1.

An object is placed at 20 cm in front of a concave mirror produces three times magnificed real image. What is the focal length of the concave mirror ?

Answer»

15 cm
6.6 cm
10 cm
7.5 cm

Solution :(a) We know that
Linear magnification `m = (f)/(f - U)`
Given, object displaced u = - 20 cm
`(therfore "all REAL image are inverted")`
So, "" `-3 = (f)/(f - (-20))`
`rArr "" -3 = (f)/(f + 20) rArr - 3F - 60 = f`
`4f = - 60 rArr f = - (60)/(4) = - 15 cm`
Since, mirror is concave f = - 15 cm.


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