Incident Ray: Definition & Overview

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  • 0:01 What is an Incident Ray?
  • 0:36 Rays of Light
  • 1:17 Reflected Rays
  • 2:02 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.

This lesson will explain incident rays. Explore what a ray is in the first place, as well as how an incident ray relates to a reflected ray. Then, test your understanding with a brief quiz.

What Is an Incident Ray?

When an incident occurs, everyone takes notice. But there's another meaning of incident: when something happens as a result of another thing. Your kitchen light being on is incident upon you flipping the switch. And conversely, if you throw a ball onto a surface, that ball is incident upon the surface. You threw it and that led to it hitting the surface.

This last usage is the one that is meant by incident ray. An incident ray is a ray of light that is pointing towards and striking a surface. But, hold on a minute! What exactly is a ray?

Rays of Light

In optics, a light ray is a model of light in which we draw the light as a straight line. Light is actually a wave, with peaks and troughs. But, try drawing that on a simple diagram! It's much simpler to take those peaks and troughs and draw a line that goes at 90 degrees to them. This will give you a line, or ray, pointing in the direction the energy is flowing.

Light rays allow us to draw clear diagrams showing the way light is moving, including reflection, light bouncing off mirrors, and refraction , the bending of light when moving from one transparent material to another. This is called ray tracing. We can make something that looks like a ray in real life by either shining a light through a slit or using a laser.

Reflected Rays

Where you have an incident ray, you usually have a reflected ray. The light has to go somewhere, and in most cases, it bounces off. And, if the surface the ray is incident upon is a mirror, you can predict the way it will bounce off.

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