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प्रश्न
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 ______.
पर्याय
–30 cm
–20 cm
–40 cm
–60 cm
MCQ
रिकाम्या जागा भरा
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उत्तर
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 –20 cm.
Explanation:
`m = h^'/h = (-v )/u` or
`u = ((-v) xx h)/h^'`
`= (-(-30 cm) xx (10 mm))/(-5 mm) `
= −60 cm
∴ `1/f = 1/u + 1/v`
= `-1/60 - 1/30`
= `-1/20` or
f = −20 cm
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