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

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.

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

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.
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