A lens can be a plastic or a glass, such that when light passes through it, the light rays get refracted.
Light rays from an object present on one side of a lens, when refracted, form an image on the other side of the lens.
Lenses can be divided into two main types-
- Convex or converging lens
- Concave or diverging lens
Image Formation by a Lens
The nature of images formed by a lens depends upon the distance of the object from the optical centre of the lens. To know the nature, size and position of the images, we need to consider at least two incident rays coming from the object whose paths after refraction are known.
The formation of images by lens is known as ray diagram.
Rules Used to Draw Ray Diagrams-
- An incident ray moving parallel to the principal axis will pass through the focus (for a convex lens) or appear to diverge from the focus (for a concave lens) after refraction.
- An incident ray passing through the focus (convex lens) or appearing to pass through the focus (concave lens) emerges parallel to the principal axis after refraction.
- An incident ray passing through the optical centre of both convex and concave lenses travels straight without any deviation.
Image Formation by a Convex Lens-
Image formation by a convex lens depends on the position of the object. The object is always placed to the left of the lens so that the incident rays travel from left to right. The object is represented by an upright arrow perpendicular to the principal axis of the lens.
Image Formation by a Concave Lens-
Rays of light falling on a concave lens always diverge after refraction. As a result, the refracted rays never happen to actually meet at a point.
They have to be produced backwards, in order to locate the point from which they appear to diverge. Thus, the image formed is always virtual, erect and diminished and located between the focus and optical centre on the same side of the lens as that of the object.
Uses of Convex Lenses:
- Convex lenses are used as magnifying glasses.
- They are used in spectacles to correct hypermetropia or long-sightedness. A long sighted person can see distant objects clearly, but is unable to focus on near objects.
- They are used in optical instruments such as microscopes, digital and video cameras, cinema projectors, telescopes, binoculars and webcams.
- The heat and light rays coming from sun are concentrated to a point using a convex lens. This heat can then be used to burn paper or dry wooden splinters.
Uses of Concave Lenses-
- They are used as an eyepiece in Galilean telescopes.
- They are also used in spectacles and contact lenses to correct myopia or short-sightedness. A short-sighted person can see near objects clearly, but has difficulty in focusing on distant object.
- They make the front glass of a torch in order to get a diverging beam of light.