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IGCSE · 3 · structured

Waves · Question 8

Waves · Original GioPhysics question with a detailed, mark-by-mark answer guide.

Demand
discriminating
Marks
9
Topics
1
Answer
Complete
Light travelling along an optical fibre by total internal reflectionairairglass core, n = 1.50light innormal
Fig. 8.1A straight length of optical fibre drawn in section as a long horizontal band with parallel walls, shaded to show glass and labelled glass core, n = 1.50, with air labelled outside it. Light entering the flat left-hand end travels up to the upper wall, reflects down to the lower wall, reflects again up to the upper wall and reaches the far end, giving a zig-zag path along the fibre. At one reflection a dashed normal is drawn perpendicular to the wall, and the angle on each side of it is marked with an arc; no angle is given a numerical value.
structured9 marks

An optical fibre has a glass core of refractive index 1.50. Light entering one end of the fibre travels along it by repeated total internal reflection at the core boundary, even where the fibre is gently curved.

  1. (a)

    State State the two conditions necessary for total internal reflection to occur.

    2 marks
  2. (b)

    Calculate Calculate the critical angle for the glass core.

    3 marks
  3. (c)

    Explain A student suggests that bending the fibre very sharply would make no difference, because total internal reflection always happens. Explain why the student is wrong.

    3 marks
  4. (d)

    Give Give one use of optical fibres other than in medicine.

    1 mark
Ready to self-mark?Reveal the detailed answer guide
Answer overviewKey answer: the light must be travelling in the optically denser medium, towards a boundary with a less dense medium the angle of incidence at the boundary must be greater than the critical angle sin c = 1 / n
01

(a)

2 marks

State State the two conditions necessary for total internal reflection to occur.

How to approach it

Answer the command word directly and use precise physical vocabulary. Include only the distinct features or facts that earn marks, without burying them in unrelated background information.

  1. 1

    the light must be travelling in the optically denser medium, towards a boundary with a less dense medium

  2. 2

    the angle of incidence at the boundary must be greater than the critical angle

02

(b)

3 marks

Calculate Calculate the critical angle for the glass core.

How to approach it

List the given quantities with units, identify the required quantity, write the governing relationship before substituting, and keep extra digits until the final line so rounding does not distort the result.

  1. 1

    sin c = 1 / n

  2. 2

    sin c = 1 / 1.50 = 0.667

  3. 3

    c = 41.8° (accept 42°)

03

(c)

3 marks

Explain A student suggests that bending the fibre very sharply would make no difference, because total internal reflection always happens. Explain why the student is wrong.

How to approach it

State the outcome first, then link cause to effect with the relevant physical principle. Each link in the reasoning should be explicit enough to earn its own marking point.

  1. 1

    bending the fibre sharply reduces the angle of incidence at the outer wall of the bend

  2. 2

    if that angle falls below the critical angle of 41.8°, the light is no longer totally internally reflected

  3. 3

    some light refracts out through the boundary and the signal is weakened or lost

04

(d)

1 mark

Give Give one use of optical fibres other than in medicine.

How to approach it

Identify what the question is testing, organise the response into distinct mark-earning points, and make every conclusion traceable to a physical principle or to the evidence supplied.

  1. 1

    telephone / cable television / internet communication — any high-speed data transmission

Private on this device

Did your answer earn the marks?

Compare the reasoning point by point, then choose what happens next.