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Question
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
Options
–30 cm
–20 cm
–40 cm
–60 cm
Solution
–20 cm
Explanation -
`"m" = "h"^'/"h" = (-υ )/"u"` or
`"u" = ((-υ ) xx "h")/"h"^'`
`= (-(-30 "cm") xx (10 "mm"))/(-5 "mm") `
= -60 cm
∴ `1/"f" = 1/"u" + 1/υ`
= `-1/60 - 1/30`
= `-1/20` or f = -20 cm
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