What is Refraction? - Definition, Causes & Examples

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  • 0:00 Definition of Refraction
  • 0:27 Definition of Light
  • 2:12 Refraction and Rainbows
  • 3:11 Refraction and Eyesight
  • 3:52 Lesson Summary
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Lesson Transcript
Instructor: Betsy Chesnutt

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

Refraction of light is responsible for producing beautiful rainbows and is necessary for your eyes to work. In this lesson, learn how refraction works and how it helps you to see.

Definition of Refraction

Have you ever seen a rainbow and wondered where all those colors came from? Or how about when you see a fish swimming just under the water, but when you reached out to touch it, it was a lot further away? Both of these examples (along with a lot of other ones) can be explained by understanding the refraction of light. Refraction occurs when light bends as it passes through a distorting material from one material to another.

Definition of Light

Light waves are a type of electromagnetic wave. Electromagnetic waves are different than waves on a string or sound waves in that they don't need anything to travel through. They're just waves of pure energy and because of this, they can travel through a lot of different materials, including empty space. They also move really fast, traveling through space at the speed of light (almost 300 million meters per second!). Although light waves can easily pass through materials like glass, water, and plastic, the waves slow down as they travel through these materials. When the light waves slow down as they pass from one material to another, that's when they're going through refraction.

When the light passes from air into a material like glass or water, it slows down and always bends toward a line drawn perpendicular to the surface of the material. This line is known as a normal line. When light passes from air into water or glass, it bends towards this normal line.

This explains why objects that you see underwater are never where they appear to be. Your eyes cannot tell that the light is not coming in a straight line, so you see an image of the object that is in a different place than the real thing.

Refraction of light

As you look at this image, notice that as light passes from air (on top) into water (on bottom) it bends down towards the normal line (shown by the dashed line).

You'll see that as light passes from water into air, it changes speeds, and this causes the light waves to bend. When you look at a pencil under the water, your eyes see an image of the pencil that's not where the pencil is actually located. Don't let your eyes fool you! The pencil only appears to bend at the point where it goes into the water because of this illusion.

Refraction And Rainbows

Sunlight actually consists of all the colors of light mixed together. When light passes into another material like glass or water, refraction causes it to bend. However, all the colors of light do not bend the same amount. At one end of the spectrum, red light bends the least, while at the other end, blue and violet light bend the most. When there are lots of water droplets suspended in the air (like after a rain storm or near a waterfall), each droplet of water bends the light as it passes through the water. The light reflects off the back of the water droplet and then refracts again as it passes out of the water back into the air. This causes the colors of light to separate and produces the familiar shape of a rainbow.

The light bounces off the back of the water droplet and then bends again as it comes out of the water into the air. Because each color of light bends a different amount, the colors become separated, forming a rainbow.

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