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Light Reflection and Refraction
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01/12/2020 12:09 pm
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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
(a) -30 cm
(b) -20 cm
(c) -40 cm
(d) -60 cm
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01/12/2020 12:12 pm
(b) -20 cm
Explanation:
Given h0 = +10 mm = + 0.1 cm
h2 = -5 mm = -0.5 cm
for real image, v = -30 cm
Now, magnification, m = hi/h0 = - v/u
1/f = 1/v + 1/u = 1/-30 + 1/-60
= -2 - 1/60
= - 1/20
∴ f = -20 cm