A concave lens has focal length of 15 cm. At what distance should be object from the lens be placed so that it form an image at 10 cm from the lens?30 cm15 cm3 cm6 cm

To solve this problem, we can use the lens formula which is 1/f = 1/v - 1/u. Here, f is the focal length of the lens, v is the image distance, and u is the object distance.

Given that the focal length f of the concave lens is -15 cm (it's negative because concave lenses have a negative focal length), and the image distance v is -10 cm (it's negative because for a concave lens, the image is always formed on the same side as the object, which is taken as the negative side).

Substituting these values into the lens formula gives us:

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

Solving this equation for u (the object distance) gives us:

1/u = 1/-15 - 1/-10 1/u = -1/30 u = -30 cm

The negative sign indicates that the object should be placed 30 cm on the same side of the lens as the light source. So, the correct answer is 30 cm.