Question

A concave mirror has a focal length 20 cm . At what distance from the mirror should a 4 cm tall object be placed so that it forms an image at a distance of 30 cm from the mirror ? Also calculate the size of image formed


pls explain me what will be the sin convention of V...and why??/

Answer 1

v= 30 cm

f= -20 cm

h=4cm

By mirror formula,

1/v + 1/u = 1/f

Putting the values,

1/30 + 1/u = 1/-20

1/u = -1/20 - 1/30

1/u = (-3-2)/60

1/u = -5/60

1/u = -1/12

u = -12 cm

M= h'/h = -v/u

h' = -vh/u

h' = -(30×4)/-12

h' = 120/2

h' = 10

So, the image is virtual and erect.

Image is larger than the object.

Object didtance = -12cm

For sign convention, refer to the attachment