topic 5 light & EM spectrum

• What does total internal reflection depend on?

  depends on the critical angle
• 2 types of reflection

  specular and diffuse
• Specular reflection

  when waves are reflected in a single direction by a smooth surface
• What surface does specular reflection use?

  smooth surface
• Result of specular reflection

  clear reflection e.g. in a mirror
• Incident rays in specular reflection

  parallel to each other
• Diffuse reflection

  occurs when waves are reflected by a rough surface and the waves are reflected in all directions
• Surface of diffuse reflection

  rough, bumpy surface
• Why does diffuse reflection happen?

  this happens because the normal is different for each incident ray, so each ray has a different angle of incidence
• Result of diffuse reflection

  when light is reflected by something rough, the surface looks matt, and you do not get a clear reflection
• Refraction in light

  slows down in denser medium

  • the ray bends towards the normal
• Refraction in sound

  speeds up in denser media

  • the ray bends away from the normal
• Law of reflection

  angle of incidence = angle of reflection
• Total internal reflection

  when a wave hits a surface it is reflected back into the material
• When can TIR occur?

  when the wave travels through a dense material like glass or water towards a less dense substance like air

  • at a large enough angle from the normal
• When does TIR happen (angles)?

  when the angle of incidence is larger than the critical angle for that particular material
• What does every material have?

  its own, different critical angle
• Critical angle

  angle at which angle of incidence is equal to critical angle
• What happens if the angle of incidence is less than the critical angle?

  most of the light is refracted into the outer layer, but some of it is internally reflected
• What happens if the angle of incidence is equal to the critical angle?

  the ray would refract along the boundary with internal reflection too
• What happens if the angle of incidence is larger than the critical angle?

  no light comes out

  • it is all internally reflected TIR
• What is a lens?

  a piece of transparent material shaped to refract light in particular way
• What does a power of lens describe?

  how much it bends light that passes through it
• A more powerful lens is what?

  more curved and bends the light more
• 2 types of lens

  converging and diverging
• Converging lens

  • bulges outwards in the middle
  • causes parallel rays of light to be brought together (converge) at focal point
  • always convex
• Diverging lens

  • thinner in middle
  • caves inwards
  • causes parallel rays of light to spread out (diverge)
• Uses of converging lens

  magnifying glasses, cameras
• Uses of diverging lens

  correcting short sightedness
• Focal length

  the distance from the centre of the lens to the focal point
• Principal focus of diverging lens

  the point where rays hitting the lens parallel to the axis appear to all come from
• Principal focus of converging lens

  where rays hitting the lens parallel to the axis all meet
• What happens to a ray as it enters a lens?

  it bends towards the normal
• What happens to a ray as it leaves any lens?

  it bends away from the normal
• How to describe a ray image?

  • bigger or smaller
  • upright or inverted
  • real or virtual
• Where are images formed?

  at points where all light rays from a certain point on an object appear to come together
• 2 types of images formed by lenses

  real and virtual
• How is a real image formed?

  when the light rays come together to form the image
• Why can a real image be captured on a screen?

  because the light rays actually meet at the place where the image seems to be
• Real image

  image formed where the refracted light rays are focused

  • inverted
  • projected on a screen