Correcting Sight (Cambridge (CIE) IGCSE Physics): Revision Note

Correcting sight

Extended Tier Only

• Converging and diverging lenses are commonly used in glasses and contact lenses to correct defects of sight

  • Converging lenses (opens in a new tab) can be used to correct long-sighted vision

  • Diverging lenses (opens in a new tab) can be used to correct short-sighted vision

Use of lenses to correct long-sightedness

• Long-sighted people have eyes that are less curved than normal or the eyeball is too short

  • This means they cannot see things that are close and can only clearly see things that are far away

Ray diagram to show long-sightedness

• The eye refracts the light rays and they are brought to a focus beyond the retina

  • In other words, the focus point is behind the retina at the back of the eye

• This can be corrected by using a convex or converging lens

The effect of a converging lens on a long-sighted eye

Use of lenses to correct short-sightedness

• People who are short-sighted have eyes that are more curved than normal or have an eyeball that is too long

  • This means they cannot see things that are far away, and only see things that are close to them

Ray diagram of short-sightedness

• This is because the eye refracts the light and brings it to a focus before it reaches the retina

  • In other words, the focus point is in front of the retina at the back of the eye

• This can be corrected by using a concave or a diverging lens

The effect of a diverging lens on a short-sighted eye