Uses of Lenses (Cambridge (CIE) O Level Physics): Revision Note

Magnifying Glasses

• If the object is placed closer to the lens than the focal length, the emerging rays diverge and a real image is no longer formed

• When viewed from the right-hand side of the lens, the emerging rays appear to come from a point on the left

  • This point can be found by extending the rays backwards (creating virtual rays)

• A virtual image will be seen at the point where these virtual rays cross

A Lens Close to an Object Forms a Magnified Image

A virtual image is formed by the divergence of rays from a point

• In this case the image is:

  • Virtual

  • Enlarged

  • Upright

• Using a lens in this way allows it to be used as a magnifying glass

• When using a magnifying glass, the lens should always be held close to the object

Correcting Sight

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

  • Converging lenses can be used to correct long-sighted vision

  • Diverging lenses can be used to correct short-sighted vision

Correcting Short-Sightedness

• People who are short-sighted have eyes that are 'too large'

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

• 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 in front of the eye

  • This causes the light to focus further back, at the retina

Correcting Long-Sightedness

• People who are long-sighted have eyes that are 'too small'

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

• This is because 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 in front of the eye

  • This causes the light to focus further forward, at the retina