Optics 2 Question 22
22. A light wave of frequency $5 \times 10^{14} Hz$ enters a medium of refractive index 1.5. In the medium the velocity of the light wave is and its wavelength is ……
(1983, 2M)
Passage Based Questions
Passage 1
Light guidance in an optical fibre can be understood by considering a structure comprising of thin solid glass cylinder of refractive index $n _1$ surrounded by a medium of lower refractive index $n _2$. The light guidance in the structure takes place due to successive total internal reflections at the interface of the media $n _1$ and $n _2$ as shown in the figure. All rays with the angle of incidence $i$ less than a particular value $i _m$ are confined in the medium of refractive index $n _1$. The numerical aperture (NA) of the structure is defined as $\sin i _m$.
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Answer:
Correct Answer: 22. $2 \times 10^{8} m / s, 4 \times 10^{-7} m$
Solution:
- $v=\frac{c}{\mu}=\frac{3 \times 10^{8}}{1.5}=2.0 \times 10^{8} m / s$
$\lambda=\frac{v}{f}=\frac{2.0 \times 10^{8}}{5.0 \times 10^{14}}=4.0 \times 10^{-7} m$
