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  4. A clock hung on a wall has marks instead of numerals on its dial. On the adjoining wall, there is a plane mirror and the image of the clock in the mirror indicates the time 7:10 . Then the time on the clock is

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Q. A clock hung on a wall has marks instead of numerals on its dial. On the adjoining wall, there is a plane mirror and the image of the clock in the mirror indicates the time $ 7:10 $ . Then the time on the clock is

ManipalManipal 2008Ray Optics and Optical Instruments

Solution:

In a plane mirror the image undergoes depth inversion hence, the time on the image is as much ahead of $12\, h\, 0\, $ min as the time on actual clock is behind $12\, h\, 0$ min.
Time on the actual clock $= 7\, h\, 10$ min
Time on the image clock
$= (12\, h$ and $0$ min) $ - (7 h$ and $10$ min) $= 4 h$ and $50$ min