An object 4 cm in size is placed at 25 cm in front of a concave mirror of focal length 15 cm . At what distance from the mirror should a screen be placed in order to obtain a sharp image? Find the size of the image.

Given, The object size, h o = + 4 cm The object distance, u = 25 cm The focal length, f = 15 cm The image distance, v = ? The height of the image, h i = ? Using mirror formula, 1 f = 1 v + 1 u ⇒ 1 - 15 = 1 v + 1 - 25 ⇒ v = - 37 . 5 cm The magnification of the lens, m = - v u = h i h o ⇒ m = - v u = - - 37 . 5 - 25 = h i 4 Therefore, the image height h i = m × h o = - 6 cm The height of the image is 6 cm . To get a sharp image, v = 37 . 5 cm The image is real, inverted and magnified.

Where should an object be placed in front of a convex lens to get a real image of the size of the object?

At the principal focus of the lens.

Between the optical centre of the lens and its principal focus.

Draw a ray diagram and describe the image formation by a convex mirror when an object is placed at (i) infinity (ii) a finite distance.

(a) at infinity When an object is placed at infinity, the image formed is virtual, erect, and highly diminished. So the image formed is a point size image. (b) at a finite distance from the mirror. When the object is placed at a finite distance from the mirror the image is formed between the pole and principal focus.