A concave mirror of focal length `20 cm` is placed `50 cm` from a wall. How far the wall should an object be placed to form a real image on the wall ?

Explanation

Here, `f = -20 cm, v = -50 cm`
Let the object `AB` be held at a distance `x` from the wall. Image `A'B'` is formed on the wall, (Fig. 11(a).16)
`u = -(50 - x) cm`
From `(1)/(v)+(1)/(u) = (1)/(f)`
`(1)/(u) = (1)/(f) - (1)/(v) = (1)/(-20) - (1)/(-50) = (-5 + 2)/(100)`
`u = - (100)/(3) cm = -33.3 cm`
As `u = -(50 - x) = -33.3`
`x = 50 - 33.3 = 16.7 cm`.
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