Question

A concave lens of focal length 20cm forms an image at a distance of 10cm away from the lens what is the distance of object from lens

Answer 1

Answer:

The answer will be 20cm(-ve)...

Answer 2

The distance of object from lens is 20cm.

Given

focal length of concave lens =  20 cm

distance of image  =  10 cm

To Find

We need to find the distance of object from lens.

Solution

Lens formula for concave and convex lens is given by,

   $\frac{1}{f}$  =  $\frac{1}{v}$ -  $\frac{1}{u}$

F = Focal length of the lens.

     positive for convex lens.

     negative for concave lens.

u = distance of the object form the middle of the lens.

v = distance of the image from the middle of the lens.

f = -20 cm              v = -10cm

$\frac{1}{-20}$  =  $\frac{1}{-10}$   -   $\frac{1}{u}$    

$\frac{1}{u}$     =  $\frac{1}{20}$  -  $\frac{1}{10}$

$\frac{1}{u}$     = -  $\frac{1}{20}$

u = -20 cm

Negative sign shows that the distance is measured opposite to the incident ray.

Hence distance of object from lens is 20 cm.

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