NEET-XII-Physics
17: Light Waves
- #36A parallel beam of light of wavelength 560 nm falls on a thin film of oil (refractive index = 1.4). What should be the minimum thickness of the film so that it strongly reflects the light?Ans : Given:
Wavelength of light used, `` \,\mathrm{\,\lambda \,}=560\times {10}^{-9}\,\mathrm{\,m\,}``
Refractive index of the oil film, `` \,\mathrm{\,\mu \,}=1.4``
Let the thickness of the film for strong reflection be t.
The condition for strong reflection is
`` 2\,\mathrm{\,\mu \,}t=\left(2n+1\right)\frac{\lambda }{2}``
`` \Rightarrow t=\left(2n+1\right)\frac{\lambda }{4\,\mathrm{\,\mu \,}}``
where n is an integer.
For minimum thickness, putting n = 0, we get:
`` t=\frac{\lambda }{4\,\mathrm{\,\mu \,}}``
`` =\frac{560\times {10}^{-9}}{4\times 1.4}``
`` ={10}^{-7}\,\mathrm{\,m\,}=100\,\mathrm{\,nm\,}``
Therefore, the minimum thickness of the oil film so that it strongly reflects the light is 100 nm.
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