Question

(5) when an object is placed at
a distance of 15 cm from a
concave mirror, its image is
formed at 10 cm in front of the
mirror. Calculate the focal
length of the mirror.​

Answer 1

Your question :

When a object is placed at a distance of 15cm from a concave mirror, it's image is formed at 10 cm in front of the. mirror. Calculate the focal length of the mirror.

Formula should be used :

1/f = 1/v + 1/u

Answer :

u = -15cm

v = - 10 cm ( - because it's a concave mirror)

f = ?

1/v + 1/u = 1/f

1/f = 1/-10 + 1/-15

1/f = 1/-10 - 1/15

1/f = -3 - 2 / 30

1/f = -5 / 30

f = -30 / 5

f = -6 cm

Answer 2

Given:-

• Object - distance ,u = -15cm

• Image - distance ,v = -10cm

To Find:-

• Focal length ,f.

Solution:-

Type of mirror concave mirror

By using mirror formula

• 1/v +1/u = 1/f

Substitute the value we get

→ 1/-10 + 1/-15 = 1/f

→ 1/f = -1/10 -1/15

→ 1/f = -3 -2/30

→ 1/f = -5/30

→ 1/f = -1/6

→ f = -6 cm

∴ The Focal length of the mirror is 6cm.