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Wave Optics — Physics STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 SciencePhysicsWave Optics1 Mark
Question
For question two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is NOT the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion (A): In Young's experiment the fringe width is directly proportional to wavelength of the source used.
Reason (R): When a thin transparent sheet is placed in front of both the slits of Young's experiment, the fringe width will increase.
✓

Answer

  1. A is true but R is false.
Explanation:

Fringe width $\beta=\frac{\lambda\text{D}}{\text{d}}$ shall remain the same as the waves travel in air only, after passing through the thin transparent sheet. Due to introduction of thin sheet, only path difference of the wave is changed due to which th ere is shift of position of fringes only, which is given as $\triangle\text{x}=\frac{\text{D}(\mu-1)\text{t}}{\text{d}}$, where $\mu$ is refractive index of thin sheet and tis its thickness.

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