Physics

• Lens


  Focuses and disperses light using refraction
• Lens types


  • Convex
  • Concave
• Convex lens


  Has an outwards bulge in the center which causes it to refract parallel rays of light inwards to a single point (principal focus)
• Converging lens


  Another name for a convex lens, causes light rays to converge
• Concave lens


  Caves in on either side which causes it to refract parallel arrays outwards, effectively dispersing the light
• Principal focus


  The point where parallel rays of light converge after passing through a lens
• Lenses are symmetrical and can work both ways, reversing the direction of light
• Focal length


  The distance between the principal focus and the center of the lens
• Shorter focal length


  More powerful lens (refracts light more strongly)
• To make a lens more powerful

  Increase the curvature or use a different material that refracts light more strongly
• Real image


  Formed when light rays actually converge to form the image
• Virtual image


  Formed when light rays only appear to converge but don't actually meet
• Real images can be captured on a screen, virtual images cannot
• The image formed on the retina is real and inverted, but our brain corrects it to appear upright
• Describing images


  • Real or virtual
  • Upright or inverted
  • Bigger or smaller than the object
• Concave/diverging lens


  Causes the light rays to diverge and spread out
• Concave lens symbol


  • Outwards pointing V shapes at either end to show the lens getting wider towards the ends
• Axis


  • Horizontal line through the middle of the lens
• Focal point/Principal focus


  • One on either side of the lens
• 2f points


  • Points twice as far away from the lens as the focal point
• Drawing ray diagram for concave lens

  1. Pick a point near the top of the object

  2. Draw incident rays - one parallel to axis, one towards center

  3. Draw virtual ray to focal point

  4. Find where rays cross to get top of image

  5. Bottom of image on axis
• Image from concave lens


  Virtual, upright, smaller than object
• Convex/converging lens


  Focuses light onto a single point
• Convex lens symbol


  • V shapes at ends point inwards as lens gets narrower towards ends
• Image from convex lens

  Real, inverted, smaller than object
• If object more than 2f from lens, image is real, inverted, smaller
• Temperature


  The average kinetic energy of particles in the object
• Physical properties used to measure temperature

  • Volume of liquid
• Liquid in glass thermometers

  1. Expansion of liquid (increase in volume)

  2. Contraction of liquid (decrease in volume)

  3. Measure temperature
• Sensitivity


  The ability of a thermometer to determine small temperature changes
• Factors affecting sensitivity


  • Expansion of liquid used
  • Diameter of inner glass tube
  • Thickness of bulb
• Mercury expands more than alcohol
• The narrower the tube, the higher the sensitivity
• The thinner the bulb, the more sensitive it becomes
• Work done = force x distance moved in direction of force
• Energy can be transferred from one object to another by work done or heat transfer.
• The SI unit of energy is the joule (J).
• Energy is measured in Joules (J)
• Distance is measured in metres (m)
• Force is measured in Newtons (N)