Question

6. An object is placed 36cm from a converging lens of focal length 24cm. If a real image which is 4cm high is formed, calculate the height of the object

Answer 1

Answer: h = 2cm

Explanation:

Lens formula (for lens) : 1/v - 1/u = 1/f

Magnification formula : M = h'/h = v/u

(where, symbols have their usual meanings)

Given,

u = -36cm (why 'minus' sign? Because of the sign convention i.e. object placed at left side of lens have negative distance)

v = ?

f = +24cm

From lens formula;

1/v - 1/-36 = 1/24

1/v = 1/24 - 1/36 = 1/72

=> v = 72cm

Now, from Magnification formula;

v/u = h'/h

=> 72/-36 = -4/h ( why '-4' because real image is always negative)

=>h = (-4 * -36)/72 = 2cm