lenses and light and magnification

• Different lenses produce different kinds of images
• Lenses


  Refract light (bend it) and change its direction
• Lenses


  • Have different shapes
  • Have opposite effects on light
• Converging lens


  Causes parallel rays of light to be bent together (converge) to a focal point
• Diverging lens


  Causes parallel rays of light to spread out (diverge)
• Principal focus of a convex lens


  The point where rays hitting the lens parallel to the axis converge
• Principal focus of a concave lens


  The point where rays hitting the lens parallel to the axis appear to come from
• Axis of a lens

  A line passing through the middle of the lens
• Focal length


  The distance from the centre of the lens to the principal focus
• You need to be able to draw proper ray diagrams to show how convex and concave lenses differ
• Rules for Refraction in a Convex Lens

  1. Incident ray parallel to axis refracts through lens and passes through principal focus

  2. Incident ray passing through principal focus refracts through lens and travels parallel to axis

  3. Incident ray passing through centre of lens carries on in same direction
• Rules for Refraction in a Concave Lens

  1. Incident ray parallel to axis refracts through lens and appears to come from principal focus

  2. Incident ray passing towards principal focus refracts through lens and travels parallel to axis

  3. Incident ray passing through centre of lens carries on in same direction
• The rules allow you to draw ray diagrams without bending the rays as they go into and out of the lens
• Real image


  Where light from an object comes together to form an image, like on a screen
• Virtual image


  Where the light appears to be coming from a different place than the object
• Drawing a Ray Diagram for an Image Through a Convex Lens

  1. Draw ray parallel to axis refracting through principal focus

  2. Draw ray through middle of lens not bending

  3. Mark where rays meet as top of image

  4. Repeat for bottom of object
• Drawing a Ray Diagram for an Image Through a Concave Lens

  1. Draw ray parallel to axis refracting to appear to come from principal focus

  2. Draw ray through middle of lens not bending

  3. Mark where refracted rays meet as top of image

  4. Repeat for bottom of object
• Concave lenses always create virtual images
• Magnifying glass


  Uses a convex lens to create a magnified virtual image
• Magnification


  The ratio of image height to object height
• Visible light is made up of a range of colours with different wavelengths
• Primary colours


  Red, green, blue
• Mixing primary colours creates white light
• Opaque object


  Absorbs some wavelengths of light and reflects others, giving it a colour
• White object


  Reflects all wavelengths of visible light equally
• Black object


  Absorbs all wavelengths of visible light
• Transparent/translucent object


  Transmits light, allowing some wavelengths to pass through
• How colour filters work

  1. Only transmit particular wavelengths of light, absorbing the rest

  2. Primary colour filter only transmits that colour

  3. Non-primary colour filter transmits both that colour and the primary colours that mix to make it