Question

a concave lens of focal length 15 cm forms an image 10cm from the lens. how far is the object placed from the lens draw the ray diagram

Answer 1

Answer:

Focal length (f ) = -15 cm

Distance of image (v) = -10 cm

1v–1u=1f

1/u = -(1/10) – (1/- 15)

1/u =1/15 – 1/10

1/u = -0.033

u = -30 cm

so the object is placed 30 cm away from the concave lens.

Answer 2

Given: focal length of concave lens, f = 15cm

           distance of image from the lens, v = 10cm

To Find: distance of the object from the lens, u.

Solution:

To calculate u, the formula used:

• 1 / focal length = 1/ image distance - 1/ object distance from lens
• 1 / f = 1/v - 1/ u          ⇒ Mirror formula
• Here, f = - 15cm

                 u = - 10 cm

        Negative indicates that the lens is concave.

Applying the above formula:

1 / -15  = (1 / -10) - (1/ u)

1/ -15 + 1 / 10 = - 1/ u

-1/15 + 1/ 10 = - 1/ u

1/ u = 1/ 15 - 1/10

      = 2 - 3 / 30

      = - 1 / 30

1 / u = - 1/ 30

u = -30 cm

Hence, the distance of the object from the concave lens is 30 cm.