3.2 Refraction, diffraction and interference

Question 1

Define coherence.

Answer 1

Coherent waves have a fixed phase difference and the same frequency and wavelength.

Question 2

Define path difference.

Answer 2

A measure of how far ahead a wave is compared to another wave.

Usually expressed in terms of the wavelength

Question 3

Why are lasers used for showing interference and diffraction?

Answer 3

They are monochromatic sources of light, hence interference and diffraction patterns are more defined.

Question 4

Describe the Young’s double slit experiment.

Answer 4

Single light source directed at two slits, which each act as a coherent light source, the light interferes constructively and destructively to create an interference pattern.

Question 5

Use wave theory to explain how a fringe pattern is formed.

Answer 5

• Slits act as coherent sources
• Waves diffract at slits
• Waves superpose
• Bright patches: constructive
• Dark patches: destructive

Question 6

Describe the interference pattern created using white light.

Answer 6

1. Bright white central maximum
2. Dark fringes
3. Bright fringes beginging with violet and ending with red
4. Repeating 2 and 3

Question 7

What are 4 safety precautions that must be followed when using a laser?

Answer 7

● Wear laser safety goggles
● Don’t shine the laser at reflective surfaces
● Display a warning sign
● Never shine the laser at a person

Question 8

How does a diffraction pattern change if the slit width became narrower?

Answer 8

• Increased fringe separation
• Lower maxima intensity

Question 9

What is diffraction?

Answer 9

The spreading out of waves when they pass through or around a gap.

Question 10

State 2 aplications of diffraction gratings

Answer 10

• Absorption spectra to identify elements in a star.
• X-ray crystallography

Question 11

Which direction does light bend when entering a more optically dense medium?

Answer 11

Towards the normal.

Question 12

When does Total Internal Reflection occur?

Answer 12

When light is at a boundary to a less optically dense medium and the angle of incidence is greater than the critical angle.

Question 13

What are the two components of an optical fibre?

Answer 13

• Cladding
• Core

Question 14

What are the purpose of cladding?

Answer 14

● Protects core from scratches which would allow light to escape and degrade the signal.
● Allows TIR as it has a lower refractive index than the core.

Question 15

How does absorption in an optical fibre affect the signal?

Answer 15

Part of the signal’s energy is absorbed by the fibre so its amplitude is reduced.

Question 16

What is pulse broadening?

Answer 16

When the received signal is wider than the original, this can cause overlap of signals leading to information loss.

Question 17

What is material dispersion?

Answer 17

When light with different wavelengths is used some wavelengths slow down more than others in the fibre so they arrive at different times causing pulse broadening.

Causes pulse broadening

Question 18

How can material dispersion be reduced?

Answer 18

Use monochromatic light.

Question 19

What is modal dispersion?

Answer 19

When rays enter the fibre at different angles, causing each ray to travel a different distance. Resulting in the rays arriving at different times.

Causes pulse broadening

Question 20

How can modal dispersion be reduced?

Answer 20

Use a very narrow fibre so the possible difference in path lengths is smaller.

Question 21

How can both absorption and dispersion be reduced?

Answer 21

Use an optical fibre repeater to regenerate the signal.

Question 22

State the advantages of optical fibres over traditional copper wires.

Answer 22

● Signal can carry more information as light has a high frequency
● No energy lost as heat
● No electrical interference
● Cheaper
● Very fast