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NEET 2010
Exam
NEET
Year
2010
Subject
Physics
Chapter
Wave Optics

NEET 2010 · Physics · Wave Optics

A lens having focal length f and aperture of diameter d forms an...

A lens having focal length ff and aperture of diameter d\mathrm{d} forms an image of intensity II. Aperture of diameter d2\frac{\mathrm{d}}{2} in central region of lens is covered by a black paper. Focal length of lens and intensity of image now will be respectively

Options

  1. Aff and 14\frac{1}{4}
  2. B3f4\frac{3 f}{4} and I2\frac{I}{2}
  3. Cf and 3I4\frac{3 I}{4}
  4. Df2\frac{\mathrm{f}}{2} and \frac{\mathrm{I}}{2}

Correct answer

C. f and 3I4\frac{3 I}{4}

Step-by-step solution

Intensity, I∝A2I \propto A^2 \Rightarrow \frac{\mathrm{I}_2}{\mathrm{I}_1}=\left[\frac{\mathrm{A}_2}{\mathrm{~A}_1}\right]^2=\frac{\pi \mathbf{r}^2-\frac{\pi \mathbf{r}^2}{4}}{\pi \mathbf{r}^2}=\frac{3}{4} ⇒\Rightarrow \quad\mathrm{I}2=34_2=\frac{3}{4}\mathrm{I}1_1 and focal length remains unchanged.
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