Question

how to find the sign of height of image ,object and image ,object distance of mirror and lens

Answer 1

An inverted image is magnified by 2 when the object is placed 22 cm in front of a concavemirror. Determine the image distanceand the focal length of the mirror. If M=2 and the image is inverted, then the di must be +44 cm.

Answer 2

For convex lens

the object distance is -ve.

image distance is +ve for real image and -ve for virtual image.

focal length is +ve.

For concave lens

the object distance is -ve

image distance is -ve

focal length is -ve.

The rule is simple,

Distances measured from optic center to the right are considered +ve and the distances measured from optic center to the left are -ve.

if you draw a line from the optic center and move towards right i.e., along the +ve x-axis then the sign is +ve.

similarly if you move towards the -ve x-axis, the distances are -ve.

for converging lens, the light rays meet at a point after refraction, thus it has positive focal length.

For diverging lens the light rays appear to meet at a point after refraction, thus virtual focal length and -ve sign.