Chapter 7 of 7

All 7 chapters
  1. 1Where light comes from, readunderstood
  2. 2Light goes in straight lines, readunderstood
  3. 3How we see, readunderstood
  4. 4Opaque, translucent, transparent, readunderstood
  5. 5Big and small shadows, readunderstood
  6. 6A shadow stick through the day, readunderstood
  7. 7Mirrors and shiny things, readunderstood

S2.6.7 Chapter 7

Mirrors and shiny things

Why can I see myself in a mirror but not a wall?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Say why a mirror shows a picture and a wall does not
  • Say how a ray of light bounces off a mirror
  • Use a mirror and a torch safely

You see a wall because light bounces off it into your eyes. You see a mirror in the same way. But a mirror bounces light in a special way.

A mirror is smooth. Smooth means with no bumps. Light that hits a smooth, flat thing bounces off in one direction. Your eyes catch it, and you see a picture of things. That picture is an image.

A wall is rough. Rough means bumpy. Light that hits a rough thing goes every way. You see the wall, but you see no image.

Draw it first

Smooth and rough, side by side

  1. Two pictures, one above the other. At the top is a flat mirror, labelled smooth mirror. Three rays of light hit it from the upper left and bounce off side by side, all in the same direction. At the bottom is a bumpy wall, labelled rough wall. Three rays hit it from the upper left. Each bump bounces its ray in a different direction.smooth mirror

    1 Draw a smooth mirror. Draw three rays of light going in.

  2. Two pictures, one above the other. At the top is a flat mirror, labelled smooth mirror. Three rays of light hit it from the upper left and bounce off side by side, all in the same direction. At the bottom is a bumpy wall, labelled rough wall. Three rays hit it from the upper left. Each bump bounces its ray in a different direction.smooth mirror

    2 Draw the three rays going out, side by side.

  3. Two pictures, one above the other. At the top is a flat mirror, labelled smooth mirror. Three rays of light hit it from the upper left and bounce off side by side, all in the same direction. At the bottom is a bumpy wall, labelled rough wall. Three rays hit it from the upper left. Each bump bounces its ray in a different direction.smooth mirrorrough wall

    3 Now draw a rough wall. Draw three rays going in.

  4. Two pictures, one above the other. At the top is a flat mirror, labelled smooth mirror. Three rays of light hit it from the upper left and bounce off side by side, all in the same direction. At the bottom is a bumpy wall, labelled rough wall. Three rays hit it from the upper left. Each bump bounces its ray in a different direction.smooth mirrorrough wall

    4 Each bump sends its ray a different way. Draw them going out.

Paper looks smooth. Up close it is rough. That is why you can read a page from any side.

Still water is smooth. You can see an image in it. Ripples make it rough. Then the image breaks up.

Two pictures of water seen from the side. In the top one, labelled still water, three rays hit a flat surface from the upper left and bounce off side by side, all the same way. In the bottom one, labelled rippled water, three rays hit a bumpy surface from the upper left. Each bump bounces its ray a different way. Labels: still water, rippled water.still waterrippled water
Still water bounces the rays side by side. Ripples send them every way.

You see an image by two bounces. Light bounces off the ball. Then it bounces off the mirror into your eye.

A lamp at the top, a ball at the left, an eye at the bottom left and a flat mirror standing on the right. An arrow goes from the lamp to the ball. A second arrow goes from the ball to the mirror. A third arrow goes from the mirror into the eye. Labels: Lamp, Ball, Eye, Mirror.LampBallEyeMirror
Lamp light bounces off the ball to the mirror. Then it bounces into the eye.

A narrow beam of light is a ray. Look at the dashed line in the picture. It stands straight up from the mirror. A ray leans away from this line. How far it leans is its angle.

A flat mirror lies along the bottom, labelled mirror. A dashed line stands straight up from the mirror. A ray, labelled light in, comes down from the upper left at 40°. It bounces off to the upper right at 40°, labelled light out. Arcs mark both angles.40°40°mirrorlight inlight out
The angle in equals the angle out. Both are measured from the dashed line.

See it three ways

With things

Hold up your right hand to a mirror. The hand in the mirror looks like a left hand.

As a picture

A letter F on paper stands on the left, labelled paper. A mirror stands in the middle, labelled mirror. On the right is the image of the F, labelled image. The image is the F turned round, so its bars point toward the mirror. The top of the F is still at the top.papermirrorimage

In words

In a mirror, up is still up. Your head is still at the top. The mirror turns things front to back. So a right hand looks like a left hand. Writing looks backwards.

Try it

Bounce a beam off a mirror

You need: a small flat mirror, a torch, or the torch on a phone, a plain wall in a dim room, a book to lean the mirror on.

Be safe: Use a torch, not the Sun. Use a mirror with no cracks and no sharp edges. If it breaks, do not touch the pieces. Call a grown up. Never shine a torch or a mirror into anyone's eyes.

  1. Stand the mirror up, leaning on the book. Face it toward the room.
  2. Dim the room. Shine the torch at the mirror from far to one side.
  3. Find the bright spot on the wall. The mirror bounced the beam there.
  4. Move the torch to a new place. Watch the bright spot move.

What you will see: The beam leaves on the other side, at the same angle. Swing the torch one way, and the spot swings the other way.

If you cannot try it, this is what happens.
A torch beam, labelled torch beam, comes down from the upper left onto a flat mirror, labelled mirror, at 60°. It bounces off to the upper right at 60°, and goes to the wall, labelled to the wall. A dashed line stands straight up from the mirror, and arcs mark both angles.60°60°mirrortorch beamto the wall

A wrong idea many people have

A mirror makes its own light.

Why it feels right: A mirror looks bright and shows you a clear picture.

The test that breaks it: Take a mirror into a room with no light at all. Look in it. You see nothing. Switch a torch on. Now you see the image.

What is true: A mirror only bounces light. With no light to bounce, it shows nothing.

Key words

smooth
with no bumps.
rough
bumpy, not flat.
image
the picture you see in a mirror.
angle
how far a ray leans from the straight up line.

Back to your guess

The answer is B: The mirror sends light back in one direction.

A mirror is smooth and shiny. It bounces light back in one direction. A wall is rough. It bounces light every way, so you see no picture.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. A ray hits the mirror at 30° to the dashed line. At what angle does it leave?

    A flat mirror lies along the bottom, labelled mirror. A dashed line stands straight up from it. A ray, labelled light in, comes down to it from the upper left. No ray leaves.light inmirror
    Tap your answer.

    1. Not this one. You may be measuring from the mirror. Measure from the dashed line, both times. Try another.

    2. Not this one. The ray does not go straight up. It leaves at the same angle it came in. Try another.

    3. Yes, that is right. The angle in equals the angle out. Both are measured from the dashed line.

    See the answer

    30°

    The angle in equals the angle out. Both are measured from the dashed line.

    • If you chose A: You may be measuring from the mirror. Measure from the dashed line, both times.
    • If you chose B: The ray does not go straight up. It leaves at the same angle it came in.

  2. From an earlier chapter

    Which of these makes its own light?

    Tap your answer.

    1. Yes, that is right. A flame makes its own light. A mirror and the Moon only bounce light.

    2. Not this one. A mirror only bounces light. In the dark you see nothing in it. Go to S2.6.1. Try another.

    3. Not this one. The Moon bounces light from the Sun. It makes none of its own. Go to S2.6.1. Try another.

    See the answer

    A candle flame

    A flame makes its own light. A mirror and the Moon only bounce light.

    • If you chose B: A mirror only bounces light. In the dark you see nothing in it. Go to S2.6.1.
    • If you chose C: The Moon bounces light from the Sun. It makes none of its own. Go to S2.6.1.

  3. Why do you see no image in a rough wall?

    Tap your answer.

    1. Yes, that is right. The bumps send light in many directions. No one picture reaches your eyes.

    2. Not this one. You can see a wall. So light must bounce off it into your eyes. Go to S2.6.3. Try another.

    3. Not this one. A wall is not a light source. It bounces light from lamps and the Sun. Go to S2.6.1. Try another.

    See the answer

    A wall bounces light every way.

    The bumps send light in many directions. No one picture reaches your eyes.

    • If you chose B: You can see a wall. So light must bounce off it into your eyes. Go to S2.6.3.
    • If you chose C: A wall is not a light source. It bounces light from lamps and the Sun. Go to S2.6.1.

  4. A sheet of foil is smooth and shiny. You crumple it, and the image breaks up. Why?

    Tap your answer.

    1. Not this one. Foil makes no light at all. It only bounces light. Go to S2.6.1. Try another.

    2. Yes, that is right. Each tiny part bounces light a different way. So no clear picture reaches your eyes.

    3. Not this one. It is still foil. But it is now rough. A rough thing sends light every way. Try another.

    See the answer

    The crumpled foil has many tiny flat parts that face different ways.

    Each tiny part bounces light a different way. So no clear picture reaches your eyes.

    • If you chose A: Foil makes no light at all. It only bounces light. Go to S2.6.1.
    • If you chose C: It is still foil. But it is now rough. A rough thing sends light every way.

  5. A friend flashes sunlight from a mirror at a driver. What do you say?

    See the answer

    Never shine sunlight from a mirror into anyone's eyes.

    It can hurt their eyes. A driver who is dazzled can also crash.

  6. Hold up your right hand to a mirror. What does the hand in the mirror look like?

    See the answer

    A left hand. Up is still up.

    A mirror turns a picture front to back. So a right hand looks like a left hand.

  7. Which is a safe way to play with a mirror and light?

    Tap your answer.

    1. Not this one. A mirror sends the Sun's light into your eye. It can hurt as much. Try another.

    2. Not this one. A mirror sends back almost all the Sun's light. It can hurt your friend's eyes. Try another.

    3. Not this one. A torch can dazzle and hurt eyes. Keep the beam away from faces. Try another.

    4. Yes, that is right. A torch on a wall shows the bounce and hurts nobody. Never look at the Sun, even in a mirror.

    See the answer

    Shine a torch at a wall, away from all eyes.

    A torch on a wall shows the bounce and hurts nobody. Never look at the Sun, even in a mirror.

    • If you chose A: A mirror sends the Sun's light into your eye. It can hurt as much.
    • If you chose B: A mirror sends back almost all the Sun's light. It can hurt your friend's eyes.
    • If you chose C: A torch can dazzle and hurt eyes. Keep the beam away from faces.

  8. A ray leaves the mirror at 25° to the dashed line. At what angle did it come in?

    A flat mirror lies along the bottom, labelled mirror. A dashed line stands straight up from it. A ray, labelled light out, leaves the mirror to the upper right. No ray comes in.light outmirror
    Tap your answer.

    1. Not this one. The two angles are equal. They do not add up. Try another.

    2. Not this one. You may be measuring from the mirror. Measure from the dashed line, both times. Try another.

    3. Not this one. You may have measured from the far side of the mirror. Measure from the dashed line, both times. Try another.

    4. Yes, that is right. The angle in equals the angle out. Both are measured from the dashed line.

    See the answer

    25°

    The angle in equals the angle out. Both are measured from the dashed line.

    • If you chose A: The two angles are equal. They do not add up.
    • If you chose B: You may be measuring from the mirror. Measure from the dashed line, both times.
    • If you chose C: You may have measured from the far side of the mirror. Measure from the dashed line, both times.

Keep it

Copy this into your exercise book.

A smooth, shiny thing bounces light one way. You see an image. A rough thing sends light every way. The angle in equals the angle out. Never shine sunlight from a mirror into eyes.

Draw: A mirror line with a dashed line standing up from it. Draw a ray in and a ray out at the same angle.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (644 KB). It opens on any phone.