Question

TA projector makes use of
A. Convex lens only
B. Concave lens only
C. A combination of convex and concave lenses
D. A glass block instead of any lens

Answer 1

Hint: To project an image on a screen using a projector, we need a lens that can magnify the given object and can form real images. A real image can be projected on a screen while a virtual image cannot be projected on a screen.

Complete step by step answer:
A projector is an optical device that can magnify images and obtain them on a screen in the magnified form. To obtain these characteristics, we need a lens that can magnify the objects as well as form real images which can be obtained on a screen.
Convex lens has the property of converging light rays incident on it. A convex lens can form both real images as well as virtual images. Real images can be obtained on a screen but a virtual image cannot be obtained on a screen. So, we can use a convex lens in a configuration in which it produces real images to be used in projecting images in a projector.
We also need magnification from a convex lens which can be obtained by placing the object beyond the focus of the lens. The following figure shows a convex lens where we can obtain a magnified image by placing the object between f and 2f.

We observe that the image formed is inverted. SO, to obtain erect images on a projector, the object that is to be magnified must be placed behind the projector by inverting it.

Note: We don’t use a concave lens in a projector because of the following reasons:
A concave lens always forms a virtual image. A virtual image cannot be obtained on a screen.
A concave lens forms images that have positive magnification which means that size of the image is smaller than the object.