Physics

• The human eye


  A specialized sensory organ that sends visual images to the brain after receiving them
• How the eye works

  1. Light passes through cornea

  2. Light enters pupil

  3. Iris controls light

  4. Light passes through lens

  5. Light hits retina

  6. Photoreceptors turn light into electrical signals

  7. Electrical signals travel to brain

  8. Brain turns signals into images
• Accommodation


  The ability of the eye to adjust its focus to clearly see objects at different distances
• Accommodation of the eye


  • Lens shape change
  • Distant object focus
  • Range of accommodation
  • Presbyopia
  • Accommodation reflex
• Myopia (Short-sightedness)


  The far point is closer than infinity
• Causes of Myopia


  • Increased curvature of cornea or lens
  • Increased length of eye
  • Increased refractive index of lens
  • Displacement of lens
• How to correct Myopia


  Use of diverging (concave) lens
• Refraction of light


  When light passes from one medium to another, the light can slow down and deviate from its original direction by a small angle
• Index of refraction


  A measure of how much the speed of light is reduced when passing through a material, given by the formula n = speed of light in vacuum / speed of light in material
• As light passes from air to a denser material


  The light slows down and bends towards the normal
• As light passes from a denser material to air


  The light speeds up and bends away from the normal
• Snell's law of refraction


  1. n1 sin(θ1) = n2 sin(θ2)

  2. Where n1 and n2 are the refractive indices of the two media, and θ1 and θ2 are the angles of incidence and refraction
• When light passes from water to air


  The angle of refraction increases, causing the object to appear shallower than it really is
• Apparent depth


  The depth at which an object appears to be, which is less than the actual depth, due to refraction
• Treading water


  Swimming with head above the surface of a 3m deep pool
• The swimmer sees a coin in the pool
• Determining how the coin appears to the swimmer

  1. Use formula

  2. Actual depth of pool is 3m

  3. Swimmer's head is above water

  4. Refractive index of water is 1.32

  5. Ray from coin bends as it enters air
• Ray from coin


  Appears shallower than actual depth
• Coin appears to be 2.26m deep, when actual depth is 3m
• Total internal reflection


  Occurs when angle of incidence is greater than critical angle
• Total internal reflection is very useful, e.g. in fibre optics and medical instruments
• Determining critical angle for diamond in air vs diamond in water

  1. Use formula N2/N1

  2. Critical angle for diamond in air is 24.4°

  3. Critical angle for diamond in water is 33°
• Angle of incidence 28°


  • Total internal reflection occurs in diamond in air
  • Total internal reflection does not occur in diamond in water
• Atom


  Outer electrons tend to be not strongly bound to the nucleus
• Dispersion of light


  White light is made up of different frequencies that bend at different angles when passing through a medium
• Electron movement


  1. Application of electrical force causes an electron to move from one atom to the next

  2. Void created in one atom can be filled by an electron from another atom

  3. This creates an electric current or flow of electrons
• Dispersion of light causes the formation of rainbows
• Conductors


  • Materials that allow the flow of electrons from atom to atom
  • Metallic bonding structure facilitates electron flow
• Converging lens


  Refracts light rays to a focal point
• Good conductors


  • Copper
  • Gold
  • Silver
  • Aluminium
• Diverging lens


  Refracts light rays to spread out
• Insulators


  Bad conductors of electricity due to strong electron bonding to nucleus
• Lens equation is used to determine focal length and other lens properties
• Insulators


  • Plastic
  • Rubber
  • Glass
  • Ceramics
• Semiconductors


  Materials like silicon that became important for devices like transistors and diodes
• Diverging lens


  Two prisms put together in a way that refracts light to spread it out instead of converging it to a focal point
• Electric charge


  Unbalance of positive and negative charges in a material can create a flow of electrons (electric current)
• Drawing ray diagrams for diverging lenses


  1. Draw ray from object point to center of lens

  2. Draw ray from object point through focal point of lens and parallel to principal axis

  3. Draw ray from object point straight through center of lens
• Electrons and protons behave like magnets, with opposite charges attracting and like charges repelling
• Drawing ray diagrams for converging lenses


  1. Draw ray parallel to principal axis through focal point

  2. Draw ray through focal point to center of lens

  3. Draw ray through center of lens