lenses

• different lenses produce...


  different kinds of image
• how do lenses form images?

  by refracting light and changing its directions
• two main types of lens

  convex and concave
• axis of a lens

  the line passing through the middle of the lens
• principal focus of a convex lens

  where rays hitting the lens parallel to the axis meet
• principle focus of a concave lens

  the point where rays hitting the lens parallel to the axis appear to all come from
• focal length


  the distance from the centre of a lens to the focal point
• focal point


  the point at which rays parallel to the optical axis reflect and meet
• incident ray


  the ray that comes from an object and strikes a surface
• optical axis


  an imaginary line that divides a lens in half
• how is a convex lens drawn?


  with a straight line and arrowheads on each side pointing away from the line

  
• convex lens diagram

  
• how is a concave lens drawn?

  with a straight line and arrowheads on each side at the line

  
• concave lens diagram

  
• rules for refraction in a concave lens

  - an incident ray parallel to the axis refracts through the lens & travels in line with the principal focus (it appears to have come from the principal focus)

  - an incident ray passing through the lens towards the principal focus refracts through the lens & travels parallel to the axis

  - an incident ray passing through the centre of the lens carries on in the same direction
• rules for refraction in a convex lens

  - an incident ray parallel to the axis refracts through the lens & passes through the principal focus on the other side

  - an incident ray passing through the principal focus refracts through the lens & travels parallel to the axis

  - an incident ray passing through the centre of the lens carries on in the same direction
• lenses can produce...

  real and virtual images
• real image


  a copy of an object formed at the point where light rays actually meet
• real image examples


  a cinema screen, an image created on the retina of the eyeball
• virtual image


  an image formed where rays of light appear to meet or come from
• virtual image examples


  mirror reflection, magnifying glass
• what type of image can be projected onto a screen?

  real images
• what type of image can only be seen through the lens?

  virtual images
• to describe an image, what three things do you need to mention?

  - whether it's magnified or diminished

  - whether it's upright or inverted

  - whether it's real or virtual
• convex lens ray diagram

  
• convex lenses always produce images that are...

  - magnified, upright, virtual

  - diminished, inverted, real
• what type of image is produced if the object is placed between the focal point and a convex lens?


  magnified, upright, virtual
• what type of image is produced if the object is placed beyond the focal point of a convex lens?

  diminished, inverted, real
• concave lens ray diagram

  
• concave lenses always produce images that are...

  - diminished

  - upright and virtual

  - inverted and real
• why do concave lenses form only diminished objects?

  because the lens causes the light rays to converge, resulting in a diminished object
• what type of image is produced if the object is placed between the focal point and a concave lens?

  diminished, upright, virtual
• what type of image is produced if the object is placed beyond the focal point of a concave lens?

  diminished, inverted and real
• how can you tell whether the image is magnified or diminished by looking at a ray diagram?


  - if the image is larger than the original object, the image is magnified

  - if the image is smaller than the original object, the image is diminished
• how can you tell if an image is real or virtual by looking at a ray diagram?

  - if the object and image are on opposite sides of the lens, it is a real image

  - if the object and image are on same sides of the lens, it is a virtual image
• how can you tell if an image is upright or inverted by looking at a ray diagram?

  - if the object and image are on the same side of the optical axis, the image is upright

  - If the object and image are not on the same side of the optical axis, the image is inverted
• magnification =


  Image size / actual size
• magnification formula triangle