KS3 · Physics

How we see things

It feels as if your eyes reach out and grab these words. They don't. Light bounces off this page into your eyes, and your eyes simply catch it.

How you see a book

Stage 1 of 5: Lamp. paused

Lamp · 1/5scrub the light →

The lamp is luminous: it makes its own light, and gives it out in all directions from its glowing part.

Follow the light. Every arrow points away from the lamp. None of them ever comes out of your eye.

Test the rule

How do you see the Moon?

It's a clear night. The Moon is the brightest thing in the sky and you can see it perfectly.

Which is closest to what you think right now?

The Moon makes its own light. It's so bright it must be glowing.My eyes send something out to the Moon, and that's how I pick it up.Sunlight hits the Moon, bounces off it, and some of that light travels into my eyes.

How sure are you?

I'm sureI think soI'm guessing

The Moon makes its own light. It's so bright it must be glowing.

This is a common misconception

It's a very natural thought, but bright doesn't mean luminous. The Moon makes no light of its own, so it is non-luminous. It only looks bright because light from the Sun, a luminous source, is shining on it.

My eyes send something out to the Moon, and that's how I pick it up.

This is a common misconception

Nothing leaves your eyes. No part of the eye gives out light. Here's the test: if eyes sent out light, you could see in a room with no light at all. You can't. Eyes cope with very dim light, but with none at all you see nothing, because seeing needs light coming in.

Sunlight hits the Moon, bounces off it, and some of that light travels into my eyes.

This matches the evidence

Exactly right. The Sun is the luminous source. Its light travels to the Moon, the Moon reflects it, and some of that reflected light ends up in your eyes. It's the same chain as the lamp and the book, just on a much bigger scale.

Luminous objects give out their own light. Non-luminous objects, like the Moon, a book or your face, are only seen because light from a luminous source bounces off them and into your eyes.

Two kinds of reflection

Specular reflection (a mirror)vsDiffuse scattering (a dull page)

Here's the surprise: the book in the lamp scene was reflecting light too.

Focus

Does every ray obey the law of reflection?

Specular reflection (a mirror)

Yes

Diffuse scattering (a dull page)

Yes, every single ray still does

The insight

Scattering is a kind of reflection. Same rule, different surface.

The surface

Specular reflection (a mirror)

Very smooth and shiny

Diffuse scattering (a dull page)

Rough and bumpy

Where the reflected light goes

Specular reflection (a mirror)

In a regular way: each ray goes off in one particular direction

Diffuse scattering (a dull page)

In all directions

Can you see yourself in it?

Specular reflection (a mirror)

Yes, it can form a clear image

Diffuse scattering (a dull page)

No, there's no reflection of you

Reflection at a mirror

Where does the mirror send the light?

Angle of incidence 40°. Normal to your eye 68°. Relationship: Angle of reflection = angle of incidence (40° here), both measured from the normal, on opposite sides of it. Your eye catches the reflected ray only when it sits on the far side of the normal at that same 40°.

Angle of incidence40°Normal to your eye68°drag the eye →

Angle of reflection = angle of incidence (40° here), both measured from the normal, on opposite sides of it. Your eye catches the reflected ray only when it sits on the far side of the normal at that same 40°.

A thin beam of light hits a flat mirror at O. The normal is an imaginary line at right angles to the mirror. Drag the eye around and watch the second reading.

Inside the eye

What happens once the light is inside your eye

Showing 1 layer: The eye

Explore

tap a part of the eye

Light comes in from the left and travels to the back of the eye.

Follow the light from the front of the eye to the back.

Predict, then check

Use the rule: to see something, light from it has to get into your eye.

A torch shines a beam across a perfectly clean, dust-free room onto the far wall. You stand to the side. What do you see?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Your eyes catch light. They never send it out.

What you need to know

For you to see anything, light from it must get into your eye.
Luminous objects give out their own light. Non-luminous objects don't, and are seen by the light they reflect.
Light travels in straight lines. We draw it as rays with arrows pointing the way it goes.
Smooth, shiny surfaces reflect light in a regular way (specular reflection). Rough, dull surfaces scatter it in all directions (diffuse scattering).
At a mirror, the angle of reflection equals the angle of incidence, both measured from the normal.
Light enters the eye through the pupil, is focused by the lens and is detected by the retina.

The big picture

You see something only when light from it gets into your eye. Luminous objects, like the Sun or a lamp, give out their own light. Non-luminous objects, like the Moon or a book, are seen because light from a source hits them and some of it bounces into your eye. Light travels in straight lines. Smooth mirrors reflect it in a regular way, where the angle of reflection equals the angle of incidence. Rough surfaces scatter it in all directions. Inside the eye, light enters through the pupil, the lens focuses it, and the retina detects it.

Key points

1Seeing works one way only: light travels from a source, sometimes via an object, into your eye. No part of the eye gives out light.

2The Moon is non-luminous. It looks bright because sunlight is shining on it.

3A dull object can be seen from many positions because it scatters light in all directions.

4Scattering is still reflection. Every ray obeys the law of reflection, but a bumpy surface sends the rays in all directions.

5The normal is an imaginary line at right angles to the mirror. The two angles sit on opposite sides of it.

6A mirror is non-luminous, and you see a reflection in it only from certain angles.

7A beam of light is only visible if dust or water in the air scatters some of it into your eye.

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Worked example

Problem

It's a sunny afternoon. Explain why you can see a white cat sitting in the garden, and why your friend at the other end of the garden can see it too.

⚠ Watch out

Drawing the arrow the wrong way round. On a ray diagram, arrows go from the light source, to the object, to the eye. An arrow coming out of an eye says the eye gives out light, and it doesn't.

🧠

Memory hook

Eyes are catchers, not torches.

✓

Check yourself

Without looking back, explain how you can see the Moon in three steps, starting from where the light begins. Then check: does every arrow in your head point towards your eye?

Flashcards

(15)

What is a luminous object?

An object that gives out its own light, like the Sun or a lamp.

What is a non-luminous object?

An object that makes no light of its own. We see it by light from a source that bounces off it.

Why does the Moon look bright if it isn't luminous?

Light from the Sun is shining on it, and some of that light reflects towards us.

How does light travel?

In straight lines, until it hits the surface of a material.

What has to happen for you to see anything at all?

Light from it, either given out or reflected, must enter your eye.

Do your eyes send anything out when you look at something?

No. No part of the eye gives out light. Eyes only detect light that comes in.

What is specular reflection?

Reflection from a very smooth, shiny surface, in a regular way. It can form a clear image.

What is diffuse scattering?

Reflection from a rough, dull surface, sending light off in all directions. You see no reflection of yourself.

State the law of reflection.

The angle of reflection equals the angle of incidence, both measured from the normal, an imaginary line at right angles to the mirror.

Why can you only see a reflection from certain angles?

A mirror sends each ray off in one particular direction instead of scattering it, so the light only reaches eyes in that direction.

What does the pupil do?

It is the hole that lets light into the eye.

What does the lens of the eye do?

It focuses the incoming light, changing automatically so near and far things look clear.

What does the retina do?

It is the back surface of the eye, covered in cells that detect light.

When can you see a beam of light from the side?

Only when something in the air, like dust or water, scatters some of the beam's light into your eye.

Why must you never look straight at a laser or the Sun?

The very intense light can damage the light-sensitive cells of the retina, even if it doesn't hurt.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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How you see a book

How do you see the Moon?

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