6.3 - Lenses

• Lenses form images by refracting light and changing its direction. There are two main types: convex and concave.
• What is a convex lens?

  A lens that bulges outwards, causing the ray of light parallel to the axis of the lens, to converge at the principal focus.
• Convex lenses:

  A) parallel

  B) axis

  C) principal focus

  D) focal

  
• Important rules for convex lenses:

  1. An incident ray travelling parallel to the axis refracts through the lens and passes through the principal focus on the other side.
  2. An incident ray passing through the centre of the lens carries on in the same direction.
  3. An incident ray passing through the principal focus before meeting the lens refracts through the lens and travels parallel to the axis.

  
• Concave lenses:

  A) principal

  B) axis

  C) focal

  D) virtual

  E) parallel

  
• Important rules for concave lenses:

  1. An incident ray travelling parallel to the axis refracts through the lens and travels in the line with the near-side principal focus.
  2. An incident ray passing through the centre of the lens carries on in the same direction.
  3. An incident ray passing through the lens towards the far-side principal focus refracts through the lens and travels parallel to the axis.

  
• Different lenses produce different types of images: real images and virtual images.
• How are real images formed?

  The light rays from a point on an object come together to form and image. Light rays pass through the same point. Can be captured on a 'screen' - like the image formed on the eye's retina.

  
• How are virtual images formed?

  The light rays from a point on an object are diverging after they leave the lens. SO the light from the point on an object appears to be coming from a completely different place. The light rays don't actually pass through that point.