How does my eye make a picture of the world, and why do some people need glasses?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Explain how the lens in the eye makes an image
Explain how glasses help people see clearly
Use a jar of water as a lens to make text look bigger
Kofi cannot see the board at the front of the class. His sister cannot read a page held close to her face. Both of them need glasses, but not the same glasses. Why?
A lens is a clear piece of glass or plastic with a curved surface. It bends light by refraction. A lens that is thicker in the middle is a converging lens, because it bends rays together.
Send parallel rays at a converging lens. Each ray bends towards the middle. They all cross at one spot, the focus. A fatter lens bends rays more, so its focus is nearer.
A converging lens makes parallel rays meet at the focus.
Your eye has a lens too. The clear front of the eye, the cornea, does most of the bending. The lens inside fine tunes it. Together they make a picture on the back of the eye.
The back of the eye is the retina. It is a screen of tiny cells that react to light. They send signals along a nerve to the brain.
Draw it first
Draw how a lens makes an image
1 Draw a lens that is thicker in the middle. Draw rays that meet at the focus.
2 In the eye, rays from the top and the bottom of an object cross in the lens. They meet on the retina as a small upside down image.
See it three ways
As a picture
In words
A diverging lens is thinner in the middle and spreads rays out. A converging lens is thicker in the middle and brings rays together.
In symbols
short sight: diverging lens. long sight: converging lens.
In a clear eye, rays from an object meet exactly on the retina. The picture is sharp. In some eyes, the rays meet in the wrong place. The picture on the retina is blurred.
In short sight, far things look blurred. The eye bends the rays too much, so they meet in front of the retina. The person sees near things clearly.
Short sight: the rays meet in front of the retina.
A diverging lens is thinner in the middle. It spreads rays out a little before they enter the eye. This moves the meeting point back onto the retina. Glasses with diverging lenses correct short sight.
A diverging lens moves the meeting point back onto the retina.
In long sight, near things look blurred. The eye does not bend the rays enough. They would meet behind the retina. The person sees far things clearly.
Long sight: the rays would meet behind the retina.
A converging lens in front of the eye helps the eye bend the rays a little more. The meeting point moves forward onto the retina. Glasses with converging lenses correct long sight.
A converging lens moves the meeting point forward onto the retina.
Try it
Use a jar of water as a lens
You need: a clear round jar or a clear plastic bottle, water, a page with writing on it.
Be safe: Keep the jar out of direct sunlight, and never point it at the Sun. Wipe up any spilled water.
Fill the jar to the top with water, so there is no bubble. Close the lid tightly.
Lay the page flat on a table, in a shaded place.
Lay the jar on its side, crossing the line of writing at right angles. Look down through the round side.
Lift the jar a little way off the page, then lower it. Then take the jar away and compare.
What you will see: The writing looks wider, but no taller. The round side of the jar bends light from side to side only. It is a converging lens in one direction.
If you cannot try it, this is what happens.
A wrong idea many people have
The picture on the back of the eye is the right way up.
Why it feels right: We see the world the right way up, so the picture in the eye must be too.
The test that breaks it: On paper draw an arrow, and a lens in the middle of a page. Draw a straight line from the tip of the arrow through the middle of the lens. Draw another from its tail. The lines cross, so the tip ends up lower than the tail.
What is true: The image on the retina is small and upside down. The brain makes sense of it, so the world looks the right way up.
Key words
lens
a clear curved piece of glass or plastic, or the clear part in the eye, that bends light.
converging lens
a lens that is thicker in the middle. It brings rays together.
diverging lens
a lens that is thinner in the middle. It spreads rays out.
retina
the screen at the back of the eye where the image forms.
short sight
seeing near things clearly but far things blurred.
long sight
seeing far things clearly but near things blurred.
Back to your guess
The answer is A: The eye bends light from the object into a small upside down picture at the back.
Light from the object goes into the eye, and the eye bends it. A small upside down picture forms at the back. The brain makes sense of it. The eye sends out nothing.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Mei looks at a tree. Which part of her eye is the screen where the picture forms?
See the answer
The retina.
The cornea and lens bend the light so it meets on the retina. The retina is the screen. Its cells send signals along a nerve to the brain.
If you chose A: The lens bends the light to make the picture. The picture forms farther back, on the retina. Go to S3.9.3.
If you chose C: The brain makes sense of the signals. The picture itself forms on the retina at the back of the eye. Go to S3.9.3.
What kind of picture does the lens of the eye make on the retina?
See the answer
A small picture, upside down.
Rays from the top of the object cross in the lens and reach the bottom of the retina. The brain makes sense of the picture, so we see the world the right way up.
If you chose A: Rays from the top of an object cross in the lens. So the picture comes out upside down and small. Go to S3.9.3.
If you chose B: The rays cross in the lens. So the top of the object ends up at the bottom. Go to S3.9.3.
Kofi sees near things clearly but the board is blurred. In his eye, the rays from the board meet in front of the retina. Which lens in glasses helps?
See the answer
A diverging lens, thinner in the middle.
The eye bends the light too much, so the meeting point is in front of the retina. A diverging lens spreads the rays a little, and the meeting point moves back onto the retina.
If you chose B: That bends rays together even more, so they would meet even farther in front of the retina. Go to S3.9.3.
If you chose C: Flat glass bends each ray going in, and back again coming out. The rays leave going the same way. The rays must be spread out a little first. Go to S3.9.3.
Amina cannot read a page held close. In her eye, the rays would meet behind the retina. Which lens in glasses helps?
See the answer
A converging lens, thicker in the middle.
The eye does not bend the light enough, so the meeting point is behind the retina. A converging lens helps bend the rays, and the meeting point moves forward onto the retina.
If you chose A: That spreads the rays, so they would meet even farther behind the retina. Go to S3.9.3.
If you chose C: Flat glass bends each ray going in, and back again coming out. The rays leave going the same way. The rays need to bend a little more. Go to S3.9.3.
Tariq fills a clear round jar with water and looks at writing through it. What does he see, and why?
See the answer
The writing looks wider. The round jar of water bends light like a converging lens.
The jar is thicker in the middle from side to side. It bends light together across the jar. The writing looks wider, but no taller.
If you chose A: Slowing light is why it bends, but the bending brings the rays together. A converging lens makes the writing look larger. Go to S3.9.3.
If you chose B: Clear things can still bend light. A round jar of water bends light like a lens. Go to S3.9.2.
From an earlier chapter
Review: a ray goes from air into glass at a slant. What happens to the ray?
See the answer
It slows down and bends towards the normal.
Light is slower in glass than in air. A ray that slows down at a slant turns towards the normal.
If you chose B: That is what happens coming out of glass. Going in, light slows down and bends towards the normal. Go to S3.9.2.
If you chose C: Only a ray along the normal goes straight on. A ray at a slant bends when it changes medium. Go to S3.9.2.
Keep it
Copy this into your exercise book.
A converging lens is thicker in the middle and brings rays together. The eye makes a small upside down image on the retina. Short sight needs a diverging lens. Long sight needs a converging lens.
Draw: An eye with two rays from an arrow. Show them crossing in the lens and meeting on the retina, upside down.
