Question

Rakhi conducts an experiment to produce an image of an object on a screen which is placed at 20 cm from the lens. She uses a convex lens of focal length 15 cm for the experiment.Where should she place the object in order to produce the sharpest image?

Answer 1

Answer:D – using a lens of longer focal length, when light rays pass through the lens, it will converge to a point further away from the lens. Hence the sharp image will be formed on the school. Alternatively, you can move the screen nearer to the lens such that the position of the screen is at the intersection of the rays.For convex lens, virtual image will form when object is in-between focus and optical center. For concave lens, virtual image will form for any location of object, and corresponding image is lying between focus and optical center.Plane mirrors, convex mirrors, and diverging lenses will always produce an upright image. A concave mirror and a converging lens will only produce an upright image if the object is located in front of the focal point.

Explanation:

Answer 2

In the given question,

the image distance(v)= +20 cm

The focal length (f) of convex lens = + 15 cm

The object distance (u)= u cm

For convex lens the mirror formula=

1/v -1/u = 1/f

1/v-  1/f = 1/u

(f -v)/v*f = 1/u

v*f/(f -v)= u

Given, v= +20 cm and f =+ 15 cm

u= (20*15)/(15-20)

u = 300/(-5)

u= (-60) cm

Hence, Rakhi should place the object at a distance of 60 cm to obtain the sharpest image.