Question

solve question no 4 solve fast

Answer 1

$\underline{\underline{\mathbb{ANSWER}}}$

GIVEN -

f = -20cm (This is a concave mirror)

m = $\frac{-1}{5}$

TO FIND-

The object distance (u)

SOLUTION-

We know that,

m = $\frac{-v}{u}$

$\frac{-1}{5}$ = $\frac{-v}{u}$

5v = u

Mirror formula,

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

Substituting 5v =  u in the formula we get

$\frac{1}{v}$ + $\frac{1}{5v}$ = $\frac{1}{-20}$

$\frac{5v+v}{5v^{2}}$ = $\frac{-1}{20}$

$\frac{6v}{5v^{2}}$ = $\frac{-1}{20}$

6v (20) = -5v²

120v = -5v²

5v² + 120 = 0

5v(v + 40) = 0

v= 0 or v = -40cm

v canno be equal to zero, Hence, v = -40cm

They asked for image distance(u)

5v = u

5(-40) = u

u = -120cm

The object is placed 120 cm from the mirror

Answer 2

the mirror is kept at 120 cm from object