AQA A Level Physics复习笔记3.3.2 Demonstrating Interference

Interference & Diffraction of a Laser


  • Interference and diffraction of lasers can be demonstrated with slits or diffraction gratings
  • For light rays, such as a laser light through two slits, an interference pattern forms on the screen


Laser light interference experiment


  • Constructive interference is shown as bright fringes on the screen
    • The highest intensity is in the middle


  • Destructive interference is shown as the dark fringes on the screen
    • These have zero intensity


Safety Issues with Lasers

  • Lasers produce a very high energy beam of light
  • This intense beam can cause permanent eye damage or even blindness
  • In schools, only class 2 lasers are allowed - these are lasers with a power output of less than 1 mW
    • However, more powerful lasers can reach outputs of more than 500 mW
    • These are known as class 4 lasers. They are so powerful they can make a person instantly blind and can even damage the skin



The four classes of laser: In a school laboratory, only Class 2 lasers may be used



  • 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


Sound & EM Wave Interference

Using Sound Waves

  • Two-source interference of sound can be demonstrated with two speakers

Sound wave interference from two speakers


  • Sound waves are longitudinal waves so are made up of compressions and rarefactions
  • Constructive interference occurs when the compression and rarefactions line up and the sound appears louder
  • Destructive interference occurs when the compression lines up with a rarefaction and vice versa. The sound is quieter
    • This is the technology used in noise-cancelling headphones


  • The diagram below shows diffracted sound waves from two point sources

An experiment demonstrating interference of sound waves


  • The two waves interfere causing areas of constructive and destructive interference
  • The lines of maximum displacement occur when all the peaks and troughs line up with those on another wave

Using Microwaves

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

Microwave interference experiment


  • Constructive interference: regions where the detector picks up a maximum amplitude
  • Destructive interference: regions where the detector picks up no signal

Worked Example

Two speakers are set up in a room and play a note of frequency 280 Hz. The waves are in phase as they leave the speaker. A student walks 3.0 m between two speakers and hears quiet and loud spots as she moves.Calculate the number of quiet spots the student hears as she walks.Speed of sound in air = 340 m s–1