How your eyes see
How does the whole world fit inside your eye?
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Explain
Light goes into your eyes.
Your brain makes the picture.
Light from things enters your eye through the dark hole in the middle, called the pupil.
The lens focuses the light onto the retina, a layer of cells at the back of your eye.
The retina sends signals along a nerve to your brain, which makes the picture you see.
The lens equation links distances: 1/f = 1/u + 1/v. Your eye changes f by changing the shape of its lens.
If the sharp image is in front of the retina, you are short-sighted. If it's behind, you are long-sighted. Glasses correct this.
Worked example: f = 2.35 cm and u = 100 cm give v ≈ 2.41 cm, right on the retina.
- Light enters your eye
- The lens focuses it
- Your brain makes a picture
Want the full story?
Most of the bending in your eye is done by the curved cornea at the front, and the lens only fine-tunes it. The image on your retina is upside down, and your brain flips it. The sim uses a simple eye that is 2.4 cm deep.
The Formula
one over focal length = one over object distance + one over image distance
- the focal length, measured in centimetres (cm)
- the distance to the object, measured in centimetres (cm)
- the distance from the lens to the sharp image, measured in centimetres (cm)
Try the numbers
Sharp image forms at: 2.41 cm
Try it at home
You need your finger, a window.
You need: your finger, a window.
Look at your finger. Now look far away.
- Hold up a finger.
- Look at your finger, and notice the far things go blurry.
- Now look past it, and see your finger blur.
What's happening: your lens changes shape to focus, and can only focus at one distance at a time.
Physics is life
Your body
Your pupils get big in the dark.
Daily life
Glasses help people see.
Nature
Cats see well at night.
Space
Telescopes are giant eyes.
Quick check
Question 1 of 3
Words to know
Retina: The layer at the back of your eye that senses light.