Question

An object of height 10 cm is placed 25 cm away from the optical centre of a converging lens of focal length 15 cm. Calculate the image distance and height of the image formed.

Answer 1

Object height, O = +10 cm

Focal length, f = +15 cm

Object distance, u = −25 cm

Image distance, v = ?

Image height, I = ?

Using lens formula,

`1/"f" = 1/"v" -1/"u"`

`1/15 = 1/"v" -1/((-25))`

Or, `1/"v" = 1/15-1/25`

`1/"v" = (5 - 3)/75`

= `1/"v"=2/75`

v = `75/2`

Also, `(-"v")/"u"="I"/"O"`

Hence, `(75/2)/-25 = "I"/10`

`75/2 xx 1/-25 = "I"/10`

`3/-2 = "I"/10`

`-1.5  = "I"/10`

I = (-1.5) × 10 =  −15 cm

Height of image is −15 cm, inverted.