Optics

Question 1

Describe the structure of a step-index optical fibre, outlining the purposes of the core and cladding (3 marks):

Answer 1

• Glass core allows light waves to be transmitted through fibre
• Cladding has a lower refractive index than core, which allows TIR to happen
• Cladding protects core from damage to surface, which leads to light escaping)

Question 2

Total Internal Reflection:

Answer 2

The complete reflection of a light ray reaching an interface with a less dense medium when the angle of incidence exceeds the critical angle.

Question 3

Critical Angle:

Answer 3

The value of the incident angle at which, the refracted angle is going to be 90 degrees. (any more than this, it wont escape the more dense medium therefore wont refract, and TIR will occur)

Question 4

Relation between speed of light wave and refractive index:

Answer 4

Greater refractive index = Light will travel slower.

Question 5

Refractive index of the blue light =1.467
Refractive index of red light = 1.459
Explain how the difference in refractive index results in a change in the PULSE of white light by the time it leaves the fibre. (2 marks).

PART TWO
Discuss two changes that could be made to reduce the effect described. (2 Marks)

Answer 5

• Blue light will travel slower due to higher refractive index
• Therefore the pulse will STRETCH (Because Red light will arrive at the end of the fibre before blue light will).

PART TWO
-Make core thinner to reduce effect of multipath dispersion
-Use monochromatic source
-Add relay/repeaters to fix pulse every so often
(Any two).

Question 6

dsintheta=nlambda : What is d?

Answer 6

Line spacing/(Grating spacing), d = 1/Lines per M
Overall, lines per metre will be a bigger value, as many small lines per mm add up to a larger metre value.
NOTE: When working out d by 1/N, the end value is still in M (lines per millemetre measured in metres), so if it asks in mm then x10^-3.

Question 7

Absolute uncertainty of grating spacing:

Answer 7

(Percentage uncertainty in decimal form eg 2.9%= 0.029)xgrating spacing. GIVE TO D.P same as grating spacing.

Question 8

Working out spacing using a ruler:

Answer 8

Use a multiple amount of fringes, not just two adjacent ones. (e.g use 20 then divide overall length between the two by 20).

Question 9

Safety Precautions:

Answer 9

• Do not look directly into beam

- Ensure beam is not regularly reflected (reflected in a mirror).

Question 10

Quinckes Tube: (4 marks)

Answer 10

• When d1=d2, waves travel same distance therefore arrive in phase, Constructive interference occurs, sound is at its loudest.
• As tube gets pulled out, path difference increases and waves no longer in phase so loudness and displacement decreases
• At one half wavelength path difference, two waves in antiphase, where sound is at its quietest (Destructive interference occurs).
• As the path difference continues to increase, the two waves become more in phase and the sound gets louder again.

Question 11

State and explain one advantage associated with a smaller diameter core.

Answer 11

• less multipath dispersion (smearing / overlap of pulses)

- less light is lost

Question 12

When a light ray in air hits the normal about to enter an optical fibre, does the ray refract towards the normal line to meet the boundary or away

Answer 12

Towards normal (then goes on to hit boundary to TIR)

Question 13

State and explain one reason why part B of the optical fibre is made as narrow as
possible.

Answer 13

• To reduce multipath dispersion

- (which would cause) light travelling at different angles to arrive at different times

Question 14

State one application of optical fibres and explain how this has benefited society.

Answer 14

-Communications
-/improve
transmission of data/high speed internet

Question 15

Explain why optical fibres used for communications need to have cladding (2 marks)

Answer 15

• Prevents scratching of the core
• cladding provides (tensile) strength for fibre
• it keeps (most) light rays in due to TIR

Question 16

c = f λ

State how each of these quantities will change, if at all, when light travels from air to glass

Answer 16

c- decrease
f- constant (fixed)
wavelength- decrease

Question 17

What is frequency?

Answer 17

Frequency is FIXED

Question 18

Explain how the glass cladding around the optical fibre’s core improves the security
of data being transmitted through it and give a reason why this is important.
(3 marks)

Answer 18

-light does not enter the cladding so cannot pass across from one fibre to another

-fibres without cladding can allow light to pass between fibres
when the surface of the fibre becomes scratched

-personal data (such as bank account information) must be transmitted along fibres from which there is no danger of leakage of light resulting in a breach of security

Question 19

State and explain two physical properties of the light produced by a laser which makes
it different from the light produced by a filament lamp. (4 marks)

Answer 19

Properties of laser light:

• Monochromatic: As waves are of single wavelength
• coherent: As waves produced have a constant phase difference (in constant phase)

Question 20

When completing an optical fibre diagram, what is important to remember about the angles of the refracted ray when drawing?

Answer 20

reflected angle = incident angle

Question 21

A pulse of monochromatic light X is transmitted a long distance along an optical fibre. Explain why the shape of the pulse at the endpoint Y has a lower amplitude than the pulse at X. (2 marks)

Answer 21

• Reduced amplitude due to energy loss (within fibre)

- Broadening of pulse due to multipath dispersion