3.5 - Refraction

Question 1

When does refraction occur?

Answer 1

When light passes a boundary between 2 different transparent media at an angle.

Question 2

What happens at the boundary between two mediums when light refracts?

Answer 2

The rays of light undergo a change in direction.

Question 3

How is the angle of direction taken in refraction?

Answer 3

From the normal (perpendicular to the surface of the boundaries).

Question 4

What happens to the direction of light when going from a less dense to more dense medium?

Answer 4

The light bends towards the normal.
(air to glass)

Question 5

What happens to the direction of light when going from a more dense to less dense medium?

Answer 5

The light bends away from the normal.
(glass to air)

Question 6

What happens to the direction of light when passing along the normal?

Answer 6

Light does not bend at all.

Question 7

Why does light refract when going to a more or less dense medium?

Answer 7

The change in direction occurs due to the change in speed when travelling in different substances (light slows down when going into a denser medium so it bends towards the normal).

Question 8

What is the refractive index? (n)

Answer 8

A property of a material which measures how much light slows down when passing through it.

Question 9

How do you calculate the refractive index? (equation)

Answer 9

n = c / cₛ
c = the speed of light in a vacuum (3 x 10⁸) (ms⁻¹)
cₛ = the speed of light in a substance (ms⁻¹)

Question 10

What is a material with a high refractive index called?

Answer 10

An optically dense material.

Question 11

Why must n always be above 1 when calculating the refractive index?

Answer 11

Because the speed of light in a substance can never be higher than the speed of light in a vacuum.

Question 12

What is Snell’s Law?

Answer 12

n₁sinθ₁ =n₂sinθ₂
n₁ = the refractive index of material 1
n₂ = the refractive index of material 2
θ₁ =the angle of incidence of the ray in material 1
θ₂ = the angle of refraction of the ray in material 2

Question 13

Where are the angles θ1 and θ2 are always taken from?

Answer 13

The normal.

Question 14

What is the angle of incidence?

Answer 14

The angle between the normal and the ray of light.

Question 15

What is the critical angle? θ𝒸

Answer 15

When the angle of incidence goes over 90 degrees and the ray changes from just refracting to total internal reflection.

Question 16

How can the critical angle be found?

Answer 16

sin θ𝒸 = n₁ / n₂

Question 17

What is total internal reflection (TIR)?

Answer 17

When the angle of incidence is greater than the critical angle and the incident refractive index n₁ is greater than the refractive index of the material at the boundary n₂.

Question 18

What are the 2 conditions for TIR?

Answer 18

• The angle of incidence > the critical angle
• The refractive index n₁ is greater than the refractive index n₂

Question 19

How do you calculate the angle of reflection?

Answer 19

critical angle - angle of incidence

Question 20

What are 2 things fibre optics are used for?

Answer 20

Communications - (telephone/internet)
Medical imaging - (endoscopes)

Question 21

What do fibre optics do?

Answer 21

Utilise the phenomenon of total internal reflection to send high speed light signals over large distances.

Question 22

What is the cladding of a fibre optic?

Answer 22

The material with a lower refractive index that surrounds the core.

Question 23

What is the core of a fibre optic?

Answer 23

An optically dense material (like plastic/glass).

Question 24

What are the 3 components of optical fibres?

Answer 24

• core
• cladding
• outer sheath

Question 25

Why is the core more optically dense?

Answer 25

So that total internal refraction occurs.
n cladding < n core

Question 26

What are the 3 purposes of the outer sheath?

Answer 26

• Prevents physical damage to the fibre
• Strengthens the fibre
• Protects the fibre from the outside from scratches

Question 27

What are 3 purposes of the cladding?

Answer 27

• Keeps signals secure and maintains quality
• Keeps the core away from adjacent fibre cores so there is no crossover signals
• Prevents core from damage

Question 28

What is material dispersion?

Answer 28

When white light is used instead of monochromatic light.

Question 29

What is pulse broadening?

Answer 29

Wen material dispersion occurs and the difference frequencies of light travel at different rates.
The pulses that emerge are longer than they should be.

Question 30

What is modal dispersion?

Answer 30

When the light pulses in the optical fibre spread out due to the different angles of incidence in the original pulse.

Question 31

Why is the core so narrow?

Answer 31

To prevent modal dispersion.