Applications of Refraction: Eyesight Correction

Glasses and contact lenses work because of refraction. Refraction is the bending of light when it passes from one material to another. Your eye uses refraction in the cornea and the eye’s lens to focus light onto the retina, forming a clear image.

How a normal eye focuses

For distant objects, incoming rays are almost parallel. The cornea and lens bend these rays so they meet exactly on the retina. For near objects, the eye lens changes shape (accommodation) to bend light more strongly so it still focuses on the retina.

Short-sightedness (myopia)

• Problem: Distant objects look blurry. The eyeball may be too long or the eye lens too strong, so the image forms in front of the retina.
• Correction: A diverging (concave) lens is used. It spreads out the incoming rays slightly so the eye’s own lens focuses them further back, onto the retina.
• Ray idea: Parallel rays → diverging lens makes them spread → eye lens then brings them to focus on the retina.

Long-sightedness (hyperopia)

• Problem: Near objects look blurry. The eyeball may be too short or the eye lens too weak, so the image would form behind the retina.
• Correction: A converging (convex) lens is used. It starts the rays bending together so the eye can focus them sooner, onto the retina.
• Ray idea: Rays from a near object → converging lens begins to bring them together → eye lens completes the focus on the retina.

Lens power and focal length

Lens strength is measured in dioptres (D). Positive values are converging; negative values are diverging. The power P and focal length f (in metres) are linked by:

$$P = \frac{1}{f}$$

Common misconceptions

• Glasses do not make the final image on a screen. They create rays that the eye then focuses on the retina.
• A diverging lens does not make rays meet; it spreads them so the eye focuses further back.
• Short-sighted means “see well at short distance” (trouble with far). Long-sighted means “see well at long distance” (trouble with near).