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  4. The glass of optical fibre has a refractive index 1.55 and cladding with another glass of refractive index 1.51 . When the surrounding medium is air, the numerical aperture will be

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Q. The glass of optical fibre has a refractive index $1.55$ and cladding with another glass of refractive index $1.51$ . When the surrounding medium is air, the numerical aperture will be

NTA AbhyasNTA Abhyas 2022

Solution:

For air, $ \, \mu _{0}=1$ . Here, $\mu _{1}=1.55, \, \mu _{2}=1.51$
$NA=\frac{\sqrt{\mu _{1}^{2} - \mu _{2}^{2}}}{\mu _{0}}$
$=\frac{\sqrt{\left(1.55\right)^{2} - \left(1.51\right)^{2}}}{1}=0.350$