Question

Pls answer it fast......I will mark as brainliest.

Answer 1

Here is your answer

first case ,focal length=30cm

second case ,when the object is moved 20cm towards the lens , the image will be formed at 60cm

Answer 2

Magnification is negative so this implies that the image is real and inverted.

Also magnification is 1 this means that the image and object are of the same size.

This is possible only if the object is kept at the centre of curvature.

So in this case image distance = object distance.

u = v = 60/2 = 30cm

u = -30cm and v = 30cm

Focal length is r/ 2

R = - 30cm so focal length is 15cm

It is a convex lens.

If object is moved 20 cm towards lens then u = -10cm

We know that

1/f = 1/v - 1/u (lens formula)

1/f + 1/u = 1/v

1/15 + -1/10 = 1/v

2-3/30 = 1/v

-1/30 = 1/v

v = - 30cm

m = v/u = -30/-10 = +3

So the image is going to be virtual as magnification is positive and object is kept between focus and optic centre.