GATE 2023 IN – Question 45
A silica-glass fibre has core index 1.47 and cladding index 1.44. The cladding is completely stripped off and the core immersed in water of index 1.33. The modified fibre’s numerical aperture is ______ (three decimal places).
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Show answer and explanation
Correct answer: 0.620 to 0.640
Explanation
The surrounding index is now water’s 1.33. Numerical aperture is $\sqrt{n_{core}^2-n_{water}^2}=\sqrt{1.47^2-1.33^2}=\sqrt{0.392}=\boxed{0.626}$. The removed cladding index does not enter.