3.3.2.1 Waves: Interference

Question 1

What is two-source interference?

Answer 1

When waves from two sources interfere to produce a pattern.

Question 2

What are the conditions needed to get clear interference patterns from two sources?

Answer 2

Two source must be monochromatic and coherent (same wavelength and frequency and fixed phase difference)

Question 3

What is coherent?

Answer 3

Two waves are coherent if they have the same wavelength and frequency and a fixed phase difference

Question 4

On the pattern what determines whether you get constructive or destructive interference?

Answer 4

How much further one wave has travelled than the other wave to get to that point (path difference)

Question 5

What is path difference?

Answer 5

The amount by which the path travelled by one wave is longer than the path travelled by the other wave

Question 6

What is a maxima?

Answer 6

Points at an equal distance from two sources, where constructive interference occurs. Also occurs at any point where the path difference is a whole number of wavelengths

Question 7

What is minima?

Answer 7

At points where the path difference is half a wavelength/ 1.5 wavelengths etc. waves arrive out of phase and total destructive interference occurs.

Question 8

When does constructive interference occur?

Answer 8

When there is a path difference of nlambda

Question 9

When does destructive interference occur?

Answer 9

When there is a path difference of (n+1/2)lambda

Question 10

How to see interference patterns with sound waves?

Answer 10

Have two speakers connected to an amplifier. The interference pattern is produced at a black line. This can be observed by having someone walk in a straight line parallel to speakers, they’ll observe quiet areas where destructive interference occurs and louder areas where constructive interference occurs.

Question 11

How to see interference patterns from microwaves?

Answer 11

You can use two microwave transmitter cones attached to the same signal generator - you also need a microwave receiver probe (you move it parallel to the transmitter cones to see a pattern)

Question 12

How can you see two-source interference of light?

Answer 12

Use two coherent light sources or shine a laser through two slits.

Question 13

Why is a laser used to generate interference patterns?

Answer 13

It is monochromatic and coherent

Question 14

What must be true about the slits used to shine a laser?

Answer 14

They must be about the same size as the wavelength of the laser so that it is diffracted, making the light from the slits act like two coherent point sources.

Question 15

What happens if you use white light instead of a laser?

Answer 15

Diffraction pattern is less intense with wider maxima. It also contains different colours with a central white fringe because white light is made up of a mixture of frequencies.FINISH THIS!¬!!!!

Question 16

How can you create two coherent light sources using a single laser?

Answer 16

Shine a laser through two slits. They have to be about the same size as the wavelength in order for light to be diffracted, making the light from the slits act like two coherent point sources.

Question 17

Describe some laser safety precautions

Answer 17

Don’t shine laser towards a person
Wear laser safety goggles
Avoid shining laser beam at reflective surface
Have a warning sign on display (?)
Turn off when not needed

Question 18

What is the double slit formula?

Answer 18

w=lambdaD/s
((fringe spacing = wavelength*distance between slits and screen)/distance between slits)

Question 19

How can you derive the double slit equation?

Answer 19

Consider path difference between the waves from each slit when light interferes constructively to form the first bright fringe. Path difference between the light waves at the first bright fringe is lambda

Question 20

What is the evidence for the wave-like nature of light?

Answer 20

Young’s double slit experiment proved that light could diffract and interfere, hence wave