Question

v=60cm,
u=20cm,
then
f=?​

Answer 1

Answer:

is it a mirror or lens?

if mirror

1/f = 1/v +1/u = 1/-20 + 1/60 =

1/f = -2/60

f = -30 cm

If lens then

1/f = 1/v - 1/u

= 1/60 - 1/-20 = 4/60

f = 15 cm

Note that u is always negative

Answer 2

$\large \rm \blue {➸ \: \: Given}$

$\sf It \: is \: given \: that:-$

$\sf \bullet \: \: v = 60cm$

$\sf \bullet \: \: u=-20cm$

$\large \rm \blue {➸ \: \: To \: find}$

$\sf We \: have \: to \: find \: the \: value \: of \: f.$

$\large \rm \blue {➸ \: \: Solution}$

$\sf \green { \dfrac{1}{f} = \dfrac{1}{v}+ \dfrac{1}{u} }$

$\red ➞ \: \: \sf \cfrac{1}{f} = \cfrac{1}{60cm} - \cfrac{1}{20cm}$

$\red ➞ \: \: \sf \cfrac{1}{f} = \cfrac{1 -3}{60}$

$\red ➞ \: \: \sf \dfrac{1}{f} = \dfrac{-2}{60}$

$\red ➞ \: \: \sf f=-30cm$

$\sf \green {\therefore Focal \: length=15cm}$