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
Betsy Chesnutt

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

Expert Contributor
Christianlly Cena

Christianlly has taught college physics and facilitated laboratory courses. He has a master's degree in Physics and is pursuing his doctorate study.

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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Additional Activities

Disappearing Test Tube Experiment

This is a hands-on experiment that engages the student to observe the refraction of light as it goes from one medium to another. The materials, procedures, and assessment of the experiment are provided in the next section.

(Note: Adult supervision is necessary when kids are performing the experiment)


  • tape or glue
  • light yellow cardboard
  • vegetable oil
  • water
  • two thin, clear beakers (100 ml and 500 ml)
  • test tube


Part 1: Disappearing Test Tube

  1. Use a table for this experiment. With tape or glue, mount the light yellow cardboard on the table to serve as background.
  2. Place the 500 ml beaker in front of the cardboard and pour a considerable amount of vegetable oil into this beaker.
  3. Then, carefully place the 100 ml beaker into this 500 ml beaker with oil.
  4. Fill the 100 ml beaker with more oil until it overfills and that the oil level surpasses the height of this beaker. Record your observations. (You should see a disappearing beaker as shown below)
  5. Next, put a test tube inside the smaller beaker as in the figure. Record your observations.
  6. Slowly fill the test tube with oil and record your observations. (You should observe that a part of the test tube slowly disappears as illustrated below. )

Part 2: Disappearing and Reappearing Test tube

  1. For this part, pour equal amounts of water and vegetable oil into the 500 ml beaker.
  2. Dip a test tube with oil into the beaker. Record your observations. (You should observe that a part of the test tube disappears and reappears)

Follow-Up Questions

  1. What phenomenon is manifested by light as it travels to different mediums? (Answer: The bending or slowing down of light, when it passes from one medium into another of different optical density is known as refraction).
  2. Name the mediums where light has traveled in this experiment. (Answer: Air, Glass, Oil, and Water)
  3. Explain how the test tube disappears when placed inside the 500 ml beaker with oil.
  4. How does the test tube disappear and reappear when placed inside the 500 ml beaker with water and oil?
  5. When viewed from above, will the test tube also disappear? (Answer: No, it won't disappear because light rays that pass the normal line do not bend.)

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