# NEET Physics Ray Optics and Optical Instruments Questions Solved

BOARD

Use the mirror equation to show that an object placed between

f and 2f of a concave mirror produces a real image beyond 2f.     (2 marks)

$\frac{1}{\mathrm{f}}=\frac{1}{\mathrm{v}}+\frac{1}{\mathrm{u}}$                                                              (1/2 mark)

For concave mirror f<0 and u<0

As object lies between f and 2f

(i) At u=-f

$⇒\frac{1}{\mathrm{v}}=-\frac{1}{\mathrm{f}}+\frac{1}{2\mathrm{f}}=-\frac{1}{2\mathrm{f}}$                                        (1/2 mark)

$⇒$=-2f                                                                 (1/2 mark)

$⇒$Hence, image distance v$\ge$-2f                             (1/2 mark)

Since v is negative, therefore, the image is real.

Alternative Method

$\frac{1}{\mathrm{f}}=\frac{1}{\mathrm{v}}+\frac{1}{\mathrm{u}}$                                               (1/2 mark)

For concave mirror                                      (1/2 mark)

f<0, u<0

so, 2f<u<f

$⇒\frac{1}{2\mathrm{f}}<\frac{1}{\mathrm{v}}<0$                                                       (1/2 mark)

$⇒$v<0    so, Image is real

Also v>2f        image is formed beyond 2f.                 (1/2 mark)

(Any alternative correct method should be given full credit.)

Difficulty Level:

• 82%
• 19%
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