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

Light Travels in Straight Lines: Shadows and Materials

C2.3 C2.6 A1.2About 35 minutes
Learning goals
  • I can explain that light travels in straight lines and very fast.
  • I can give evidence that light travels in straight lines.
  • I can explain how a shadow forms.
  • I can sort materials as transparent, translucent or opaque.
  • I can do a fair test to see how a shadow changes size.

Words to know

ray (beam)
A straight line of light.
shadow
A dark area where light is blocked by an object.
transparent
Lets almost all light pass straight through. You can see clearly through it, like a clean window.
translucent
Lets some light through, but you cannot see clearly through it, like wax paper.
opaque
Blocks light. No light passes through, like wood or cardboard.
fair test
An experiment where you change only one thing and keep everything else the same.

Shadow tag

On a sunny winter day, your shadow stretches across the snow. It copies every move you make. Why does it have your shape? Why can't you get away from it? The answer is in how light travels.

Light travels in straight lines

Light moves out from a light source in straight lines. A straight line of light is called a ray or a beam.

Here is some evidence (proof):

  • A flashlight beam in a dark, dusty room or on a foggy night is straight.
  • Sunbeams shine through gaps in the clouds or trees in straight lines.
  • You cannot see around a corner. Light does not curve around it to reach your eyes.
  • Shadows have the same shape as the object that makes them.

Light is also super fast. It travels about 300 000 kilometres in one second. Light from the Sun takes only about 8 minutes to reach Earth.

Beams of sunlight shine in straight lines through the misty air between tall pine trees.
Look at the sunbeams: each one is perfectly straight. This is evidence that light travels in straight lines. AI-generated image
card 1card 2card 3holes in a straight line: light gets through
The three-card test. When the holes line up in a straight line, you can see the flashlight through all three cards. Move one card a little and the light is blocked.

How a shadow forms

When light hits an object that it cannot pass through, the object blocks the light. Light cannot bend around the object, because it travels in straight lines. So there is a dark area behind the object with no light. That dark area is a shadow.

  • The shadow has the same outline as the object.
  • The shadow is always on the side away from the light source.
lampobjectwallshadow
Light travels in straight lines from the lamp. The object blocks some of the light, so a shadow forms on the wall.

Transparent, translucent and opaque

Different materials let different amounts of light through.

  • Transparent: lets almost all light pass straight through. You can see clearly through it. Examples: clean window glass, clear plastic wrap, clean water, air.
  • Translucent: lets some light through, but scatters it, so you cannot see clearly through it. Examples: wax paper, tissue paper, a frosted bathroom window, a thin curtain.
  • Opaque: blocks the light. No light passes through. Examples: wood, cardboard, metal, a brick, your body.

Opaque objects make dark shadows. Translucent objects make light, faint shadows. Transparent objects make almost no shadow.

MaterialHow much light passes through?GroupShadow
clear plastic wrapalmost alltransparentalmost none
wax papersometranslucentfaint
cardboardnoneopaquedark
clean window glassalmost alltransparentalmost none
aluminum foilnoneopaquedark
Materials and light

Shadows change size

A shadow can grow or shrink.

  • Move an object closer to the light, and its shadow gets bigger.
  • Move it farther from the light (closer to the wall), and its shadow gets smaller.

Outside, shadows change during the day too. In the morning and late afternoon the Sun is low in the sky, so shadows are long. Near noon the Sun is highest, so shadows are short.

Worked example

Let's read a shadow test

Zoe shone a flashlight at a wall 100 cm away. She held a toy 6 cm tall at different distances from the flashlight and measured the height of its shadow on the wall.

  • toy 50 cm from the flashlight: shadow 12 cm
  • toy 30 cm from the flashlight: shadow 20 cm
  • toy 10 cm from the flashlight: shadow 60 cm
  1. What did Zoe change? Only the distance between the toy and the flashlight.
  2. What stayed the same? The same toy, the same flashlight, the wall in the same place. So this is a fair test.
  3. Look for a pattern: as the toy moved closer to the flashlight (50 → 30 → 10 cm), the shadow got bigger (12 → 20 → 60 cm).
  4. Compare: at 10 cm the shadow was 60 cm tall. That is 10 times as tall as the toy itself (6 × 10 = 60)!

The closer the object is to the light source, the bigger its shadow.

Just for fun

What if light could bend around corners? You could read a comic book from inside a closed cupboard, and hide-and-seek would be impossible. Good thing light travels in straight lines!

Hands-on activity

Shadow lab

You need: a flashlight, a wall or a large sheet of paper taped to a wall, a small toy, a ruler or measuring tape, and test materials: clear plastic wrap, wax paper, tissue paper, cardboard, aluminum foil, a plastic cup, a sheet of printer paper. For part 1: three index cards, a hole punch, modelling clay.

Part 1: Three-card test. A grown-up punches a hole in the same spot on each card (about 1 cm in from the middle of one long edge, as far as the hole punch reaches). Stand each card up in a lump of clay, hole at the top, so the holes line up. Shine the flashlight through the first hole and look from the other end. Then slide the middle card 2 cm sideways. What happens?

Part 2: Material sort. In a dim room, hold each material between the flashlight and the wall. Sort them into transparent, translucent and opaque by looking at the shadow.

Part 3: Shadow size. Do Zoe's test. Keep the flashlight and the wall in the same place. Hold the toy at 3 different distances from the flashlight and measure the shadow each time. Record your results in a table.

What to look for: light shines through only when the holes are lined up straight; plastic wrap is transparent, wax paper and tissue are translucent, cardboard and foil are opaque; the shadow grows as the toy moves closer to the flashlight.

Safety: a grown-up uses the hole punch. Don't shine the flashlight into anyone's eyes.

Your turn

1

How does light travel?

2

Give one piece of evidence that light travels in straight lines.

3

What is a shadow?

4

Is wax paper transparent, translucent or opaque?

5

Is a wooden door transparent, translucent or opaque?

6

To make a shadow bigger, should you move the object closer to the light or farther from it?

7

When are your shadows longest outside: at noon or late in the afternoon?

More practice: warm-up

1

Transparent, translucent or opaque: a clean window?

2

Transparent, translucent or opaque: a cardboard box?

3

Transparent, translucent or opaque: tissue paper?

4

Which makes the darkest shadow: an opaque, a translucent or a transparent object?

5

Is your shadow on the same side as the light or on the side away from the light?

More practice: on your own

1

Why does a shadow have the same shape as the object?

2

Why can't you see a friend who is hiding behind a corner of the school?

3

A bathroom window is frosted. Why is this useful?

4

In Zoe's test, why did she keep the wall in the same place every time?

5

Light travels about 300 000 km in one second. Is that faster or slower than a car? Explain.

Challenge

1

Mia makes a shadow puppet show. Her puppet's shadow is too small. Give two ways she could make it bigger.

2

Ali holds up a clear plastic cup, a white plastic grocery bag and a cereal box in front of a flashlight. Predict what the shadows will look like.

3

A hockey arena's lights are high up, right above the ice. Why are players' shadows on the ice short and close to their skates?

Check your understanding

Quiz

1.

How does light travel?

2.

A shadow forms when...

3.

Which material is translucent?

4.

Which material is opaque?

5.

You move a toy closer to a flashlight. Its shadow gets...

6.

Light from the Sun reaches Earth in about...

For parents and teachers

Big idea: Ontario Grade 4 expectation C2.3 asks children to describe properties of light, including that light travels in a straight path. Shadows are the clearest evidence. This lesson also starts C2.6 (how different materials interact with light): transparent, translucent and opaque. The shadow-size test is a fair test (A1.2).

How to help: Make shadow puppets after supper. Ask "How can we make the shadow bigger? Fuzzier? Why does it have that shape?" On sunny days, compare the length of your shadows in the morning and at lunchtime.

Common difficulties:

  • Thinking a shadow is a "reflection" or a thing in itself. It is simply where light is missing.
  • Mixing up transparent and translucent. Ask "Can you see clearly through it?" (transparent) or "Only blurry light?" (translucent).
  • Thinking moving the object closer to the wall makes the shadow bigger. It's the opposite: closer to the light makes it bigger.

Safety: you handle the hole punch. Remind your child never to look at the Sun when talking about shadows outside.