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Manipal MET2015PhysicsWave Optics

A thin film of soap solution ( _s=1.4 ) lies on the top of a glass plate ( _g=1.5 ). When incident light is almost normal to the plate, two adjacent reflection maxima are observed at two wavelengths 420 nm and 630 nm . The minimum thickness of the soap solution is

Options

  1. A420 nm
  2. B450 nm
  3. C630 nm
  4. D1260 nm

Correct answer

B. 450 nm

Step-by-step solution

For the reflection at the air-soap solution interface, the phase difference is For reflection at the interface of soap solution to the glass also there will be a phase difference of . The condition for maximum intensity =2 t=n ...(i) array rlrl & For n , & n ₁ & =(n-1) ₂ & & n(420) & =(n-1) 630 & & n(630-420) & =630 & n(210) & =630 & & n & =3 array This is the maximum order where they coincide, From Eq. (i), This is the maximum order where they coincide, From Eq. (i), array ll & 2 1.4 t=3 420 & t= 2 420 2 1.40 =450

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