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 While light is incident on a glass plate of thickness 5000 \mathrm{~A}^{0} and refractive index 1.5 the wavelength in the visible region \left(4000 A^{0}-7000 A^{0}\right) that are strongly reflected by the plate is

Option: 1

4290 \mathrm{~A}^{0}


Option: 2

6000 \mathrm{~A}^{0}


Option: 3

4000 \mathrm{~A}^{0}


Option: 4

Both (A) &(B)


Answers (1)

best_answer

Light of wavelength \lambda is strongly reflected if 2 \mu \mathrm{t}=(2 n+1) \lambda / 2

\text { If } \lambda=4000 \mathrm{~A}^{0} \quad \ldots(1)
solving it for n we get \mathrm{n=3.25} similarly for \mathrm{\lambda=7000 \mathrm{~A}^{0} }

\mathrm{we\: get \: \mathrm{n}=1.66}

Hence, within \mathrm{4000 A^{0}} and \mathrm{7000 A^{0}} the integer can take value 2 and 3  from equation (i) \mathrm{\lambda=4 \mu \mathrm{t} /\left ( 2n+1 \right )}

Substituting \mathrm{n=3} and \mathrm{n=4} we get
\mathrm{\lambda=6000 \mathrm{~A}^{0}} and \mathrm{4290 \mathrm{~A}^{0}}.

So these two wavelengths are reinforced.

Posted by

vishal kumar

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