An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focussed image can be obtained ? Find the size and the nature of the image.

An object of size 7.0 cm is placed at 27 cm in front of a concave mirr

1.	An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focussed image can be obtained ? Find the size and the nature of the image.
Answer» Solution :u = -27 CM, F = -18 cm (As the mirror is concave), h = 7.0 cm From mirror formula `(1)/(v) + (1)/(u) = (1)/(f) rArr (1)/(v) + (1)/((-27)) = (1)/((-18)) rArr (1)/(v) = - (1)/(18) + (1)/(27) = (-3 + 2)/(54) = (-1)/(54)` or v = - 54 cm. The screen MUST be placed at a distance of 54 cm from the mirror in front of it. `m = - (v)/(u) = -(-54)/(-27) = -2`. The image is real, inverted and enlarged. Size of the image h. = m.h `= (-2) xx 7` cm = - 14 cm. Thus, the image is of 14 cm length and is an inverted image i.e., formed below the PRINCIPAL AXIS.