C. Matter and Energy · Light and Sound

Making Light or Bouncing It: Emit or Reflect?

C2.2 A1.2About 30 minutes

Learning goals

I can tell the difference between things that emit their own light and things that reflect light.
I can explain that the Moon reflects light from the Sun.
I can explain that we see most objects because light bounces off them into our eyes.
I can name living things that make their own light.
I can test an idea in a dark room and explain what I found.

Words to know

emit
To give off your own light (or sound, or heat).

reflect
To bounce light (or sound) back off a surface.

reflector
An object made to reflect light back brightly, like the reflector on a bike.

luminous
Gives off its own light, like the Sun or a lamp.

non-luminous
Does not give off its own light. We see it only when light bounces off it.

bioluminescence
Light made by a living thing.

The Moon mystery

On a clear night, the Moon can be so bright that it lights up the snow and makes shadows. So is the Moon a light source like the Sun?

No! The Moon does not make its own light. The Moon is a big ball of rock. Sunlight shines on it and bounces off it toward Earth. Scientists say the Moon reflects light from the Sun.

A bright full Moon shines over a snowy field at night, and the trees cast long shadows on the snow. — The Moon looks bright, but it only reflects sunlight. Its reflected light is strong enough to make shadows on the snow. AI-generated image

The Sun emits light. The Moon only reflects sunlight back toward Earth, like a giant rocky mirror. (Not to scale: the real Sun is much bigger and much farther away.)

Emit or reflect?

Everything you see is in one of two groups:

Things that emit (give off) their own light. Scientists call them luminous. Examples: the Sun, other stars, a lit lamp, a phone screen, a firefly.
Things that reflect light from somewhere else. They do not make their own light. Scientists call them non-luminous. Examples: the Moon, a mirror, your bike reflector, a book, a tree, your face.

Here is the big surprise: most things you see do not make light at all. You can see your book only because light from a lamp or the Sun bounces off the book and goes into your eyes.

The lamp emits light. The light bounces off the book and goes into the eye. That is how we see the book.

Living things that glow

Some living things emit their own light. This is called bioluminescence.

Fireflies flash their light on summer evenings in many parts of Ontario.
Some fish, jellyfish and other sea creatures glow deep in the ocean, where sunlight cannot reach.
Some mushrooms glow very faintly in dark forests.

Be careful: a cat's eyes may seem to glow in the dark when car lights shine on them. But the cat's eyes are only reflecting the light. They do not make light.

Emits its own light	Reflects light from another source
Sun	Moon
firefly	cat's eyes at night
flashlight	bike reflector
phone screen	mirror
lit candle	safety stripes on a winter jacket

Things that emit light and things that only reflect it.

Reflectors keep us safe

Many winter jackets, backpacks and boots have reflective stripes. In the dark, when car headlights shine on them, the stripes reflect the light back toward the driver, so the driver sees you. Road signs and crossing guards' vests use the same idea. A reflector cannot glow by itself in a dark room. It needs a light shining on it.

Worked example

Let's sort: emit or reflect?

Sort these: the Moon, a firefly, a bike reflector, a flashlight that is on, a mirror, a star.

Ask: Would it still shine in a room with no other light at all? If yes, it emits light. If no, it only reflects light.
The Moon: without the Sun shining on it, we could not see it. → reflects.
A firefly: it flashes in the dark by itself. → emits.
A bike reflector: it shines only when headlights hit it. → reflects.
A flashlight that is on: it shines in a dark room. → emits.
A mirror: in a dark room you cannot see it at all. → reflects.
A star: stars are like our Sun. They make their own light. → emits.

Emit: the firefly, the flashlight, the star. Reflect: the Moon, the bike reflector, the mirror.

Just for fun

Funny but true: every "moonbeam" you have ever seen is really second-hand sunlight. The Moon is the biggest borrower in the sky, and it never gives the light back to the Sun!

Hands-on activity

Dark room test: emit or reflect?

You need: a room you can make very dark (a closet or a bathroom with no window), a flashlight, a mirror, a bike reflector or a jacket with reflective stripes, a spoon, and a phone or tablet (switched on).

Question: Can we see a mirror and a reflector in total darkness?
Predict: write what you think will happen.
With a grown-up, go into the dark room and close the door. Wait one minute for your eyes to get used to the dark. Can you see the mirror? The reflector? The spoon? The phone screen?
Now turn on the flashlight and shine it on each object (not in anyone's eyes). What changes?
Record your results in a chart with two columns: in the dark and with the flashlight.
Conclude: which objects emit light? Which only reflect it?

What to look for: in total darkness, only the switched-on phone screen can be seen; the mirror, reflector and spoon are invisible (unless they are right beside the screen and catch a little of its glow, which is reflected screen light!). With the flashlight, the reflector shines brightly and the mirror and spoon shine. So the screen emits light, and the others only reflect it.

Safety: a grown-up stays with you in the dark. Move slowly, and never shine the light into eyes.

Your turn

Does the Moon emit its own light or reflect light? From where?

Emit or reflect: a lit lamp?

Emit or reflect: a mirror?

How do you see a book in your room?

Name a living thing that emits its own light.

Why do jackets have reflective stripes?

More practice: warm-up

Emit or reflect: the Sun?

Emit or reflect: a bike reflector?

Emit or reflect: a firefly?

Emit or reflect: a shiny spoon?

What is the word for light made by living things?

More practice: on your own

Can you see a mirror in a room with no light at all? Explain.

At night, a cat's eyes seem to glow in the headlights. Do they emit light? Explain.

Is a bright TV screen luminous or non-luminous? Explain.

Why can you see the Moon at night, even though the Sun has set for you?

Sam says, "My white shirt glows, so it is a light source." Is Sam right?

Challenge

Design a test to find out if a sticker emits light or only reflects it. What would you do, and what result would show it reflects?

Why do crossing guards wear vests with reflective stripes, even though the stripes make no light?

Deep in the ocean there is no sunlight. Why is bioluminescence useful to animals that live there?

Check your understanding

Quiz

Which one emits its own light?

The Moon shines because it...

How do we see a chair in a room?

A bike reflector in a totally dark room will...

Which word means "gives off its own light"?

Which object is non-luminous?

For parents and teachers

Big idea: Ontario Grade 4 expectation C2.2 asks children to distinguish between objects and living things that emit their own light and those that reflect light from other sources. The key idea is that we see most things because light reflects off them into our eyes. The dark-room test is a simple investigation with a question, prediction, observations and a conclusion (A1.2).

How to help: At night, look at the Moon together and ask "Where does its light come from?" When you drive or walk in the dark, point out reflective signs and stripes.

Common difficulties:

Calling the Moon a light source. It looks like one, but it only reflects sunlight.
Thinking we see because light goes out from our eyes. Light travels into our eyes.
Glow-in-the-dark toys: these soak up light and give it off slowly for a while afterwards. It's fine to tell your child they are a special case, but don't let them confuse the main lesson.

Safety: stay with your child in the dark room, and keep flashlight beams away from eyes.