To find the distance at which a 4 cm tall object should be placed from a concave mirror with a focal length of 20 cm, we can use the mirror formula:

1/f = 1/v - 1/u

Where:

f = focal length of the mirror (20 cm)

v = image distance (unknown)

u = object distance (unknown)

We can rearrange the formula to solve for v:

1/v = 1/f + 1/u

Substituting the given values:

1/v = 1/20 + 1/u

Now, let's solve for v when the object height (h) is 4 cm:

Magnification (m) = -v/u = h'/h

Since the object height (h) is positive and the image height (h') is negative (due to being inverted by a concave mirror), we have:

m = -h'/h = -4/v

Rearranging the equation:

v = -4/m

Now, substituting the value of m for a concave mirror, which is -1:

v = -4/(-1) = 4 cm

Therefore, the object should be placed 4 cm away from the concave mirror.

Main Avatar