Question

what should be the value of distance 'd' so that final image is formed on the object itself.(focal length of the lenses are written on the lense)​

Answer 1

Answer:

Explanation:

According to the question, the object distance will be same as that of the image distance since the image will lie on the object.

Hence, image distance v = -(10+d/2)

[-ve since convex lense ]

       

Object distance, u = (10+d/2)

NOW, effective focal length f = - 1/(10+d/2) + 1/(10+d/2)

So, by using the formulae of effective focal length we get.

f= 1/10 - 1/20 + d/10*20

Thus, equating both sides we get -

1/10 - 1/20 + d/10*20 = - 1/(10+d/2) + 1/(10+d/2)

1/10 - 1/20 + d/10*20 = 0

d/10*20 = 1/ 20

d= 10 cm