Introduction

Mirror

Plane Mirror

If the reflecting surface of a mirror is flat and smooth, this type of mirror is called a plane mirror. Plane mirrors are used as looking glass at home and in the bathroom. To draw a mirror we first draw a straight line as a reflecting surface. on the other side, we shade to indicate the non-reflecting or polished surface of the mirror.

Characteristics of image formed by plane mirror

The image formed by a plane mirror is erect (straight), virtual (that cannot be obtained on a screen), of the same size as the object, and formed at a distance equal to the object's distance. The image thus formed on a plane mirror is laterally inverted (or left appearing right and right appearing left).

Spherical Mirror

We use plane mirrors as a looking glass in our homes, parlors, etc. But all mirrors do not have plane surfaces. So mirrors have surfaces depressed in the middle while some mirrors have raised surfaces in the middle. Mainly, there are two types of spherical mirrors:

1. concave mirror
2. convex mirror

Before proceeding further lets us learn what is concave and convex surfaces. Consider a spoon as shown in the figure below. The inner surface of the spoon is depressed which is called concave surface. Look at the outer surface which is raised at the center, called a convex surface.

Concave Mirror

The mirror which has raised outer part and the uniformly depressed middle part is called a concave mirror. The parallel beam of light is converged at a point after reflection on a concave mirror. This point is called the focus. A concave mirror is also called a converging mirror because it converges a parallel beam of light incident on its surface. Similarly, rays of light coming from its focus are reflected back parallel to its principal axis.

Convex Mirror

The mirror which has a depressed outer part and uniformly raised middle part is called a convex mirror. A parallel beam of light diverges after reflection on the surface of a convex mirror. So, it is also called a diverging mirror.

The rays of light reflected back from a convex mirror appear to be coming from a point. This point is called the focus of the convex mirror. It forms an erect and diminished image. The image of large objects located at a far distance is formed at near with diminished size.

Real and virtual image

The image that can be obtained on a screen is called a real image. It is formed by the actual intersection of the reflected rays. An image that cannot be obtained on a screen is called a virtual image. It is formed by the apparent intersection of the reflected rays but they don't intersect each other in real. A concave mirror can form a real image but convex and plane mirrors from virtual images.

Click here to read the difference between real and virtual images. or navigate to the solution section of this note

Terms related to Spherical Mirror

Some terms are frequently used while doing different activities using spherical mirrors. They are explained as below:

Pole of mirror

The geometrical center of the reflecting surface of a mirror is called the pole of the mirror. It is denoted by P. All the distances are measured from this point.

Center of curvature

A spherical mirror (Concave or Convex) is a part of a whole sphere of glass. The center of the sphere from which the mirror is formed is called the center of curvature. It is denoted by C.

Radius of curvature

The radius of the sphere in the spherical mirror is called the radius of curvature. It is the distance between the center of curvature (C) and the pole of the mirror (P). It is denoted by R.

Principal axis

A parallel beam of light incident on a concave mirror gets converged at a point after reflection. This point is called the principal focus. It lies on the principal axis in front of the mirror. The parallel beam of light is diverged after reflection on the surface of the convex mirror but appears to be coming from a point. This point is called the focus of the convex mirror. The focus of a convex mirror lies behind the mirror and it is denoted by F.

Focal length

The distance between the principal focus (F) and the pole of a mirror (P) is called the focal length. It is denoted by (f) and measured in meters.

Image formed by spherical mirror

The diagram that represents the image formed by a spherical mirror is called a ray diagram. Ray diagram ascertain the size, location, and distance of the image formed by a spherical mirror.

Rules for drawing ray diagrams in a concave mirror

To learn the procedure to draw a ray diagram for a concave mirror read this question or navigate to the exercise section

Ray diagram for concave mirror

The size, position, and nature of the image formed by a concave mirror depend upon the position of the object. So, the nature of the image of an object placed at different positions from a concave mirror is described as below:

Object at infinity

When an object is placed at infinity, its image is formed at focus. The image is real, inverted, and highly diminished in size.

Object at a point beyond C

When an object is placed at a point beyond C, its image is formed between C and F. The image is inverted, real and diminished.

Ray diagram for Convex mirror

Before drawing a ray diagram for the convex mirror, first look at the rules for drawing a ray diagram in a convex mirror. You can also navigate to the exercise section. The image of an object is formed behind the convex mirror. The image is formed by a convex mirror that is always virtual, erect, and diminished.

Object at infinity

When an object is placed in front of a convex mirror at infinity, the image is formed at its focus behind the mirror. The image is virtual, erect, and highly diminished.

Object between P and infinity (Object in front of a convex mirror)

When an object is placed at any point in front of a convex mirror, its image is formed between P and F behind the mirror. The formed image is erect, virtual, and diminished.

Uses of spherical mirror

Uses of concave mirror

1. Concave mirrors are used in torch lights, searchlights, and head lights of vehicles where the light needs to be focused at the distant objects.
2. Doctors use concave mirrors to observe the interior part of some organs like ear, nose, mouth, throat, etc.
3. It is used to see the enlarged image of our face for doing make-up or shaving.
4. It is used as reflectors in telescopes.
5. It is used in solar cookers. It converges the sunlight at a point where the cooker is placed.

Uses of convex mirror

1. It is used as side mirrors of vehicles to view objects behind the vehicle.
2. It is used in street lamps to spread the light in larger area.

Refraction of Light

When we see the rivers, lakes, ponds, well, etc., the real depth of water seems to be less than its real depth. Similarly, if we dip a straight stick up to its half-length in water, it is seen to be bent. Why does it seem so?

Well, the medium through which the light travels is called an optical medium. Air, water, kerosene, glass, plastic, etc. are called an optical mediums because light can travel through them. Light can travel even travel without a medium. Light always travels in a straight line in an optical medium. However, it bends at the interface of two mediums when it travels from one optical medium to another optical medium. The process of bending light when it travels from one optical medium to another is called the refraction of light.

Cause of refraction

The speed of light is different in different media. The higher the density of the medium, the lesser is the speed of light and vice versa. As the speed of light is different in different media, it bends when it travels from one optical medium to another. The medium whose density is relatively high is called a denser medium and that having a relatively low density is called a rarer medium.

Laws of refraction

1. When a ray of light travels from a rarer to a denser medium obliquely, it bends towards normal. Conversely, when light ray travels from denser to a rarer medium, it bends away from normal.
2. Incident ray, normal and refracted ray lie on a same plane.
3. They ray of light falling normally to the surface does not bend at all.

• A mirror is an instrument that forms an image by reflecting the rays of light falling on its surface coming from a source of light.
• There are mainly two types of mirrors:
  1. plane mirror
  2. spherical mirror
• If the surface of a mirror is a plane, it is called a plane mirror. It always forms a virtual image.
• There are two types of spherical mirrors:
  1. concave mirror
  2. convex mirror
• If the outer part of the mirror is raised and the middle part is uniformly lowered, the mirror is called a concave mirror. As a concave mirror converges the parallel rays of light at a point, it is also called a converging mirror.
• If the outer part of the mirror is lowered and the middle part is uniformly raised, the mirror is called a convex mirror. Since a parallel beam of light diverges after reflection from the convex mirror, it is also called a diverging mirror.
• A parallel beam of light reflected from a concave mirror converges at a point. But in a convex mirror, a parallel beam of light diverges after reflection in such a way that it appears as coming from a point which is called the focus.
• The image which can be obtained on a screen is called real image and that cannot be obtained on a screen is called a virtual image.
• A concave mirror forms all types of images such as real or virtual, enlarged or diminished according to the position of the object.
• The convex mirror always forms a virtual and diminished image.
• Concave mirrors are used in torch, search lights, as headlights of motors, as looking glass in parlors and saloons, in telescopes or in solar stoves, etc.
• Convex mirrors are used to make looking glass of vehicles, reflectors in street lamps, etc.
• The process of bending light when it passes from one medium to another is called the refraction of light.
• Light bends towards normal when it passes from rarer to denser medium and bends away from normal when it passes from denser to rarer medium. The rays of light falling normally pass without bending.