Question

an object is kept in front of the converging lens focal length is 15 at 20 cm. find the distance of image?

Answer 1

the object is placed at a position which is between f and 2f

so the image will be formed between infinity and 2f.

logic is had the object been at f …the image will be at infinity and if its at 2f the image will come to 2f

using the mirror rrelation

1/v + 1/u = 1/f where u, v , f are object distance, image distance and focal length respectively.

u= 20 cm f=15 cm (the sign convention is coordinate convention with mirror at origin and object on positive x-axis)

so 1/v = 1/f - 1/u , = 1/15 - 1/20

= 5/(15x20)= 1/60 therefore v= 60 cm

so the image will be formed at 60 cm

with magnification m= v/u = 60/20 = 3

the image will be 3 times larger and real