12. Critical Angle and TIR

Question 1

How do you work out the critical angle?

Answer 1

sin θc = n2 / n1

θc = critical angle
n2 = refractive index of less optically dense material
n1 = refractive index of more optically dense material

Question 2

What is total internal reflection?

Answer 2

At angles of incidence greater than the critical angle, refraction cannot happen so light is reflected back into the material

Question 3

What is an optical fibre?

Answer 3

A very thin flexible tube of glass or plastic fibre that can carry light signals over long distances and round corners using TIR

Question 4

What are step-index optical fibres?

Answer 4

They have a high refractive index core surrounded by cladding with a lower refractive index to allow TIR

Question 5

What does the cladding in an optical fibre used for?

Answer 5

Protecting the fibre from scratches which would otherwise allow light to escape

Question 6

What are the advantages of optical fibres?

Answer 6

1. The signal can carry more information because light has a high frequency
2. The light does not heat up the fibre - almost no energy is lost as heat
3. There is no electrical interference
4. Fibre-optic cables are cheaper to produce
5. The signal can travel a long way, very quickly with minimal signal loss

Question 7

What are the two causes of signal degradation?

Answer 7

1. Absorption

2. Dispersion

Question 8

How does absorption cause signal degradation?

Answer 8

Some of the signal’s energy is absorbed by the material so the amplitude of the signal is reduced, causing the loss of some information

Question 9

How does dispersion cause signal degradation?

Answer 9

Causes pulse broadening - the received signal is wider than the initial signal. Broadened pulses can overlap each other, leading to information loss

Question 10

What are the two types of dispersion?

Answer 10

1. Modal

2. Material

Question 11

What is modal dispersion caused by?

Answer 11

Light rays entering the optical fibre at different angles

Question 12

How can modal dispersion be reduced?

Answer 12

Using a single-mode fibre in which light is only allowed to follow a very narrow path

Question 13

What is material dispersion caused by?

Answer 13

The different amounts of refraction experienced by different wavelengths of light

Question 14

How can material dispersion be reduced?

Answer 14

Using monochromatic light

Question 15

How can signal degradation be reduced?

Answer 15

Using optical fibre repeaters to regenerate the signal every so often