3.3.2.1 - Interference

Question 1

Crest + crest?

Answer 1

Constructive interference.

Question 2

Crest + trough?

Answer 2

Destructive interference.

Question 3

What do waves have to be to be in phase?

Answer 3

Both at the same point in the wave cycle, and have the same displacement and velocity.

Question 4

One complete cycle =

Answer 4

360 degrees or 2 pi.

Question 5

When are two sources coherent?

Answer 5

Two sources are coherent if they have the same wavelength and frequency and a fixed phase difference between them.

Question 6

What does whether you get constructive or destructive interference depend on?

Answer 6

Depends how much further one wave has travelled than the other wave to get to that point.

Question 7

What is the path difference?

Answer 7

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

Question 8

When can you get constructive interference?

Answer 8

At any point an equal distance from two sources that are coherent and in phase.
Where the path difference is a whole number of wavelengths.

Question 9

Where do you get destructive interference?

Answer 9

Where the path difference is half a wavelength or more half a wavelength value.

Question 10

When is there a diffraction pattern for a laser?

Answer 10

When the wavelength is about the same size as the aperture.

Question 11

What is a laser made up of?

Answer 11

Monochromatic light.

Question 12

What is monochromatic light?

Answer 12

Same wavelength and frequency, so therefore all the same colour, white.

Question 13

What do you see when a laser is diffracted through a slit?

Answer 13

You’ll see a central bright fringe, with dark and bright fringes alternating on either side.

Question 14

What are the light and dark fringes caused by?

Answer 14

Constructive and destructive interference.

Question 15

What happens in young’s double split experiment?

Answer 15

A laser is shone through two slits onto a screen. The slits have to be about the same size of the wavelength of the laser light so that it is diffracted - then the light from the slits acts like two coherent point sources.

Question 16

What causes the pattern of young’s double split?

Answer 16

You get a pattern of light and dark fringes, depending on whether constructive interference is taking place.

Question 17

What are the dangers of lasers?

Answer 17

The light of a laser is focused to a very direct, powerful beam of monochromatic light. Therefore if you look at it directly, your eye’s lens would focus it onto your retina which would be permanently damaged.

Question 18

How can you avoid damage due to lasers?

Answer 18

• Never shine the lasers towards a person.
• Wear safety goggles.
• Avoid shining on reflective surfaces.
• Turn off when not in use.

Question 19

In young’s double split equation what does w mean?

Answer 19

Fringe spacing.

Question 20

In young’s double split equation what does s mean?

Answer 20

Spacing between slits.

Question 21

In young’s double split equation what does D mean?

Answer 21

Distance from slits to screen.

Question 22

What happens when white light is shone through a single narrow slit:

Answer 22

All the wavelengths are diffracted by different amounts. This means that instead of getting clear fringes, you get a spectra of colours.

Question 23

How can you do a similar experiment using microwaves?

Answer 23

Also replace the screen with a microwave receiver probe. If you move the probe up and down, perpendicular to direction of microwave generator you will get an alternating pattern of strong and weak signals - just like the dark and light fringes on the screen.

Question 24

What do you need to replace in order to do a similar experiment with microwaves?

Answer 24

Replace the laser and slits with two microwave transmitter cones attached to the same signal generator.

Question 25

How was young’s double split experiment used to show that light was made up of waves?

Answer 25

Newton and Huygens both had theories of light, but Huygens idea was confirmed as young’s double split experiment showed that light could both diffract and interfere.