Question

A lens produces a real image of a real object.

a) Is the image inverted or upright?

b) Is the lens diverging or converging?

c) Is the image enlarged or reduced in size?

d) If two convex lenses identical in size and shape aremanufactured from glass with two different indices of refraction,would the focal length of the lens with the greater index ofrefraction (lens 1) be larger or smaller than that of the otherlens (lens 2)?

Answer 1

Concepts and reason

The concept used to solve this problem is lens properties.

Initially use the properties of image to find the orientation of image. Then use the lens properties to find the nature of the lens. Finally use the relation of focal length and refractive index to compare the focal length.

Fundamentals

The properties of lens are as follows:

The diverging lens always produced is diminished virtual and upright image.

The converging lens can produce both real and virtual image.

The focal length of the lens is inversely proportional to the refractive index of the material.

The property of images formed by lens are as follows:

Real image is always inverted and Virtual image is always upright.

Real image is diminished or enlarged according to position of object.

Virtual image formed by diverging lens is always diminished.

(a)

The lens produced a real image of real object. From the properties of image formed, a real image is always inverted. Thus, the image is inverted.

(b)

The image produced by the lens is real. Diverging lens produces only virtual image. The converging lens produces real image. Thus, the lens is converging.

(c)

The lens produced a real image of real object. The converging lens can produce a real image of any size that is enlarged, same or diminished according to the position of the real object. The position of the object with respect to lens is not given so the size of the image cannot be determined.

(d)

The convex lenses are of same size and shape except different refractive indices of glass they are made of. The focal length is inversely proportional to the refractive index. So, the focal length of the lens with greater index of refraction is smaller. The rays converge more for a greater refractive index glass and thus the point where they converge is short than for another lens.

Ans: Part a

The image is inverted.