How To Make A Rainbow – Simple Science Experiments

How Are Rainbows Formed?

Sunlight or white light is a mixture of all the rainbow colors. The different rainbow colors are essentially mixed together to result in a white color.

You may wonder why when you mix all the different colors of paint together result in black instead of light. That is because light mixing is additive while pigment mixing is subtractive.

When light passes from one medium to another medium of a different density (e.g. from air to water), the light bends. This is known as refraction of light​1​ (More light experiments).

How much a light is refracted depends on the wavelength of the light. The shorter the wavelength, the more it bends. Because lights of different colors have different wavelengths, each color is refracted by different amounts.

Therefore, white light is split up after passing from air to water and the individual colors become visible. This dispersion of light allows us to see the spectrum of colors that form a rainbow​2​.

Refraction of light from air to water
Dispersion of light happens when light travels from air into water

In nature, a rainbow is formed when sunlight is refracted on entering a droplet of water, reflected inside the back of the droplet and finally refracted again on leaving the droplet.

Illustration of how we can see rainbow created by refraction of sunlight, illustrating how to make a rainbow.

Seeing A Rainbow

To see a rainbow caused by sunlight, the sunlight needs to come from behind us because rainbows only appear in the part of the sky directly opposite the sun.

Red has the longest wavelength among the visible colors. So it tends to bend the least and the angle between the incident light and our line of sight for red is approximate 42 degrees. As a result, red appears on the top of the rainbow

Violet has the shortest wavelength. It tends to bend the most. To see violet, the angle between the incident light and our line of sight is roughly 40. So violet appears at the bottom of the rainbow.

Diagram explaining the angle that can create rainbow, How are rainbows formed.

Why Are Rainbows Arched?

Rainbows are not two dimensional. A rainbow is seen at an angle that is between 40-42 degrees from the incident sunlight. So all the water drops that produce each color in the rainbow lie on a three dimensional cone. Our eyes are at the tip of the cone.

That is why rainbows are arched. The circular shape below the ground is invisible.

How are rainbows formed. Diagram showing why rainbows are arched.
Rainbow in the sky over the sea. How to make a rainbow?

Rainbow Experiment

Now are you ready to make some rainbows? Here are 3 easy ways to make your own rainbows at home. 

Warning: Adult supervision is vital in experiments involving sunlight. Do not look directly at the sun or at the reflection of sun in a mirror.

Rainbow Prism science experiment

1. Rainbow Prism

Active Time: 15 minutes
Total Time: 15 minutes

One of the easiest and most visually stunning ways to see an artificial rainbow is by using a prism. Creating rainbows using a prism is really fun.

Materials

  • strong sunlight

Tools

  • glass prism
  • white cardboard / paper (optional)
  • adult supervision

Instructions

  1. Place a piece of white paper on the ground under the sunlight.
  2. Put the prism on or above the paper.
  3. Rotate and move the prism around until you see rainbow colors on the paper.

    prism makes rainbow

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make rainbow on the wall

2. Rainbow On The Wall

Active Time: 15 minutes
Total Time: 15 minutes

No prism? No problem. Using a small mirror and a glass, you can make a beautiful rainbow on the wall.

Materials

  • water
  • sunlight

Tools

  • a clear glass
  • small mirror
  • adult supervision

Instructions

  1. Fill the glass with water.
  2. Put the mirror into the water inside the glass at an angle.

    make rainbow on the wall
  3. Position the glass so that sunlight shines directly at the mirror. You may have to shift the mirror to find the right angle.
  4. Look for a reflection on the wall. It would be easier to see if the room is dark.
  5. Adjust the angle of the mirror until you see a rainbow on the wall.

    make rainbow on the wall

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make rainbow at home

3. Make A Rainbow Using Flashlight

Active Time: 30 minutes
Total Time: 30 minutes

You can still make a rainbow without natural light. Try this experiment using a whitelight flashlight.

Materials

  • water

Tools

  • clear glass
  • flashlight that emits strong focused white light. Flashlights that emit only yellowish light won’t work. Flashlights that don’t emit focused light also won’t work. Look for one with parabolic reflecting mirrors inside to focus the light, or one with a strong LED light.
  • 2 pieces of white cardboard
  • adult supervision

Instructions

  1. Fill the glass with water.
  2. Cut a narrow rectangle in the middle of one piece of paper.
  3. Stick that paper to the outside of the glass so that the slit is at the center.

    rainbow setup
  4. Place the other piece of paper on the floor on the other side of the glass to catch the rainbow.
  5. In a pitch dark room, point the flashlight at the slit.

    make rainbow at home
  6. Start from the glass and slowly move the flashlight farther and farther from the glass.
  7. Adjust the distance and angle of the flashlight until you see a rainbow on the paper on the other side.

    make your own rainbow at home

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More Making a Rainbow Experiments

If you’re a rainbow enthusiast, want more rainbow experiments, or wonders what makes a rainbow, here are a few fun science projects for you.

Learn More About Rainbow


References

  1. 1.
    Jiang W, Chen RT, Lu X. Theory of light refraction at the surface of a photonic crystal. Phys Rev B. June 2005. doi:10.1103/physrevb.71.245115
  2. 2.
    Whitaker RJ. Physics of the Rainbow. The Physics Teacher. May 1974:283-286. doi:10.1119/1.2350374

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