Grade by Grade
C. Matter and Energy · Light and Sound

Bending Light: Refraction, Lenses and Rainbows

C2.3 C2.6 A1.2 A1.4About 35 minutes
Learning goals
  • I can explain that light bends when it passes from one material into another.
  • I can use refraction to explain why a straw looks broken in a glass of water.
  • I can explain how a lens or a water drop can make things look bigger.
  • I can explain that white light is made of many colours, and that refraction splits it into a rainbow.
  • I can predict, observe and explain what happens in a refraction investigation.

Words to know

refraction
The bending of light when it passes from one material into another, like from air into water.
lens
A curved piece of clear glass or plastic that bends light. Eyeglasses and magnifying glasses have lenses.
magnify
To make something look bigger than it is.
prism
A clear, solid shape (often a triangle) that bends light and can split white light into colours.
spectrum
The band of colours that white light is made of, like in a rainbow.

The broken straw

Put a straw in a clear glass of water and look at it from the side. The straw looks broken or bent right at the top of the water! Take it out. It is perfectly straight. What is going on? Light is playing a trick on you.

A yellow pencil leaning in a glass half full of water, seen from the side.
Look at the water line: the pencil seems to jump sideways where it enters the water. The pencil is straight; the light bends. AI-generated image

Light bends: refraction

You know that light travels in straight lines. But when light passes from one material into another, for example from air into water or from air into glass, it bends a little at the place where the two materials meet. This bending is called refraction.

Why? Light travels at different speeds in different materials. It is slower in water and glass than in air. When it changes speed at a slant, it changes direction, a bit like a shopping cart that turns when one wheel goes from the smooth floor onto a carpet first.

Inside each material, light still travels in straight lines. It only bends at the boundary between them.

airlooks brokenwaterglass
A pencil in a glass of water. Light from the part under the water bends as it leaves the water, so the pencil looks broken at the water line.

Refraction all around us

  • Pools and lakes look shallower than they really are. Light from the bottom bends as it leaves the water. That is why you must never guess how deep the water is. Always check before you go in.
  • Lenses are curved pieces of glass or plastic that bend light on purpose. A magnifying glass makes small things look bigger. Eyeglasses and contact lenses bend light to help people see clearly.
  • A drop of water is curved, so it can act like a tiny magnifying glass.
  • Rainbows! (see below)

White light is made of colours

Sunlight looks white, but it is really a mix of many colours. When sunlight passes through a prism or through raindrops, each colour bends by a slightly different amount, so the colours spread apart. We see a band of colours called a spectrum: red, orange, yellow, green, blue and violet (some people also name indigo, between blue and violet).

To see a rainbow in the sky, the Sun must be behind you and the raindrops (or the spray from a garden hose) in front of you.

white lightprismredorangeyellowgreenblueviolet
White light bends as it enters and leaves the prism. Each colour bends a slightly different amount, so the colours spread out into a spectrum. Red bends the least and violet bends the most.

Safety with lenses

  • Never use a magnifying glass to look at the Sun.
  • Never aim a magnifying glass at sunlight onto paper, leaves or skin. A lens can focus sunlight into a tiny, very hot spot that can burn skin or start a fire.
  • Do not leave glass balls, water bottles or magnifying glasses in a sunny window.
Worked example

Let's explain the disappearing coin

Put a coin in the bottom of an empty mug. Move your head back until the rim of the mug just hides the coin. Stay still. A helper slowly pours water into the mug. Suddenly the coin appears! Why?

  1. Without water: light from the coin travels in a straight line. The rim of the mug blocks that line, so the light cannot reach your eye.
  2. With water: the light from the coin travels up through the water. When it leaves the water and goes into the air, it bends (refraction).
  3. The bent light now travels over the rim and reaches your eye.
  4. Your brain thinks light always travels straight, so you see the coin a little higher than it really is.

Refraction. The water bends the light from the coin over the rim and into your eye, so you can see a coin that was hidden.

Just for fun

Why did the straw go to the doctor?

It looked broken, but the doctor said, "Don't worry, you're just refracted!"

Hands-on activity

Refraction investigation

You need: a clear glass or jar, water, a pencil or straw, a coin, a mug, clear plastic wrap, a newspaper or flyer with small print, and (on a sunny day) a glass of water and a sheet of white paper.

For each test, predict first, then observe, then explain using the word refraction.

  1. Broken pencil: put the pencil in a glass half full of water. Look from the side, then from above.
  2. Disappearing coin: do the coin trick from the example with a helper.
  3. Water drop magnifier: lay plastic wrap flat over the flyer. Put one drop of water on the wrap over a small word. What happens to the word?
  4. Make a spectrum: on a sunny day, set a full glass of water on a windowsill in the sunlight so it hangs a little over the edge, with white paper on the floor below. Move the paper until you see colours.

What to look for: the pencil looks broken at the water line; the coin appears when water is added; the word under the drop looks bigger; a small band of colours appears on the paper. Each is caused by light bending as it passes between water (or glass) and air.

Safety: a grown-up places the glass on the windowsill (a clear plastic cup is safer, since a glass can fall and break). Wipe up spills. Never look at the Sun, and don't leave the glass of water in a sunny window afterwards.

Your turn

1

What is refraction?

2

Why does a straw look broken in a glass of water?

3

What is a lens?

4

Name two things that use lenses.

5

What colours can you see in a rainbow?

6

Where must the Sun be for you to see a rainbow?

More practice: warm-up

1

Does light bend when it passes from air into water?

2

Does a magnifying glass make things look bigger or smaller?

3

Sunlight looks white. What is it really made of?

4

What is the name for a clear triangle shape that splits light into colours?

5

True or false: you can use a magnifying glass to look at the Sun.

More practice: on your own

1

Why is it dangerous to guess how deep a lake is by looking at it?

2

Why can a drop of water make tiny print look bigger?

3

Why does a rainbow have colours in bands instead of looking all white?

4

How do eyeglasses help some people see?

5

Why should you never leave a magnifying glass in a sunny window?

Challenge

1

Light travels in straight lines. How can it also bend? Explain both ideas together.

2

You are in a pool and a toy is on the bottom. You reach for where it looks to be, but miss. Explain what happened.

3

On a sunny afternoon, Ava wants to make a rainbow with the garden hose. Should she face the Sun or stand with her back to it? Why?

Check your understanding

Quiz

1.

The bending of light when it passes from air into water is called...

2.

A spoon in a glass of water looks...

3.

What does a magnifying glass do?

4.

Sunlight is really...

5.

To see a rainbow, the Sun should be...

6.

Why should you never aim a magnifying glass at sunlight onto paper?

For parents and teachers

Big idea: Ontario Grade 4 expectation C2.3 includes that light can be refracted (bent), and C2.6 asks how materials such as water and glass interact with light. Lenses and rainbows are everyday examples. Each test asks for a prediction, an observation and an explanation (A1.2), and the safety note about lenses connects to A1.4.

How to help: At the dinner table, put a straw or spoon in a glass of water and ask why it looks broken. If you wear glasses, let your child look at how the lenses are curved.

Common difficulties:

  • Mixing up reflection and refraction. Reflection = light bounces back. Refraction = light goes through and bends.
  • Thinking light bends all the time. It bends only when it passes from one material into another.
  • Thinking rainbows are "in" the raindrops. The colours were in the sunlight all along; the drops spread them apart.

Safety: supervise all use of magnifying glasses outdoors, and never let them be aimed at the Sun or used to focus sunlight. Water safety: teach that pools and lakes are deeper than they look.