Short sight, long sight and glasses
Look at a far tree and a near book with a normal eye, a short-sighted eye and a long-sighted eye. See where the image forms, then choose the glasses that bring it onto the retina.
Core concept
A normal eye focuses images on the retina; a short-sighted (myopic) eye focuses far objects in front of it and is corrected by a concave lens, and a long-sighted (hypermetropic) eye focuses near objects behind it and is corrected by a convex lens
Topic: Light
Light enters the eye through the cornea and the pupil, and the eye lens focuses it into an image on the retina, at the back of the eye. Cells in the retina send the image to the brain through the optic nerve. The ciliary muscles change the shape of the lens so we can see both near and far things clearly. In a short-sighted (myopic) eye, the eyeball is too long or the lens too strong, so the image of a far object forms in front of the retina and looks blurred; near things are clear. A concave (diverging) lens spreads the light a little and corrects it. In a long-sighted (hypermetropic) eye, the eyeball is too short, so the image of a near object forms behind the retina; far things are clear. A convex (converging) lens corrects it. In this lab the image's distance from the retina is shown in millimetres: minus means in front.
What you will learn
- Describe how the eye forms an image on the retina.
- Explain the causes of myopia and hypermetropia.
- Choose the lens that corrects each defect.
- Explain why the wrong glasses blur your sight.
Before you start
- Know how convex and concave lenses bend light.
Materials
- A convex lens and a concave lens (from old glasses or a lens kit), a sheet of paper as a screen
Equipment
- An eye chart (Snellen chart)
Procedure
- With the normal eye, look at the far tree and the near book, and record.
- With the short-sighted eye, look at the far tree, without glasses and with each lens.
- With the long-sighted eye, look at the near book, without glasses and with each lens.
How this simulation is simplified
- The model leaves out accommodation: the eye looks at one distance at a time without changing its lens.
- Each defect puts the image 2 mm off the retina, and each lens moves it 2 mm.