Question

14. 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

$\huge{\underline{\tt{\blue{Answer-}}}}$

u = -15cm, v = -10cm

f = ?

We know that

1/v + 1/u = 1/f

⇒ 1/(-10) + 1/(-15) = 1/f

⇒ 1/f = -(1/10) – (1/15) =  (-3 – 2)/30 = - (5/30) = -(1/6)

∴ f = - 6 cm

∴ The focal length of the concave mirror is 6 cm

Answer 2

Question :–

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.

Given :–

• u = -15 cm
• v = -10 cm

To Find :–

• Focal length of the mirror.

Solution :–

We know that,

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

So,

→ $\frac{1}{ - 10} + \frac{1}{ - 15} = \frac{1}{f}$

→ $- \frac{1}{10} - \frac{1}{15}$

We have to find L.C.M. of the denominators,

★ L.C.M. is refer to the attachment ★

→ $\frac{ - 3 - 2}{30 }$

→ $- \frac{ \cancel5}{\cancel{30}}$

→ $- \frac{1}{6}$

Therefore,

• f = -6 cm

Hence,

The focal length of the mirror is 6 cm.