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 is the equation for fringe spacing?

Answer 7

Question 8

What do the letters represent in this equation?

Answer 8

Question 9

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

Answer 9

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

Question 10

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

Answer 10

• Increased fringe separation
• Lower maxima intensity

Question 11

What is diffraction?

Answer 11

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

Question 12

What is the equation for a diffraction grating?

Answer 12

Question 13

What do the letters represent in this equation?

Answer 13

Question 14

State 2 aplications of diffraction gratings

Answer 14

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

Question 15

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

Answer 15

Towards the normal.

Question 16

What is the equation for refractive index of a substance?

Answer 16

Question 17

What do the letters represent in this equation?

Answer 17

Question 18

What is Snell’s law?

(Law of refraction)

Answer 18

Question 19

What do the letters represent in this equation?

Answer 19

Question 20

What is the equation for the critical angle?

Answer 20

Question 21

What do the letters represent in this equation?

Answer 21

Question 22

When does Total Internal Reflection occur?

Answer 22

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

Question 23

What are the two components of an optical fibre?

Answer 23

• Cladding
• Core

Question 24

What are the purpose of cladding?

Answer 24

● 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 25

How does absorption in an optical fibre affect the signal?

Answer 25

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

Question 26

What is pulse broadening?

Answer 26

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

Question 27

What is material dispersion?

Answer 27

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 28

How can material dispersion be reduced?

Answer 28

Use monochromatic light.

Question 29

What is modal dispersion?

Answer 29

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 30

How can modal dispersion be reduced?

Answer 30

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

Question 31

How can both absorption and dispersion be reduced?

Answer 31

Use an optical fibre repeater to regenerate the signal.

Question 32

State the advantages of optical fibres over traditional copper wires.

Answer 32

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