A concave mirror gives an image three times as large as its object placed at a distance of 20 cm from it. For the image to be real, the focal length should be

Question

A concave mirror gives an image three times as large as its object placed at a distance of 20 cm from it. For the image to be real, the focal length should be (A) 10 cm (B) 15 cm (C) 20 cm (D) 30 cm

Tardigrade Admin · Accepted Answer

Here, for real image, m = - 3 or - (v/u) = - 3 ∴ : :: :: v = 3 u Also, u = - 20 cm , so v = - 60 cm , f = ? Using mirror formuIa, (1/v) + (1/u) = (1/f) (1/-60) + (1/-20) = (I/f) - (4/60) = (1/f) ; : : ∴ : : f = 15 cm

Q. A concave mirror gives an image three times as large as its object placed at a distance of $20 c m$ from it. For the image to be real, the focal length should be

10 cm

15 cm

20 cm

30 cm

Here, for real image, $m = - 3$ or $- \frac{v}{u} = - 3$
$∴ v = 3 u$
Also, $u = - 20 c m,$ so $v = - 60 c m, f = ?$
Using mirror formuIa, $\frac{1}{v} + \frac{1}{u} = \frac{1}{f}$
$\frac{1}{- 60} + \frac{1}{- 20} = \frac{I}{f}$
$- \frac{4}{60} = \frac{1}{f}; ∴ f = 15 c m$

Q. A concave mirror gives an image three times as large as its object placed at a distance of 20cm from it. For the image to be real, the focal length should be