Concave Mirror Image Formation

Concave Mirror Image

A concave mirror is the part of a spherical mirror of which the internal curved surface is the reflecting surface. In a concave mirror, the reflecting surface seems to be away from the source of light. 

Due to the curved shape, the incident light is converged (i.e., reflected inwards). Due to this, a concave mirror is also called a converging mirror. Concave mirrors are used for focusing light.

Reflection from a concave mirror follows the laws of reflection. The normal to the point of incidence is drawn along the radius of the mirror, i.e., it is drawn by joining the centre of curvature with the point of incidence. 

The normal to the reflecting surface of the mirror is different for each point of the mirror and this causes the convergence of rays after reflection.


How is the Image Formed in a Concave Mirror?

The formation of an image that occurs in a concave mirror mainly depends on the distance between the object and the mirror. Both real and virtual images are formed by the concave mirror. 

When the object is placed very close to the mirror, a virtual and magnified image is formed. 

When the distance between the objects in the mirror is increased, the size of the image gets diminished and a real image is formed. 

The real image of a concave mirror can be projected on the screen. The focal point and the centre of curvature of a concave mirror lie in front of the mirror.

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Important Terms

  • Pole: The centre of the reflecting surface of the concave mirror is called its pole. The pole lies on the surface of the concave mirror and is denoted by P.

  • Centre of Curvature: Centre of curvature of the mirror is measured through the centre of the sphere in which the concave reflecting surface is a part. 

It lies external to the reflecting surface of the mirror and is denoted by C. For a concave mirror, the centre of curvature lies in front of the mirror.

  • The Radius of Curvature: It is the radius of the sphere which forms the reflective part. It is denoted by R.

  • Principal Axis: The principal axis is a straight line that passes through the pole and the centre of curvature of the mirror. It is always perpendicular to the pole.

  • Principal Focus: After reflection, the reflected rays which are parallel to the principal axis converge at a common point on the principal axis. This point is called the principal focus. It is denoted by F.

  • Focal Length: The distance between the pole and the principal focus is called the focal length. It is denoted by f.

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Image Formation By a Concave Mirror

  • When the object is placed at infinity.

  • When the object is placed beyond the centre of curvature of the mirror.

  • When the object placed exactly at the centre of curvature C.

  • When the object is placed between the focus and centre of curvature.

  • When the object is placed exactly at F.

  • When the object is placed between F and pole of the mirror, P.


I. When the Object is at Infinity

Two rays originating from the object and parallel to the principal axis reflects from the mirror. After reflection, the rays converge to form an image at point F. The image formed will be highly diminished, real, point size and inverted.

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II. The Object is Placed Beyond C

Two rays emerge from the object, one of which is parallel to the principal axis and the other moves towards the centre of curvature of the mirror. 

After reflection, the image is formed between the centre of curvature C and the focus F. The image formed is diminished, inverted and real.

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III. The Object is Placed at C

Two rays emerge from the object. One of the rays travels parallel to the principal axis and the other passes through the focus. 

After reflection, the image is formed at C. The image formed is of the same size as that of the object; it is real and inverted.

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IV. Concave Mirror Object Between C and F

Two rays emerge from the object. One of the rays is parallel to the principal axis and the other ray passes through the focus of the mirror. 

This is the image created beyond the centre of curvature C of the mirror. The size of the image is magnified; it is real and inverted.

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Concave Mirror Image Formation Table

Position of Object

Position of Image

Nature/Size of the Image

Use

At infinity

At the focus

Real, inverted & diminished

As a collector of radiation in solar heating devices

Beyond the centre of curvature

Between the focus and the centre of curvature

Real, inverted & diminished

NA

At the centre of curvature

At the centre of curvature

Real, inverted & of the same size as the object

As a reflecting mirror behind a projector lamp

Between the centre of curvature and focus

Beyond the centre of curvature

Real, inverted & magnified

In floodlights

At focus

At infinity

Real, inverted & magnified

As a reflecting mirror in car, headlights, searchlights, etc

Between the pole of the mirror and the focus

Appears behind the mirror

Virtual, erect & magnified

As a shaving mirror, or makeup mirror and dentist's mirror

FAQ (Frequently Asked Questions)

Q1. What Kind of Image is Formed By a Concave Mirror?

Ans: Real and virtual images are formed by concave mirrors. The images formed can be upright (if virtual) or inverted (if real). 

The position of the image can be behind the mirror (if virtual) or in front of the mirror (if real). The size of the image can be enlarged, diminished or of the same size as the object.

Q2. Is a Concave Mirror Image Real?

Ans: A real image is formed when the rays are converged. When the rays diverge from a point, a virtual image is formed. When the object is positioned at a distance from the mirror, real images can be formed by a concave mirror. 

However, the distance of the object should be less than the focal point. The image is real and inverted.

Q3. How Does Light Reflect Off a Concave Mirror?

Ans: When the incident light hits a concave mirror, the light rays get reflected inwards towards the focal point F. 

The light rays, which get reflected by the same angle, hit the small part of the surface. 

These light rays get reflected inwards and meet at the focal point.

Q4. What are the Rules of the Concave Mirror?

Ans: The rules of a concave mirror are:

If the incident rays travel parallel to the principal axis, it passes through the focal point after reflection.

In case the incident ray passes through the focal point, it will travel parallel to the principal axis.