Question

l. An object is placed at a distance of 20 cm before a concave mirror. A real
image is obtained at a distance of 40 cmn from the mirror. Calculate the
focal length of the mirror
1) 16.3
3) 14.3
4) -17.3
28-13.3​

Answer 1

Answer:

-13.33 cm

Step-by-step explanation:

u=-20 cm

v=-40 cm (real image is formed)

mirror formulae

1/f = 1/u + 1/v

1/f = -1/20 - 1/40

f= -40/3

=-13.33 cm

Answer 2

$\Large\mathrm {\fcolorbox {cyan} {black} {\pink{Answer}}}$

$\Large\frac{1}{f}$ = $\Large\frac{1}{u}$ + $\Large\frac{1}{v}$

${\:}$

$u \: = \: - 20 \: cm \\ v \: = \: - 40 \: cm$

$\Large\frac{1}{f}$ = $\Large\frac{1}{-20}$ + $\Large\frac{1}{-40}$ =

$\Large\frac{2+1}{-40}$ = $\Large\frac{3}{-40}$

$\large{focal\:length}$ = $\Large\frac{-40}{3}$ =

$\large\orange{–13.33cm\:is\:focal\:length.}$