3.3.2 Demonstrating Interference

• 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

• 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

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 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 two waves interfere causing areas of constructive and destructive interference

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