is defined as a reflective surface that shows a clear image. So, a mirror is... a mirror. It's certainly not the most helpful definition.

It would, however, be helpful to look at a mirror under a microscope and compare it to a regular surface. If you do that, you'll find that mirrors are super smooth. Although your dining room table might feel nice and smooth when you run your hand along it, it really isn't. If you magnified it many times over, you would see a landscape of peaks and valleys, knobs and sharp points. Surfaces only appear smooth on the large scale of humans.

Because of this, when light hits it, there's no telling where it will bounce off to. But as you make a surface smoother and smoother, those valleys and peaks start to get smaller and smaller. Pretty soon you have a surface that's so smooth that the light bounces off it in nice, predictable ways. And this is a mirror.

What are those predictable ways? Well, we can explore that using ray tracing.

What is Ray Tracing?

Light is a wave... well, sometimes it's a particle. It's kind of both actually. Yes, physics is confusing. But we often draw light as something else entirely: a ray. A ray is just a path of light drawn as a straight line, coming from a source. It's a good way to show how light moves. Instead of drawing every little photon of light and how it moves, we can just draw a few to illustrate the point.

Here are some ray diagrams to give you the idea. It really gives you a sense for what the light is doing in a particular situation. So, what exactly is light doing when it hits a mirror?

ray diagrams

The Law of Reflection

The law of reflection tells us how light bounces off surfaces. The law of reflection says that the angle light hits a surface (called the incident angle) is equal to the angle that light bounces away from the surface (called the reflected angle). You can see how this works in this diagram.

The angle light hits the surface is equal to the angle light bounces away
Diagram of law of reflection

The law of reflection works for every surface, whether a mirror or your dining room table. But the problem with your dining room table is that you never know whether a light ray will hit a peak or a valley and which direction that part of the surface will be pointing. So, the light ends up bouncing off in lots of seemingly random directions. This is called diffuse reflection.

With a mirror, however, the surface is nice and smooth. So, parallel rays will follow the law of reflection as expected and bounce off still parallel. And that allows a mirror to create an image.

Example

When you look at a mirror, you see an image of yourself. It appears as though the image is behind the mirror. And that's because the light rays hit your body, bounce off the mirror, and then head back to your eyes. The rays that reach your eyes have bounced off the mirror and appear to be coming from behind the mirror.

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