3.3.2.3 - Refraction At A Plane Surface

Question 1

Refractive index of a material =

Answer 1

C / Cs (speed of light/speed of light in that material)

Question 2

What is the refractive index of air?

Answer 2

1.

Question 3

What is snell’s law?

Answer 3

n1 sin 01 = n2 sin 02

n = refractive index.

Question 4

What causes light to be refracted along the boundary?

Answer 4

When light is reflected at the critical angle.

Question 5

What causes light to be totally internally reflected?

Answer 5

When the angle is greater than the critical angle.

Question 6

What values can sin take?

Answer 6

Smaller than 1.

Question 7

Sin c =

Answer 7

1/ refractive index

n2 / n1

Question 8

What are optical fibres used for?

Answer 8

To carry signals over long distances and round corners.

Question 9

Why is cladding needed for optical fibres?

Answer 9

Optical fibres themselves have a high refractive index but are surrounded by cladding with a lower refractive index to allow total internal reflection. Moreover cladding protects the fibre from scratches which could let light escape.

Question 10

Why is the fibre narrow?

Answer 10

So that light always hits the boundary between the fibre and cladding at an angle bigger than the critical angle.

Question 11

What two things cause signal degradation?

Answer 11

Absorption and dispersion.

Question 12

What does absorption cause?

Answer 12

Loss in amplitude.

Question 13

How is absorption caused?

Answer 13

As signal travels, some of its energy is lost through absorption by the material the fibre is made from.

Question 14

What does dispersion cause?

Answer 14

Pulse broadening.

Question 15

What are the two types of dispersion?

Answer 15

Modal and material.

Question 16

How does Modal dispersion occur?

Answer 16

Light rays enter the fibre at different angles, and so take different paths. The rays which take a longer path take longer to reach the other end than those that travel down the middle of the fibre.

Question 17

How and why can modal dispersion be stopped?

Answer 17

A single-mode fibre only lets light take one path, so it stops modal dispersion.

Question 18

What causes material dispersion?

Answer 18

As light consists of different wavelengths that travel at different speeds in the fibre, this causes some light wavelengths to reach the nod of the fibre faster than others.

Question 19

What can stop material dispersion?

Answer 19

Using monochromatic light.

Question 20

Why is pulse broadening a problem?

Answer 20

Broadened signals can overlap each other and confuse the signal.

Question 21

What can be used to reduce both signal absorption and dispersion?

Answer 21

An optical fibre repeater can be used to boost and regenerate the signal every so often.