An electromagnetic wave enters a fibre-optic cable from air - AQA - A-Level Physics - Question 18 - 2018 - Paper 1
Question 18
An electromagnetic wave enters a fibre-optic cable from air. On entering the cable, the wave slows down to three-fifths of its original speed.
What is the refractiv... show full transcript
Worked Solution & Example Answer:An electromagnetic wave enters a fibre-optic cable from air - AQA - A-Level Physics - Question 18 - 2018 - Paper 1
Step 1
Calculate the Speed Ratio
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Answer
The speed of light in a vacuum (or air) is approximately c. When the wave enters the cable, it slows down to three-fifths of its original speed, hence the speed in the cable, v, can be expressed as:
v=53c
Step 2
Apply the Refractive Index Formula
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Answer
The refractive index (n) is defined as the ratio of the speed of light in vacuum to the speed of light in the medium. This can be expressed as:
n=vc
Substituting the value of v:
n=53cc=35=1.67
Step 3
Conclusion
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Answer
Based on the calculations, the refractive index of the core of the fibre-optic cable is 1.67. Therefore, the correct answer is D.
