AQA A Level Physics

Revision Notes

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

Eye Injury Hazard, downloadable AS & A Level Physics revision notes

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

Laser Warning, downloadable AS & A Level Physics revision notes

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 experiment, downloadable AS & A Level Physics revision notes

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

Water waves interference fringes, downloadable AS & A Level Physics revision notes

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, downloadable AS & A Level Physics revision notes

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

Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top