Question

a concave mirror gives an image three times larger as the object placed at a distance of 20 CM from it. for the real image is the focal length is​

Answer 1

Answer:

u=20

magnification 3times

u=3v

m= -v/u

m= -v/u

3= -v/u

3u= -v

-v/u=1/3

-v=1

u=3

1/f= 1/v+1/u

1/f=1/-1+1/3

1/f=-2/3

f=-3/2

hence focal length is -3/2

Answer 2

Answer:

Thus, For the image to be real, the focal length should be -15 cm

Explanation:

=> According to the question, a concave mirror gives an image three times as large as the object placed at a distance of 20cm from it.

Thus, object distance, u = -20cm

As image is real. thus, m = -3

=> As per the Magnification formula for mirror :

m = -v/u

-3 = -v/(-20)

-v = -3 × -20

v = -60 cm  

According to Mirror formula:

1/v + 1/u = 1/f

1/(−60) +1/ (−20) = 1/f

∴ 1/f = (−1−3) /60

= −4/60  

f= −15 cm

Thus, For the image to be real, the focal length should be -15 cm