Demonstrating Interference (AQA A Level Physics): Revision Note

Interference & diffraction of a laser

• Lasers are the ideal piece of equipment to analyse diffraction and intensity patterns because they form light that is:

  • coherent (have a constant phase difference and frequency)

  • monochromatic (have the same wavelength)

• The diffraction pattern produced by a laser on a screen is made up of:

  • areas of constructive interference - the bright strips or fringes

  • areas of destructive interference - the dark fringes

• Other sources of light, such as a filament bulb or a sodium lamp, are non-coherent, so they produce white light

Safety issues with lasers

• Lasers produce a very high-energy beam of light

• This intense beam can cause permanent eye damage or even blindness

Precautions

• It's important to use lasers safely and follow the guidelines:

  • Never look directly at a laser or its reflection

  • Don’t shine the laser towards a person

  • Don't allow a laser beam to reflect from shiny surfaces into someone else's eyes

  • Wear laser safety goggles

  • Place a ‘laser on’ warning light outside the room

  • Stand behind the laser

Sound & EM wave interference

Using sound waves

• Two-source interference can be demonstrated with two speakers emitting a coherent sound

• Sound waves are longitudinal waves made up of compressions and rarefactions

  • Constructive interference occurs when the compressions and rarefactions from each wave line up and the sound appears louder

  • Destructive interference occurs when a compression from one wave lines up with a rarefaction from the other and vice versa

    • The two waves cancel each other out, so zero sound is heard

  • This is the technology used in noise-cancelling headphones

Using microwaves

• Two-source interference for microwaves (and other electromagnetic waves) can be detected with a moveable microwave detector

• The detector picks up a maximum amplitude or intensity in regions of constructive interference 

• The detector picks up a minimum or zero amplitude, so no signal in regions of destructive interference 

Intensity variation with amplitude

• By definition, the intensity of a wave (its power per unit area) is proportional to the energy transferred by the wave

• The intensity of a wave at a particular point is related to the amplitude of the wave at that point

• The energy transferred by a wave is proportional to the square of the amplitude

• Therefore, the intensity of a wave is proportional to the square of the amplitude 

$I\propto A^2$

• Where:

  • $I$ = intensity of the wave in W m^–2

  • $A$ = amplitude of the wave in metres (m)