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
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