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An image 2/3rd the size of the object is formed
by a concave mirror at a distance of 20 cm from​

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student-nameBhavaya Sharma asked in Science

An image 2/3rd the size of the object is formed by a concave mirror at a distance of 20cm from it.Find the focal length of the mirror

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student-nameSeelak Punia answered this

22 helpful votes in Science, Class XII-Science

Since concave mirror forms a real image therefore image distance is -ve, i.e., -20 cm

Since magnification for real image is always -ve therefore magnification (m)= -2/3

using m= -v/u

-2/3= - (-20/u)

-2/3= 20/u

-2/3x20 = 1/u

-1/30 = 1/u

u = -30

Now using mirror formula

1/f = 1/v + 1/u

1/f = 1/-20 + 1/-30

1/f = -1/20 - 1/30

1/f = -3 -2 /60

1/f = -5/60

1/f = -1/12

f = -12

Therefore focal length of the mirror is -12 cm