An object is placed in front of a concave mirror of focal length 20cm. The image formed is 3 times the size of the object. Calculate two possible dictances of the object from the mirror.

Asked by Srishti Mallick | 8th Oct, 2013, 08:14: PM

Expert Answer:

The mirror formula is

1/f = (1/v) + (1/u)    ….. (1)

Given

f = focal length = -20cm

v = image distance

u = object distance

we will assume that the following is given

hi = image height

ho = object height

hi = 3ho

or

hi / ho = 3

 

Now,

hi / ho = -v/u = 3

we get,

v = -3u

so, using this in eq. (1), we get

-1/20 = -1/3u + 1/u

or

-1/20 = 2 / 3u

or

u = -13.33 cm

Thus, the object’s distance / position of the object will be

u = -13.33 cm

 

Answered by Faiza Lambe | 8th Oct, 2013, 09:45: PM

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