Demonstrating Interference (AQA AS Physics)

• A laser is a focused beam of monochromatic light
• This makes it the ideal piece of equipment to analyse diffraction and intensity patterns from its diffraction through slits and diffraction gratings

When monochromatic laser light is diffracted through a diffraction grating, a diffraction pattern of bright and dark fringes can be observed on the screen

• The diffraction pattern on the 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, are non-coherent, so produce white light

• 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

Placing a laser warning sign outside of the door is one precaution that can be taken when using lasers

Using Sound Waves

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

Sound wave interference from 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. So the sound is quieter
  • This is the technology used in noise-cancelling headphones

Using Microwaves

• Two-source interference for microwaves can be detected with a moveable microwave detector

A microwave interference experiment creates a diffraction pattern the same as that of a laser beam

• 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