lenses

• lenses form images from refracting light and changing its direction. there are two types of lenses, convex and concave
• a convex lens bulges outwards, it causes rays of light parallel to the axis to be brought together in the principle focus
• a concave kens caves inwards, causes parallel rays of light to spread out
• the axis of a lens is a line passing through the middle of the lens
• the principle focus of a convex lens is where rays hitting the lens parallel to the axis all meet
• the principle focus on a concave lens is the point where rays hitting the lens parallel to the axis appear to all come from - you can trace them back until they all appear to meet up at a point behind the lens.
• there is a principle focus in each side of the lens. the distance from the centre of the lens to the principle focus is called the focus length
• rules for refraction in a convex lens

  1. an incident ray parallel to the axis refracts through the lens and passes through the principle focus on the other side
  2. an incident ray passing through the principle focus refracts through the lens and travel parallel to the axis
  3. an incident ray passing through the centre of the lens carries on in the same direction
• rules for refraction in concave lenses

  1. an incident ray parallel to the axis refracts through the lens and travels in line with the principle focus
  2. an incident ray passing through the lens towards the principle focus refracts through the lens and travel parallel to the axis|
  3. an incident ray passing through the centre of the lens carries on in the same direction
• lenses diagrams