Seneca
Sign up

Lenses

Physics: AQA A Level, topic 10.1.3

Converging lenses

A converging or convex lens takes rays of light and focuses them at a point. The rays converge.

Converging light

Converging light

  • If rays enter a converging lens parallel to its axis they will cross each other at a single point on the other side of the lens.
  • The is called the focal point.
  • If the rays do not come in parallel to the axis the image is formed in a different place.
Image location

Image location

  • The equation relating the image location to the object location and the focal point is:
  • 1/the focal length = 1/the image distance + 1/the object distance
  • 1f=1u+1v\frac{1}{f}=\frac{1}{u}+\frac{1}{v}
    • Where u is the image distance, v is the object distance and f is the focal length.

Diverging Lenses

A diverging or concave lens causes light to bend away from the axis, the light diverges.

Diverging light

Diverging light

  • If parallel rays enter the lens they are diverted away from the axis.
  • This creates the illusion that the object is closer to the lens than it is.
  • If the new rays are traced back they form an image at the focal point.
Calculation

Calculation

  • The focal length can be calculated from the image and the object distance with the equation:
  • 1/the focal length = 1/the image distance + 1/the object distance
  • 1f=1u+1v\frac{1}{f}=\frac{1}{u}+\frac{1}{v}
    • Where u is the image distance, v is the object distance and f is the focal length.
Image location

Image location

  • The diverging lens can appear confusing as the image and the object are on the same side of the lens.
  • The image formed is virtual as the light rays do not actually meet, they just appear to.

Lens Power

The power of a lens is related to how 'quickly' it can focus light.

Bending light

Bending light

  • A lens will change the direction of light so that all the rays cross at a point and create an image.
  • The incoming rays are parallel to the axis they will focus at the focal point.
  • The closer the focal point is to the lens the more powerful the lens.
Calculation

Calculation

  • The equation for the power of a lens is given by:
  • Power = 1the  focal  length\frac{1}{the \;focal\; length}
  • P=1f=1u+1vP=\frac{1}{f}=\frac{1}{u}+\frac{1}{v}
    • P is the power.
    • u is the image distance.
    • v is the object distance.
    • f is the focal length.

Test yourself on Lenses

  • Which of these statements / equations are correct about converging lenses?

  • What is the equation linking the focal length and the image and object distances?

  • Which of these statements are true about diverging lenses?

  • What is the equation for the focal length of a lens?

  • Light rays will meet when passing a diverging lens. True or false?

Practise this topic for free