Question

a lens of focal length is 15 cm forms an erect image three times the size of the object what is the distance between the object ​

Answer 1

Solution :-

• focal length =15 cm

As per the given condition

• hi = + 3 ho ( as the image is erect )

Magnification is given by the formula ,

➝ m = v / u = hi / ho

here ,

• v = image distance
• u = object distance
• hi = height of image
• ho = height of object

hence ,

➝ v = hi x u / ho

➝ v = 3 ho x u / ho

➝ v = 3 u

As per lens formula ,

➝ 1 /f = 1 / v + 1 / u

➝ 1 / f = 1 / 3u + 1 / u

➝1 / f = 1 + 3 / 3 u

➝ 1 / f = 4 / 3 u

➝ u = 4f / 3

➝ u = 4 x 15 / 3

➝ u = 60 / 3

➝u = 20 cm

The distance between the object and the lens is 20 cm