Answer:

-20

Explanation:

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Verified answer

Answer: (2) 20 cm

The focal length of the concave mirror can be written as

1/f = 1/v + 1/u

Where

• f = focal length of the mirror
• v = distance of the image from the pole
• u = Distance of the object from the pole

Given:

• The image is formed in front of the mirror, so the image is real and is at a distance of 30 cm from the pole.

∴ v = -30cm

Magnifying factor (m) = m/u = -y/x

• y = Image height
• x = Object’s height
• m = -5/10

Since m = v/u

-5/10 = v/u

-5/10 = (-30)/u

∴ u = 60cm

Since object distance is always negative, so u = -60cm

Apply mirror formula as

(1/ v) + (1/u) = 1/f

1 /(- 60) + 1/(- 30) = 1/f

-1/60 – 1/30 = 1/f

-90/1800 = 1/f

1 /(- 20) = 1/f or

f = – 20 cm

The focal length lies in front of the mirror. Therefore, the sign is negative.