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Figure 4 shows a ray of monochromatic light incident at angle A from air onto the end of a straight optical fibre - AQA - A-Level Physics - Question 3 - 2021 - Paper 1
Question 3
Figure 4 shows a ray of monochromatic light incident at angle A from air onto the end of a straight optical fibre. This ray undergoes total internal reflection at t... show full transcript
Worked Solution & Example Answer:Figure 4 shows a ray of monochromatic light incident at angle A from air onto the end of a straight optical fibre - AQA - A-Level Physics - Question 3 - 2021 - Paper 1
Step 1
Calculate the speed of the light ray in the optical fibre.
Answer
To find the speed of light in the optical fibre, we can use the formula:
v = rac{c}{n}
where:
- (speed of light in a vacuum)
- (refractive index of the core)
Calculating gives: v = rac{3.0 imes 10^8 ext{ m/s}}{1.47} \approx 2.04 \times 10^8 ext{ m/s}
Step 2
Calculate A, in degrees, for the optical fibre shown in Figure 4.
Answer
To calculate the critical angle, we use Snell's law:
ext{sin}(C) = rac{n_{2}}{n_{1}}
where:
- (refractive index of the core)
- (refractive index of the cladding)
Thus: ext{sin}(C) = rac{1.41}{1.47} \approx 0.957
Now, taking the inverse sine gives:
As angle A must be less than the critical angle:
Step 3
Draw, on Figure 5, what happens to the ray within the optical fibre. Explain your answer.
Answer
A diagram should be drawn to show the ray bending as it encounters the optical fibre boundary. The ray will partially reflect at the core-cladding boundary based on its angle of incidence.
- If the angle of incidence is greater than the critical angle (73.6°), some light will be reflected back into the core while some light may refract into the cladding.
- If the angle of incidence is less than the critical angle, the ray will continue in the core.
Overall, as the fibre bends, it may cause variations in how the light travels, likely resulting in some light being refracted out into the cladding.