Rainbows STEM


Rainbows STEM is a hands-on lesson that gives students the opportunity to see light in action as they use prisms to simulate rainbows. Students will learn about the properties of light, specifically reflection and refraction. They will also discover that rainbows are an illusion.

There are suggestions in the “Options for Lesson” section of the classroom procedure page for additional activities or ideas. As an introduction to rainbows, our lesson on the human eye is a great starting place. You can also check out the associated Learn Bright video for a quick introduction.

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What our Rainbows STEM lesson plan includes

Lesson Objectives and Overview: Rainbows STEM explores the circumstances that cause a rainbow to show in the sky. Students will learn to differentiate between reflection and refraction. In addition, they will learn that rainbows are an optical illusion. By the end of the lesson, they will simulate a rainbow using a prism. This lesson is for students in 5th grade and 6th grade.

Classroom Procedure

Every lesson plan provides you with a classroom procedure page that outlines a step-by-step guide to follow. You do not have to follow the guide exactly. The guide helps you organize the lesson and details when to hand out worksheets. It also lists information in the yellow box that you might find useful. You will find the lesson objectives, state standards, and number of class sessions the lesson should take to complete in this area. In addition, it describes the supplies you will need as well as what and how you need to prepare beforehand.

Options for Lesson

You can check out the “Options for Lesson” section of the classroom procedure page for additional suggestions for ideas and activities to incorporate into the lesson. While rainbows are familiar to all students, the science behind making rainbows may not be as familiar. Rainbows are an optical illusion. One option for introducing the lesson is to help students understand optical illusions and how the eye perceives different shapes or colors. As an introduction, the Learn Bright lesson on the human eye is a great starting place. Check out our video The Human Eye for Kids for a quick introduction, or complete the entire lesson on the human eye.

Teacher Notes

The paragraph on this page provides a little more information or guidance on what to expect from the lesson. It explains that this lesson is designed to provide students with a hands-on experience to understand the basic principles of light waves and their properties. You can use the blank lines to record any thoughts or ideas you have as you prepare.


Optical Illusions

The Rainbows STEM lesson plan includes three content pages. It is hard to imagine that anyone at some point hasn’t marveled at a rainbow in the sky after a hard rain. However, have you ever thought about the science of rainbows? There is more to rainbows than the beauty they project. For example, did you know that rainbows are actually optical illusions? An optical illusion is something that happens when the eye sees something and sends a message to the brain that the object is real even though it does not exist in reality.

For example, suppose you look down a long, straight road (like an interstate highway) or down a long, narrow railroad track. It appears as if the road or track narrows or merges eventually. In reality, the sides of the road or train tracks are parallel at equal distance, or equidistant, to each other.

Here’s another example. If you are at the beach and look far across the ocean, it may seem as if the water and sky merge, forming a straight line. This explains why early sailors thought the earth was flat. They believed that if they sailed out far enough, they would reach the edge and fall over it—like the edge of a table! But this was just an optical illusion. Similarly, rainbows are an optical illusion.

Rainbows don’t exist in a specific location in the sky. In other words, there is no point where the rainbow begins or ends, even though it appears to be touching the ground. The appearance of a rainbow depends on where you are standing in relation to the sun and sunlight. The rainbow you see is not necessarily visible to someone else standing nearby. The position of the sun and water droplets must be just right. The sun must be behind the observer, low in the sky, and at less than a 42° angle to the horizon. We should first discuss light waves to understand rainbows and the science behind how they form.


Light waves are a form of energy from the sun or sunlight (in the case of rainbows). Visible light is electromagnetic radiation that is visible to the human eye. Examples of visible light are the sun, ordinary household lightbulbs, moonlight, and light from a flashlight. So if there is visible light, does this mean there is also invisible light or light rays that the human eye can’t detect? The answer is yes! Both microwaves or radio waves cannot be seen by the human eye and are examples of invisible light. So how do we know waves are real if we can’t see them? We’re glad you asked.

Let’s consider some items you probably use daily, such as cell phones. We can observe both visually and audibly that when a cell phone rings and we answer it, there are both sounds and images. This is because cell phones work by transmitting radio waves from towers or base stations and then into your phone. You don’t see the radio waves but can observe the result—voice and image.

Another example is when you put a bag of popcorn into your microwave. Microwaves heat food by using electromagnetic radiation. We don’t see the waves, but we can smell fresh popcorn! When we observe a rainbow, we observe light waves in action.

Visible Spectrum of Light

Rainbows form when light waves travel through water droplets. The rainbow effect occurs because the light is reflected back inside the water droplets. First, the water droplets bend the waves because water droplets are denser than the air. The bending of light is called refraction. Then the light waves separate when the light reflects off the inside of the water droplets.

This is called dispersion. The colors you see are the wavelengths separating as the light exits the water droplets. Most of the time, people see rainbows after rain when the sun reappears from the clouds. Earlier, we said a rainbow is an optical illusion. You don’t see the rainbow, but the colors formed when light separates in the water droplets. How many colors do you see?

The human eye is incredibly complex. The visible spectrum, or what the eye can see in the rainbow, consists of seven colors. These are red, orange, yellow, green, blue, indigo, and violet. Would it surprise you to learn there are actually several more colors in the rainbow? Some scientists believe there may be upwards of a million colors in a rainbow that are not in the visible spectrum!

After Rain Come Rainbows

Rainbows are most commonly visible after rain. On average, they last about an hour. However, rainbows can form whenever excessive water is in the atmosphere, like fog or dew. Since rainbows form when light passes through droplets of water, that means one can form any time droplets and light are present. Here is a weird fact. Did you know rainbows can form at night?

Remember that rainbows form when light passes through water droplets. So when a full moon is present, and the sun reflects off the moon at night, a rainbow forms that we call a moonbow. Moonbows tend to form in tropical areas where water droplets are heavy in the atmosphere. However, the darkness makes visible light difficult for the eye to perceive. As a result, moonbows appear faint in the night sky, sometimes as a white light.

What about the shape of rainbows? Rainbows appear to form a perfect arc or a semicircle that touches down at two different points on the earth. You learned earlier that a rainbow is an optical illusion. Does a rainbow touch the earth at two points in an arc shape? The answer may surprise you—no, rainbows don’t form an arc. They actually make a perfect circle. If you are in an airplane or hovering in a helicopter, you will see the rainbow as a circle, not an arc. The rainbow doesn’t touch the earth at all. Again, it’s your eyes playing a trick on you!

So what causes rainbows? Though it may seem a lot of science goes into making a rainbow, the simple answer is this: Rainbows need a source of light (the sun if you are outdoors) and water droplets in the atmosphere. Light enters the water droplet. The light bends because the air is less dense than the water droplets. Once light enters the droplets, it reflects off the inside. Next, the light separates into the wavelengths of color. Finally, the light that exits from the water droplet makes the rainbow that is only visible if you are standing in the right place at the right time. And now you know how rainbows are formed!


The Rainbows STEM lesson plan includes three worksheets: an activity worksheet, a practice worksheet, and a homework assignment. Each one will help students solidify their grasp of the material they learned throughout the lesson. You can refer to the classroom procedure guidelines to know when to hand out each worksheet.


The activity page includes two experiments. For the first one, students will practice using a prism to see how light refracts into the rainbow of colors. (You can find prisms at any retailer that sells science supplies.) For the second experiment, students will get to make their own prisms using the supplies on the worksheet page. They must follow the directions to turn a water glass and cardboard into a prism. If you want, you can also incorporate the advanced experiment that uses glasses of different shapes and sizes.


For the practice worksheet, students will compare and contrast the diagrams at the top of the page using a Venn diagram. One image shows light entering a triangular prism. The other is of light entering a water droplet.


Students will mark a series of statements as either true (T) or false (F). There are 14 total statements in the section. Once they finish marking true or false for each one, they will cut along the dotted lines and make a matching game.

Worksheet Answer Keys

The final two pages of the lesson plan document are answer keys for the practice and homework worksheets. Correct responses are in red to make it easier to compare them to your students’ answers. Given the nature of the practice worksheet, the answers are sample responses. Your students’ work may vary. If you choose to administer the lesson pages to your students via PDF, you will need to save a new file that omits these pages. Otherwise, you can simply print out the applicable pages and keep these as reference for yourself when grading assignments.

Additional information


5th Grade, 6th Grade


Science, STEM

State Educational Standards

NGSS.4-PS4-2, NGSS.MS-PS4-2, NGSS.4-PS3-2

Lessons are aligned to meet the education objectives and goals of most states. For more information on your state objectives, contact your local Board of Education or Department of Education in your state.