A concave lens forms the image of an object kept at a distance 20 cm in front of it, at a distance 10 cm on the side of the object. (a) What is the nature of the image ? (b) Find the focal length of the lens.

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Solution

Object distance ,u=-20 cm
image distance ,v=-10 cm
(a)
The image is formed on the same side as the object .Hence ,it is a virtual image .Also ,since the lens is a concave lens the image will be erect and diminished
(b)
Lens formula is
1/f=1/v-1/u
1/f=1/-10-1/-20=-1/20
f = -20 cm

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(1) An object of size 7 cm is placed at 25 cm in front of concave mirror of focal length 15 cm. At what distance from the mirror should a screen be placed, so that we can get a sharp and clear image. Find nature and size of the image.

(2) An object of height 5 cm is held 20 cm away from converging lens of focal length 10 cm. Find the position, nature and size of the image formed.

(3) An object is placed at a distance of 10 cm from a convex lens of focal length 12 cm. Find the position and nature of the image.

An object is placed at a distance of 20 cm in front of a concave lens of focal length 20 cm. Find :

(a) the position of image, and

(b) the magnification of image.

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A concave lens forms the image of an object kept at a distance 20 cm in front of it, at a distance 10 cm on the side of the object. (a) What is the nature of the image ? (b) Find the focal length of the lens.

Object distance ,u=-20 cm image distance ,v=-10 cm (a) The image is formed on the same side as the object .Hence ,it is a virtual image .Also ,since the lens is a concave lens the image will be erect and diminished (b) Lens formula is 1/f=1/v-1/u 1/f=1/-10-1/-20=-1/20 f = -20 cm

Object distance ,u=-20 cm
image distance ,v=-10 cm
(a)
The image is formed on the same side as the object .Hence ,it is a virtual image .Also ,since the lens is a concave lens the image will be erect and diminished
(b)
Lens formula is
1/f=1/v-1/u
1/f=1/-10-1/-20=-1/20
f = -20 cm