WAVES

Question 1

Describe Transverse Wave

Answer 1

Particles oscillate perpendicular to direction of energy propagation.

Question 2

Describe Longitudinal Wave.

Answer 2

Particles oscillate parallel to direction of energy propagation.

Question 3

Define Amplitude.

Answer 3

Maximum displacement of particle from the equilibrium position.

Question 4

Define Wavelength.

Answer 4

Shortest distance between two points in phase.

Question 5

Define frequency.

Answer 5

Number of oscillations per second (of a particle.)

Question 6

Define Time period.

Answer 6

Time for one complete oscillation.

Question 7

Define Refraction.

Answer 7

Refers to bending of a wave when it changes speed (usually when passing from one material to another with different optical density.)

Question 8

Define the normal.

Answer 8

Line at 90⁰ to the surface.

Question 9

Define angle of incidence.

Answer 9

The angle between light Ray and normal.

Question 10

FAST Acronym

Answer 10

Fast
Away from the normal
Slow
Towards the normal

Question 11

Define Critical Angle.

Answer 11

Angle of incidence that provided Angle of refraction of 90⁰

Question 12

State conditions for Total Internal Reflection.

Answer 12

Angle of incidence greater than critical angle.
Light travelling from material with higher refractive index into material with lower refractive index.

Question 13

Describe how an Optical fibre works.

Answer 13

Thin glass fibre that transmits light or infrared radiation. Trapped inside by TIR. Possible if each reflection has angle of incidence greater than critical angle.

Question 14

Describe step index optical fibre

Answer 14

Has a optically dense core of uniform refractive index.
Lower optically dense cladding around the core.
Outer sheath.
Named a step index optical fibre as each outer layer has a greater refractive index.

Question 15

Describe Modal dispersion

Answer 15

Occurs when monochromatic light pulses spread out in optical fibre.
Because each part of wavefront has different angle of incidence hence different angle of reflection.
So each part of wavefront undergoes different amount of TIR causes pulse broadening.

Question 16

Describe Material Dispersion.

Answer 16

When white light used instead of monochromatic light.
It splits into all parts of the spectrum.
Eg.
Violet light shortest wavelength hence moves slowest in the fibre.
Means angle of incidence smallest hence angle of reflection smallest.
Takes longer to get to end of fibre resulting in pulse broadening.

Question 17

Define Polarisation.

Answer 17

When a particle oscillations occur in only one of the directions perpendicular to the direction of wave propagation.

Question 18

Why can only transverse waves get polarised?

Answer 18

As transverse waves oscillate in any plane perpendicular to the propagation direction.

Question 19

What happens when a transverse wave is polarised?

Answer 19

Vibrations restricted to one direction.
Vibrations still perpendicular to direction of proportional energy transfer.

Question 20

Why can’t longitudinal waves be polarised?

Answer 20

Because they travel parallel to direction of travel.

Question 21

What happens when wave passes through filter with transmission axis perpendicular to wave?

Answer 21

None of it passes through.

Question 22

How do Polaroid glasses work?

Answer 22

Contains lens with vertically orientated polarising filters.
Light reflected off horizontal plane undergoes partial plane Polarisation.
Means more proportion of light horizontally polarised.
Therefore glasses useful in reducing glare.

Question 23

How does Polarisation relate to broadcasting?

Answer 23

Radio and TV broadcasters either horizontally or vertically polarised.
Therefore reception aerial needs to be orientated in the correct plane to pick up signal correctly.

Question 24

Define Absorption.

Answer 24

When fibre absorbs part of the signal energy. Causes reduced amplitude, leads to loss of info transmitted.

Question 25

How to reduce absorption in optical fibres?

Answer 25

Use extremely transparent core.
Use a optical repeated to regenerate the pulse.

Question 26

Define pulse broadening.

Answer 26

Occurs via modal and material dispersion. When the signal gets broader.

Question 27

How to reduce pulse broadening?

Answer 27

Core as narrow as possible to reduce the possible differences in path length.
Monochromatic light so speed of pulse constant.
Single mode fibre so only single wavelength of light passes through the core.