Generally these defects are treated with spectacle lenses which refract…

The idea

When the eye has a focusing defect, the image does not form sharply on the retina. Spectacle lenses correct this by refracting light before it enters the eye, changing the direction of the rays so that they converge at the retina. In short-sightedness, or myopia, the eye focuses light in front of the retina, often because the eyeball is too long or the lens is too strong. A concave, diverging lens spreads the rays slightly before they enter the eye, so the eye's lens can bring them to a focus on the retina. In long-sightedness, or hyperopia, the eye focuses light behind the retina, often because the eyeball is too short or the lens is too weak. A convex, converging lens bends the rays inwards before they enter the eye, helping them focus on the retina. The lens power is chosen to match the defect.

What earns the marks

• Focusing defects occur when light from an object does not converge precisely on the retina, producing a blurred image.
• Spectacle lenses refract light rays before they enter the eye, altering the direction in which the rays travel.
• A concave, diverging lens is used to correct short-sightedness by spreading the rays so they focus further back, on the retina.
• A convex, converging lens is used to correct long-sightedness by bending the rays inwards so they focus sooner, on the retina.
• The power of the lens is selected so that the corrected rays focus exactly on the retina, giving a clear image.
• Correcting the defect restores sharp vision because the image now forms on the light-sensitive retina rather than in front of or behind it.

Common mistakes and how to put them right

• Mixing up which lens corrects which defect.

  Put it right: Concave, diverging lenses correct short-sightedness and convex, converging lenses correct long-sightedness.

• Saying the spectacle lens makes the image larger or smaller.

  Put it right: The lens changes the direction of light rays so they focus on the retina.

• Believing the lens changes the shape of the eyeball.

  Put it right: The lens only refracts light entering the eye and does not alter the eye's structure.

Exam tips

• State the type of lens and the direction in which it changes the light rays, then link this to the image forming on the retina.
• Use ray diagrams to show the incoming rays, the effect of the spectacle lens and the final focus point on the retina.
• Compare the two defects clearly: name the defect, say where the image would form without correction, then explain how the lens moves the focus to the retina.