# 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

#### 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

### 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