Question

2. The object distance will be when is real image is formed by a
converging lens of focal length 10cm magnified twice:

a. 5cm
b. 10cm
C. 15cm
O O
d. 20cm

Answer 1

Answer:

I think it is C 15 cm sure about that.

Answer 2

Answer:

• Correct option is (c) 15 cm

Explanation:

Given that, real image is formed by a converging lens ( convex lens ) of focal length 10 cm , that is magnified twice

therefore,

• Focal length of converging lens, f = 10 cm

[ focal length of converging lens is always positive ]

• magnification, m = - 2

[ when real image is formed then, magnification is negative ]

Let,

position of image be v and position of object be u

then,

Using formula for magnification of lens

→ m = v / u

→ - 2 = v / u

→ v =  -2 u

Using lens formula

→ 1/f = 1/v - 1/u

→ 1/(10) = 1/(-2 u)  -  1/u

→ 1/10 = ( -1 -2 )/( 2 u )

→ 1/10 = -3 / (2 u)

→ 2 u = -30

→ u = - 15 cm

Therefore,

• position of image is 15 cm infront of the lens .