Question

A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram. 

Answer 1

Focal length f = −15 cm   ...(negative due to sign convention)
Image distance v = −10 cm   ...(negative due to sign convention)
Object distance u = ?
Applying the lens formula:

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

⇒ `1/"u" = 1/"v" - 1/"f"`

⇒ `1/"u" = 1/(-10) - 1/(-15)`

⇒ `1/"u" = -1/10 + 1/15`

⇒ `1/"u" = (-3 + 2)/30`

⇒ `1/"u" = (-1)/30`

u = − 30 cm

The object should be placed at a distance of 30 cm from the concave lens and on the left side.