Light: Mirrors and Lenses
Class 08 ScienceSpherical mirrors are a common type of curved mirrors which are shaped like a part of a hollow glass sphere. Mirrors, whose reflecting surfaces are spherical are called spherical mirrors.
The reflecting surface of the spherical mirror may be curved inwards or outwards. A spherical mirror, which has a reflecting surface that curves inwards, is called a concave mirror. A spherical mirror which has a reflecting surface that curves outwards is called a convex mirror.
Images Formed by Mirrors
In the concave mirror, when the object is placed close to the mirror, the image is erect but larger than the object in size, that is, enlarged. However, when the object is moved farther away, the image becomes inverted. Initially, the image is enlarged in size and then keeps getting smaller.
In case of a convex mirror, the image is always erect and smaller than the object, that is diminished. However, the size of the image decreases slightly as the object is moved away from the convex mirror.
Spherical mirrors (concave and convex) behave differently from plane mirrors. A plane mirror always forms an erect image of the same size as the object. However, in the concave and convex mirrors, the size of the image changes as the distance of the object from the mirror changes. In addition, in the case of a concave mirror, the image also gets inverted when the object is taken away from the mirror. Lateral inversion of the image is seen in all three types of mirrors.
Side-view Mirrors on Vehicles
These mirrors are convex. They always form an erect image of the traffic behind and smaller than the actual vehicles. Also, since the convex mirror is curved outside, it provides a much wider area of the road behind.
Ray Diagrams
We represent light by straight lines with arrows, or rays. Rays indicate the path along which light travels. The ray of light that falls on the mirror is called the incident ray. The ray of light that comes back from the mirror is called the reflected ray.
From the point where the incident ray strikes the mirror, draw a line making an angle of 90° to the line representing the mirror. This line is known as the normal to the reflecting surface at the point of incidence, O.
The angle between the normal and the incident ray is called the angle of incidence (i). The angle between the normal and the reflected ray is known as the angle of reflection (r).
Laws of Reflection
- The angle of incidence (i) is equal to the angle of reflection (r).
- The incident ray, the normal to the mirror at the point of incidence, and the reflected ray, all lie in the same plane.
The laws of reflection are valid for all kinds of mirrors - plane and spherical.
When multiple parallel beams of light fall upon a plane mirror, the multiple reflected beams are also parallel. However, when multiple beams of light fall upon a concave mirror, the multiple reflected beams get closer, that is, they converge. Whereas, in the case of a convex mirror, the multiple reflected beams spread, that is, they diverge.
In the case of spherical mirrors, even though each ray of light follows the laws of reflection, the curved surface of spherical mirrors causes the parallel beam of rays to either converge (concave) or diverge (convex) on reflection depending on the shape of the mirror.
Lens
A lens is a piece of transparent material, usually made of glass or plastic, which has curved surfaces. Like mirrors, lenses can also be convex or concave. Unlike mirrors, lenses allow light to pass through them.
A lens which is thicker at the middle as compared to the edges is called a convex lens. A lens which is thicker at the edges as compared to the middle is called a concave lens.
When an object is placed behind a convex lens at a small distance from it and seen through the lens, the object appears erect and enlarged in size. As the distance between the object and the convex lens increases, the object appears inverted. It is initially enlarged in size and then diminishes in size.
An object placed behind a concave lens and seen through the lens, always appears erect and diminished in size. Its size changes, as its distance from the lens increases.
The convex lens converges the light falling on it while the concave lens diverges the light. A convex lens is also called a converging lens while a concave lens is called a diverging lens.