6.3.1 Convex & Concave Lenses

Lenses (Physics only)

• If light passes through the centre of the lens, it does not change direction.

• Lenses are generally drawn as a dashed vertical line.

  

• Focal points are points on either side of the lens where light can converge.

• Convex lenses can have virtual or real images (appear to be on the same or opposite side as the real object respectively).

• Concave lenses can only have virtual images.

Concave Lenses (Physics only)

• A concave lens "caves" inward.

• They are thinner at the centre than at the edges.

• Spreads light outwards.

  • Light appears to have come from the focal point:
  • Draw a horizontal ray from the top of the object to the lens.
  • Draw a faint line from the focal point to the point where the ray hits the lens.
  • The ray exits the lens along the same direction as the faint line.
  • It is used to spread out light further:
  • Example: To correct short-sightedness.

• As light is focused in front of the retina, it needs to be spread out slightly to be able to be focused onto the retina.

Convex Lenses (Physics only)

• A convex lens is normally wider at the centre.
• They focus light inwards.
• Horizontal rays focus on the focal point. Used for magnifying glasses, and binoculars.
• Used to correct long-sightedness, as it focuses the rays closer.