Chapter 2 of 8

All 8 chapters
  1. 1Signs of life, readunderstood
  2. 2Seeing small things, readunderstood
  3. 3Animal and plant cells, readunderstood
  4. 4What each part does, readunderstood
  5. 5Cells with special jobs, readunderstood
  6. 6Cells, tissues and organs, readunderstood
  7. 7Diffusion, readunderstood
  8. 8Living things with one cell, readunderstood

S3.1.2 Chapter 2

Seeing small things

How can we see things that are too small for our eyes?

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

  • Make a magnifier from a drop of water on clear plastic
  • Explain how a microscope lets us see cells

Every living thing is made of tiny parts called cells. A cell is the smallest part of a living thing that can do the jobs of life.

Most cells are far too small to see with your eyes alone. You need something to make them look bigger. A lens can do that.

A lens is a piece of clear material with a curved surface. It bends the light that goes through it. A lens that is thicker in the middle makes things look bigger.

Draw it first

Draw a drop of water that makes things bigger

  1. A side view in three steps. In the first, a sheet of clear plastic lies over a print. In the second, a drop of water with a curved top sits on the plastic. In the third, an eye looks down at the drop, and a green line with an arrow runs from the print up through the middle of the drop to the eye. Labels: clear plastic, print, water drop, eye.clear plasticprint

    1 Draw a flat sheet of clear plastic over a line of print.

  2. A side view in three steps. In the first, a sheet of clear plastic lies over a print. In the second, a drop of water with a curved top sits on the plastic. In the third, an eye looks down at the drop, and a green line with an arrow runs from the print up through the middle of the drop to the eye. Labels: clear plastic, print, water drop, eye.water dropclear plasticprint

    2 Draw a drop of water on the sheet. It is curved on top and flat underneath.

  3. A side view in three steps. In the first, a sheet of clear plastic lies over a print. In the second, a drop of water with a curved top sits on the plastic. In the third, an eye looks down at the drop, and a green line with an arrow runs from the print up through the middle of the drop to the eye. Labels: clear plastic, print, water drop, eye.eyewater dropclear plasticprint

    3 Draw an eye above the drop. Draw the line of light from the print, through the drop, to the eye.

A drop of water is thicker in the middle than at its edge. So it works as a small lens. Look at print through the drop and it looks bigger.

A drop makes things look about two or three times bigger. To see a cell you need much more. For that we use a microscope.

A microscope has two lenses. The lens near your eye is the eyepiece. The lens near the thing you look at is the objective lens. Each one makes the thing look bigger.

A microscope from the side. At the bottom is a base with a light, and green arrows show light going up. Above the light is a flat stage, with a thin clear slide lying on it and a small dark speck on the slide. Above the slide is the objective lens at the bottom of a tube. At the top of the tube is the eyepiece. Labels: eyepiece, objective lens, slide, stage, light.eyepieceobjective lensslidestagelight
A microscope. Light goes up through the slide and into the lenses.

You put the thing you want to see on a slide. A slide is a thin piece of glass or plastic. The light shines up through it. Only a very thin slice lets the light through.

The lenses multiply. Say the eyepiece makes things look 10 times bigger, and the objective lens 40 times bigger. Together they give 10 × 40 = 400 times. This is the magnification.

See it three ways

With things

Ana has a microscope. The eyepiece is marked 10× and the objective lens is marked 40×.

As a picture

A table of the lenses in a microscope and how much each one magnifies. The eyepiece magnifies 10 times. The objective lens magnifies 40 times. Together they magnify 400 times. Labels: lens, magnifies, eyepiece, objective lens, together, 10 times, 40 times, 400 times.lensmagnifieseyepiece10 timesobjective lens40 timestogether400 times

In words

Each lens makes the thing look bigger. The two numbers are multiplied to give the total magnification.

In symbols

10 × 40 = 400

A table of how wide some small things are, in millimetres. A pinhead is 1 mm. A plant cell is about 0.05 mm. A red blood cell is about 0.008 mm. A bacterium is about 0.002 mm. Labels: thing, width, pinhead, plant cell, red blood cell, bacterium, 1 mm, 0.05 mm, 0.008 mm, 0.002 mm.thingwidthpinhead1 mmplant cell0.05 mmred blood cell0.008 mmbacterium0.002 mm
About how wide some small things are. Cells differ, so these are rough sizes.

Look through a microscope at a thin skin from an onion, and you see rows of cells. Each one has a straight edge and a dark spot inside. The spot is the nucleus.

A round field of view, as seen down a microscope, full of plant cells that fit together like bricks. Each cell has a dark spot, its nucleus. Lines point to one cell and to one nucleus. Labels: cell, nucleus, drawn, not to scale.cellnucleusdrawn, not to scale
Plant cells, as a microscope shows them. The picture is not to scale.

A thin scrape from the inside of a cheek shows animal cells. They have no hard straight edge. They are soft and rounded, and each has a nucleus.

A round field of view full of animal cells. Each is a soft oval with a gap around it and a dark spot, its nucleus. Lines point to one cell and to one nucleus. Labels: cell, nucleus, drawn, not to scale.cellnucleusdrawn, not to scale
Animal cells, as a microscope shows them. The picture is not to scale.

Try it

Make a water drop magnifier

You need: a piece of clear plastic, such as from a clean bag or bottle, water, a stick or a straw, a page with small print, a leaf or a feather.

Be safe: Never look at the Sun through the drop. Work indoors or in the shade. Ask a grown up to cut the plastic with scissors.

  1. Lay the clear plastic flat on top of a line of small print.
  2. Dip the stick in water. Let one drop fall on the plastic over the print.
  3. Look straight down at the drop. Then move your head a little nearer and a little farther. Find where the print looks clearest.
  4. Try a leaf, or a feather, under the plastic in the same way.

What you will see: Through the drop, the print looks bigger. The drop works best when it is small and round. A flat puddle does not magnify.

If you cannot try it, this is what happens.
Two pictures of the same word, cell. On the left the word is small, in a box, labelled without the drop. On the right the word is twice as big inside a circle, labelled through the drop. Labels: without the drop, cell, cell, through the drop.cellcellwithout the dropthrough the drop

A wrong idea many people have

To see a tiny thing better, hold it closer to your eye.

Why it feels right: A near thing looks bigger, so closer seems better.

The test that breaks it: Hold this page right up to your nose. The words are blurred. Your eye cannot focus on a thing that is too close. Hold a drop lens between, and the words are clear and bigger.

What is true: Your eye cannot focus on a very near thing. A lens makes a small thing look bigger and keeps it clear.

Key words

cell
a tiny part that living things are made of.
lens
a curved piece of clear material that bends light.
microscope
a tool with two lenses that shows very small things.
slide
a thin piece of glass or plastic that holds the thing you look at.
magnification
how many times bigger a thing looks.

Back to your guess

The answer is B: A curved piece of clear material, such as a drop of water, can make small things look bigger.

A curved lens bends light and makes things look bigger. Holding a thing too close to your eye only makes it blurred.

Check yourself

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

  1. Why does a drop of water on clear plastic make print look bigger?

    Tap your answer.

    1. Yes, that is right. A drop is thicker in the middle than at its edge. It bends light and makes the print look bigger.

    2. Not this one. The letters do not change. The drop bends the light on its way to your eye, so they look bigger. Go to S3.1.2. Try another.

    3. Not this one. A flat sheet of plastic is just as close, and it does not magnify. The curve of the drop is what matters. Go to S3.1.2. Try another.

    See the answer

    The drop is curved and bends the light, so it works as a lens.

    A drop is thicker in the middle than at its edge. It bends light and makes the print look bigger.

    • If you chose B: The letters do not change. The drop bends the light on its way to your eye, so they look bigger. Go to S3.1.2.
    • If you chose C: A flat sheet of plastic is just as close, and it does not magnify. The curve of the drop is what matters. Go to S3.1.2.

  2. A microscope has an eyepiece of 10 times and an objective lens of 40 times. How many times does it magnify in all?

    Tap your answer.

    1. Not this one. You added the two numbers. The lenses work one after the other, so the numbers are multiplied. Go to S3.1.2. Try another.

    2. Yes, that is right. Multiply the two magnifications. 10 × 40 = 400.

    3. Not this one. You divided. Each lens makes the thing bigger again, so you multiply. Go to S3.1.2. Try another.

    See the answer

    400 times

    Multiply the two magnifications. 10 × 40 = 400.

    • If you chose A: You added the two numbers. The lenses work one after the other, so the numbers are multiplied. Go to S3.1.2.
    • If you chose C: You divided. Each lens makes the thing bigger again, so you multiply. Go to S3.1.2.

  3. Where does the light in a microscope come from?

    Tap your answer.

    1. Not this one. The eyepiece is where your eye looks. It gives out no light. The light goes into it. Go to S3.1.2. Try another.

    2. Not this one. A lens only bends light. It makes none. The light starts below the stage. Go to S3.1.2. Try another.

    3. Yes, that is right. Light shines up through a thin slide and into the objective lens. It goes up the tube to the eyepiece and your eye.

    See the answer

    From below the stage. It shines up through the slide.

    Light shines up through a thin slide and into the objective lens. It goes up the tube to the eyepiece and your eye.

    • If you chose A: The eyepiece is where your eye looks. It gives out no light. The light goes into it. Go to S3.1.2.
    • If you chose B: A lens only bends light. It makes none. The light starts below the stage. Go to S3.1.2.

  4. Why must a slide hold only a very thin slice?

    Tap your answer.

    1. Yes, that is right. The light shines up through the slide. A thick piece blocks it, and then you see only a dark patch.

    2. Not this one. The stage can hold something quite thick. The problem is light, not room. Go to S3.1.2. Try another.

    3. Not this one. Slicing does not change the cells. It lets light through so that you can see them. Go to S3.1.2. Try another.

    See the answer

    Light has to pass through it to reach the lens.

    The light shines up through the slide. A thick piece blocks it, and then you see only a dark patch.

    • If you chose B: The stage can hold something quite thick. The problem is light, not room. Go to S3.1.2.
    • If you chose C: Slicing does not change the cells. It lets light through so that you can see them. Go to S3.1.2.

  5. A friend says, "A quick look at the Sun through a drop lens is safe." What is true?

    Tap your answer.

    1. Not this one. Size does not make it safe. Any lens can gather sunlight into your eye. Go to S3.1.2. Try another.

    2. Not this one. Cloud thins the light but does not make it safe. Never look at the Sun through any lens. Go to S3.1.2. Try another.

    3. Yes, that is right. A lens gathers light. Sunlight is strong enough to harm your eyes. Never look at the Sun through a lens or a drop.

    See the answer

    It is never safe. A lens can harm your eyes, even in a moment.

    A lens gathers light. Sunlight is strong enough to harm your eyes. Never look at the Sun through a lens or a drop.

    • If you chose A: Size does not make it safe. Any lens can gather sunlight into your eye. Go to S3.1.2.
    • If you chose B: Cloud thins the light but does not make it safe. Never look at the Sun through any lens. Go to S3.1.2.

  6. From an earlier chapter

    A bean seed lies dry and still in a jar. Is it alive?

    Tap your answer.

    1. Not this one. A resting seed is not growing now, but it is alive. It grows again with water and warmth. Go to S3.1.1. Try another.

    2. Yes, that is right. A dry seed respires very slowly. It is alive, and it wakes up when it has water and warmth.

    3. Not this one. A thing that is not alive cannot become alive. The seed was alive all the time. Go to S3.1.1. Try another.

    See the answer

    Yes. It is alive and resting.

    A dry seed respires very slowly. It is alive, and it wakes up when it has water and warmth.

    • If you chose A: A resting seed is not growing now, but it is alive. It grows again with water and warmth. Go to S3.1.1.
    • If you chose C: A thing that is not alive cannot become alive. The seed was alive all the time. Go to S3.1.1.

  7. A plant cell is 0.05 mm wide. A microscope magnifies 400 times. How wide does the cell look, in millimetres?

    See the answer

    20 mm

    Multiply the width by the magnification. 0.05 × 400 = 20.

Keep it

Copy this into your exercise book.

A drop of water on clear plastic is a lens. It makes print look bigger. A microscope has two lenses, and their magnifications multiply. Never look at the Sun through a lens.

Draw: Draw a drop of water on plastic over print. Add an eye above and a line of light to it.

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 (754 KB). It opens on any phone.