Spherical Mirrors

Uses of concave mirror:

Shaving Mirror

When a concave mirror is held near the face such that the face is between the pole and the focus of the concave mirror, the mirror forms an upright and magnified image. So even the tiny hairs on the facecan easily be seen. For this a concave mirror of large focal length ( so that the face always lie between its focus and the pole) and large aperture ( so as to view the entire face) is used.


Doctor's Mirror

If a parallel beam of light is incident on a concave mirror, it is focuses the beam of light to a point. This fact enables us to use it as a doctor's head mirror to concentrate the beam of light on a small area of the body part to be examinedsuch as teeth, nose, throat and ear etc.

For this a parallel beam of light is made to fall on the concave mirror attached to the band tied at the fore head of doctor examining the body part.


Head lights

In torch light, search light and head lights of the vehicles a concave highly polished metallic surface is used as a reflector to obtain a parallel beam of light to focus at the longer distance.

For this, the source of light (i.e., bulb) is placed at the focus of the concave reflector ( mirror). The rays of light incident on the concave reflector from the bulb after reflection from a parallel beam.

Uses of Convex Mirror

Street Lamp

A convex polished metallic surface is used in street lamp as a reflector so as to diverge light over a larger area.

Rear View Mirror

Convex mirrors are used as rear view mirrors in vehicles. 
A driver prefers to use a convex mirror as a rear view mirror because of the following two reasons

1. A convex mirror always produces an erect image of the objects.

2. The images formed in a convex mirror is highly diminished or much smaller than the objec, due to which a convex mirror gives a wide field of view of the traffic behind.

Shop Security Mirrors

By placing a big convex mirror at a strategic point in the shop, the shop owner or the manager can keep an eye on the customers to look for thieves and shop lifters among them.

Uses of Convex Lenses

• Convex lenses are used in spectacles to correct the defffective vision called hepermetropia or longsightedness.

• Convex lens is used for making a simple camera.

• Convex lens is used as a magnifying glass (or magnifying lens)(by palmists, watchmakers, etc.).

• Convex lens is used in making microscopes, telescopes and slide projectors (or film projectors).

Uses of Concave Lenses

• Concave lenses are used in spectacles to correct the defect of vision called myopia (or shortssightedness).

• Concave lens is used as eye-lens in Galilean telescope.

• Concave lenses are used in combination with convex lenses to make high quality lens systems for optical instruments.

• Concave lens is used in wide-angle spyhole in doors.

Types of Mirror:

Plain Mirror: A mirror having a flat surface is called plane mirror.

Formation of image in plane mirror:

• A plane mirror always forms virtual and erect image.

• The distance of image and that of object is equal from the mirror.

• The image formed by a plane mirror is laterally inverted

Spherical Mirror: Mirrors having curved reflecting surface are called spherical mirrors. A spherical mirror is a part of a sphere.

Types of Spherical Mirror:

Concave Mirror: Spherical mirror with reflecting surface curved inwards is called concave mirror.

Convex Mirror: Spherical mirror with reflecting surface curved outwards is called convex mirror.

Important terms in the case of spherical mirror:

Pole: The centre of reflecting surface of a spherical mirror is known as Pole. Pole lies on the surface of spherical mirror. Pole is generally represented by ‘P’.

Centre of Curvature: The centre of sphere; of which the reflecting surface of a spherical mirror is a part; is called the centre of curvature of the spherical mirror. Centre of curvature is not a part of spherical mirror rather it lies outside the mirror. Centre of curvature is denoted by letter ‘C’.

In the case of concave mirror centre of curvature lies in front of the reflecting surface. On the other hand, centre of curvature lies behind the reflecting surface in the case of convex mirror.

Radius of Curvature: The radius of sphere; of which the reflecting surface of a spherical mirror is a part; is called the Radius of Curvature of the spherical mirror. The radius of curvature of a spherical mirror is denoted by letter ‘R’.

Similar to centre of curvature, radius of curvature lies in front of concave mirror and lies behind the convex mirror and is not a part of the mirror as it lies outside the mirror.

Aperture: The diameter of reflecting surface of a spherical mirror is called aperture.

Focus or Principal Focus: Point on principal axis at which parallel rays; coming from infinity; converge after reflection is called the Focus or Principal Focus of the spherical mirror. Focus is represented by letter ‘F’.

Principal Axis: Imaginary line passing through the centre of curvature and pole of a spherical mirror is called the Principal Axis.

In the case of a concave mirror, parallel rays; coming from infinity; converge after reflection in front of the mirror. Thus, the focus lies in front of a concave mirror.

In the case of a convex mirror, parallel rays; coming from infinity; appear to be diverging from behind the mirror. Thus, the focus lies behind the convex mirror.

Focal length: The distance from pole to focus is called focal length. Focal length is denoted by letter ‘f’. Focal length is equal to half of the radius of curvature.

Or, f=R/2

Or, R=2f