Question

An object is placed at 20 cm from the pole of a concave mirror. it forms real image at a distance of 60 cm from the pole. Find the focal length of the concave mirror.​

Answer 1

Answer:

u=−20cm, f=20cm

From mirror formula

f1=v1+u1

201=v1+−201

v1=201+201

v1=202=v=10cm.

Answer 2

$\LARGE{ \underline{\underline{ \sf{Required \: answer:}}}}$

GiveN:

• Object distance = -20 cm
• Image distance = -60 cm
• Image formed was real and inverted.

To FinD:

• Focal length of the concave mirror?

Step-by-step Explanation:

You must be wondering, Why I took the object and image distance with negative sign. It is because they are measured opposite to the direction of light ray. Refer to the attachment.

Now using the mirror's formula:

$\large{ \boxed{ \rm{ \frac{1}{v} + \frac{1}{u} = \frac{1}{f} }}}$

Plugging the values of v and u,

⇒ 1/-60 + 1/-20 = 1/f

⇒ -1/60 - 1/20 = 1/f

⇒ -1- 3/60 = 1/f

⇒ -4/60 = 1/f

⇒ -1/15 = 1/f

⇒ f = -15 cm

Hence:

• Focal length of the concave mirro is -15 cm.
• Negative sign indicates opposite to the direction of incident light.