refraction and reflection

• Refraction


  The change in direction of a wave at the boundary between two materials with different densities
• Refraction of waves


  1. Different materials have different densities

  2. Light waves may change speed and direction upon entering a denser or less dense medium
• Refraction can cause optical illusions as the light waves appear to come from a different position to their actual source
• Density


  A measure of compactness and the ratio of mass to volume
• Density of a material


  Affects the speed that a wave will be transmitted through it
• In general, the denser the transparent material, the more slowly light travels through it
• Light ray passing from air into glass

  Slows down and bends towards the normal
• Light ray passing from glass into air


  Speeds up and bends away from the normal
• Normal


  An imaginary line at right angles to the boundary between two materials
• Frequency


  The number of waves produced each second
• Wavelength


  The length of a single wave, measured from one wave peak to the next
• Wave speed


  Proportional to frequency and wavelength
• If a wave slows down, its wavelength will decrease</b>
• As a wave travels into a denser medium, it slows down and the wavelength decreases, but the frequency remains the same
• As the right hand side of an incoming wave slows down before the left hand side, the wave changes direction
• Waves can be reflected and refracted in the right circumstances
• Reflection and refraction of light explains how people see images, colour and even optical illusions
• Waves


  Vibrations that transfer energy from place to place without the transference of matter, including sound and light
• Reflection of waves


  • Waves can be reflected at the boundary between two different materials
  • The reflection of sound causes echoes
• Law of reflection


  Angle of incidence = angle of reflection
• Angle of incidence


  Angle between the normal and the incident ray
• Angle of reflection


  Angle between the reflected ray and the normal
• Normal


  Imaginary line at 90 degrees to the reflecting surface
• Plane


  Flat, two-dimensional surface
• All waves will reflect and refract in the right circumstances
• Specular reflection


  Reflection from a smooth, flat surface where light is reflected in a single direction
• The reflection and refraction of light explains how people see images, colour and even optical illusions
• Virtual image


  Image from which rays of light appear to come but do not do so in reality
• Diffuse reflection


  Reflection from a rough surface where light is scattered in different directions
• Investigate the reflection of light by different types of surface and the refraction of light by different substances

  1. Set up a ray box, slit and lens so that a narrow ray of light is produced

  2. Place a 30 centimetre (cm) ruler near the middle of a piece of plain A3 paper. Draw a straight line parallel to its longer sides. Use a protractor to draw a second line at right angles to this line. Label this line with an 'N' for 'normal'

  3. Place the longest side of a rectangular acrylic polymer block against the first line. With the normal near the middle of the block, carefully draw around the block without moving it

  4. Use the ray box to shine a ray of light at the point where the normal meets the block. This is the incident ray

  5. Move the ray box or paper to change the angle of incidence. The aim is to see a clear ray reflected from the surface of the block and another clear ray leaving the opposite face of the block

  6. Using a pencil on the paper, mark the path of: the incident ray with a cross, the reflected ray with a cross, the ray that leaves the block with two crosses - one near the block and the other further away

  7. Remove the block. Join the crosses to show the paths of the light rays

  8. Repeat steps 2 to 7 for a rectangular glass block
• Incident ray


  Light ray moving towards a surface or boundary
• Angle of incidence


  Angle between the normal and the incident ray
• Reflected ray


  Light ray leaving a surface or boundary
• The light rays should obey the law of reflection
• Hazards


  • Ray box gets hot
  • Semi-dark environment
• Acrylic polymer


  A type of transparent plastic
• To what extent do the results show the light rays obeying the law of reflection?