Light and Sound

• Light, and all electromagnetic waves, is a transverse wave, and can therefore be reflected and refracted.
• Refraction is wave changing direction when it moves from one material into another.

  This happens because the wave changes speed when moving from one material to another.
• n = sin(i)/sin(r)
• n = refractive index, aka optical density


  It is a measure of how much a material slows down light waves, and therefore, how much they refract.
• The best way to determine refractive index is:

  1. Shine a ray of light through the material, at an angle
  2. Measure the angle of incidence, from the normal, with a protractor
  3. Measure the angle of refraction, from the normal, with a protractor
  4. Plot a graph of sin(i) against sin(r)
  5. Gradient = refractive index
• Going into a material with:

  • higher optical density
  • lower speed of light
  • larger index of refraction

  The refracted ray bends towards the normal
• Going into a material with:

  • lower optical density
  • higher speed of light
  • smaller index of refraction

  The refracted ray bens away from the normal
• 1. When a ray goes from dense to less dense, it refracts away from the normal.
  2. When the ray refracts at 90 to the normal, the angle of incidence = the critical angle (c)
  3. When angle of incidence > critical angle, the ray undergoes total internal reflection
• When light goes from glass to air, the light bends away from the normal (The angle of refraction is larger than the angle of incidence).

  If you make the angle of incidence bigger, the angle of refraction also gets bigger.

  At a certain angle, the refracted ray lies along the interface between the glass and the air. This angle is called the critical angle.

  If the angle of incidence is increased further, the light is reflected inside the glass. This is called total internal reflection.
• n = refractive index

  c = critical angle

  sin(c) = 1/n
• The critical angle is:

  • The angle of incidence
  • Above which gives total internal reflection
• Explain why refraction happens at the boundary between the deep water and the shallower water.

  The wave slows down in shallow water

  The wave meets the boundary at an angle

  Part of the wave is slower than the rest, leading to a change in direction
• Optical fibres are made of plastic or glass, with an outer cladding that has a lower refractive index than the inner core.

  Light is transmitted down the fibres by total internal reflection.

  Optical fibres can bend to carry light to hard to reach places.
• In a right-angled prism, light is totally internally reflected. The image appears as a reflection at 90 to and below the object.

  These are better than mirrors because in total internal reflection no light is lost.
• Amplitude

  The maximum energy of the wave
• Frequencies between 20 and 20000 Hz are known as sound.

  Frequencies less than 20Hz are known as infrasound.
• Infra roughly means "below"

  Infrasound has a frequency "below sound"
• Frequencies between 20 and 20000 Hz are known as sound.

  Frequencies greater than 20000 Hz are known as ultrasound.
• Ultra roughly means "greater than"

  Ultrasound has a frequency "greater than sound"
• Distance = time x wave velocity
• To calculate distance between an emitter and a detector.

  Distance = time x wave velocity
• To calculate distance from an echo.

  2 x distance = time x wave velocity

  OR

  Distance = (time x wave velocity) / 2

  Because the wave has travelled the distance twice in the measured time.
• Microphones can detect sound waves travelling through the air and convert them to electrical signals.

  Sound waves can be displayed using an oscilloscope.

  Oscilloscopes display sound waves as a trace on a screen.

  X-axis = time, y-axis = amplitude.
• 1. Connect oscilloscope to microphone
  2. Adjust the oscilloscope to get a steady trace
  3. Adjust time base to give a minimum of one complete cycle on the screen
  4. Measure the number of squares for one complete cycle
  5. Multiply the number of squares by the time base to find time period, T
  6. Use frequency (f) = 1 / time period (T)
• The system that uses ultrasound to find fish is called sonar.
• Describe an experiment to measure the speed of sound in air with microphones.

  Use speed = distance/time

  Measure the distance between two microphones using tape measure

  Distance must be more than 1m

  Measure the time it takes the sound to travel from microphone A to microphone B using an electronic timer

  Repeat and take an average
• Describe an experiment to measure the speed of sound in air with 2 students.

  Use speed = distance/time

  Measure the distance between two students using a trundle wheel

  Distance must be more than 100m

  Measure the time between seeing the clap and hearing the clap using a stopwatch

  Repeat and take an average
• Explain how optical fibres are used in endoscopes.

  Optical fibres bend

  Some fibres carry light to the inside of the patient

  Some fibres transmit the reflected light

  Light passes up/down fibres by total internal reflection

  Image is analysed by computer
• When playing a drum, how is sound produced?

  Particles in the drum vibrate.

  Vibrations are passed onto particles in the air.

  Vibrations are parallel to the direction the wave travels.
• All waves can be refracted when they travel from one medium into another. The line where the two materials join is called the boundary.

  Light is refracted when it goes from air into water, and when it goes from water into air.

  This is why it is difficult to see things clearly when you are looking into water.

  Anglerfish, which spit at insects above the water, learn to take account of refraction when aiming their jet of water.