A 10 mm long awl pin is placed vertically in front of a concave mirror. A 5 mm long image of the awl pin is formed at 30 cm in front of the mirror. The focal length of this mirror is
Here, the sized of the object.
h = 10 mm = 1 cm
Size of the image h₁ = 5 mm = 0.5 cm
Distance of image from the pole of mirror v = – 30 cm
Let distance of object = u
Focal length = f
Using mirror formula
1/v + 1/u = 1/f
Therefore, the value of “u” is needed for that magnification formula will be used:
Magnification (m) = h₁/h = – v/u
1/f = 1/v + 1/u
Putting all the values in mirror formula:
Share
[B] – 20 cm
Putting all the values in mirror formula:
1/(-30) + 1/(-60) = 1/f
Or, 2+1/-60 = 1/f
1/f = 3/-60
Or, f = -20 cm