SPHERICAL MIRRORS

A mirror whose reflecting surface is a part of an imaginary hollow sphere is known as a spherical mirror.

Spherical mirrors are of two types:

(1) Concave mirrors

(2) Convex mirrors

Concave mirror: It is a spherical mirror whose reflecting surface is towards the centre of the imaginary sphere of which the mirror is a part.

Convex mirror: It is a spherical mirror whose reflecting surface is away from the centre of the imaginary sphere of which the mirror is a part.

Example: The inner shining surface of a steel spoon serves as a concave mirror and the outer shining surface of the steel spoon serves as a convex mirror.

ACTIVITY 1

(1)    Take a large shining spoon. Try to view your face in the inner shining surface of the spoon.

(2)    Bring the spoon close to your face. Observe the image.

(3)    Now move the spoon away from you. Observe the image.

(4)    Now try to view your face in the outer shining surface of the spoon.

(5)    Bring the spoon close to your face. Observe the image.

(6)    Now move the spoon away from you. Observe the image.

Terms related to spherical mirror

(1) Centre of curvature (C)

The centre of curvature of a spherical mirror is the centre of the imaginary hollow sphere of which the spherical mirror is a part.  It is denoted by C.

The centre of curvature is not a part of the spherical mirror. The centre of curvature of a concave mirror lies in front of the spherical mirror and centre of curvature of a convex mirror lies at the back of the mirror.

(2) Pole (P)

The pole of a spherical mirror is the centre of reflecting surface of the mirror. It is denoted by the point P

(3) Radius of curvature (R)

The radius of curvature of a spherical mirror is the radius of the imaginary hollow sphere of which the spherical mirror is a part. It is denoted by R

(4) Principal axis

The principal axis of a spherical mirror is the straight line passing through the centre of curvature C and pole P of the spherical mirror, produced on both sides.

(5) Aperture

The aperture of a spherical mirror is the diameter of the reflecting surface of the mirror

(6) Principal focus of a concave mirror (F)

The principal focus of a concave mirror is a point on the principal axis at which the incident rays parallel to the principal axis after reflection from the concave mirror actually meet at a point on the principal axis.  It is denoted by the letter F

(7) Principal focus of a convex mirror (F)

The principal focus of a convex mirror is a point on the principal axis at which the incident rays parallel to the principal axis after reflection from the convex mirror appears to meet at a point (or appears to diverge from a point)  on the principal axis.  It is denoted by the letter F

(8) Focal Length (f)

It is the distance between the principal focus (F) and pole P of the mirror. It is denoted by letter f.

Relation between focal length (f) and radius of curvature

Provided the aperture of a spherical mirror is much smaller than the radius of curvature, the focal length f can be related to radius of curvature R as

REFLECTION OF LIGHT BY A PLANE MIRROR

Image formed by a point object

Image formed by a finite object

Following are the important characteristics of images formed by a plane mirrors:

(1)  The image formed is always virtual. Such a image cannot be taken on a screen.

(2)  The image formed is always erect.

(3) The size of the image is same as the size of the object.

(4)  The image formed in a plane mirror as far behind the mirror, as the object is in front of the mirror.

(5) The image formed in a plane mirror is laterally inverted i.e. the left side of the object becomes the right side of the image and vice-versa.

LAWS OF REFLECTION OF LIGHT

First Law: The incident ray, the reflected ray and the normal at the point of incidence, all lie in the same plane.

Second Law: The angle of reflection (r) is equal to the angle of incidence (i).

These laws of reflection are valid for all types of reflecting surfaces. The surface may be smooth or rough. It may be a plane mirror, curved mirror, cylindrical or spherical mirror. The surface may be a spoon or a wall or a book. These laws of reflection are valid for all types of surfaces.

When light falls on smooth surfaces, regular reflection will occur. This type of reflection gives rise to image formation.

When light falls on rough surfaces, irregular reflection will occur. This type of reflection gives rise to scattering of light. Such a surface can be seen from all possible directions.

REFLECTION OF LIGHT

When light travelling in a given medium strikes any surface, a part of the incident light bounces back into the same medium. This phenomenon is called reflection.

Light reflected from a surface

Thus in reflection, the path of light rays changes its direction without any change in the medium of light.

Reflection is of two types:

Regular reflection

When the reflecting surface is smooth and well polished, the parallel rays of light incident on it are reflected parallel in one particular direction. This is known as regular reflection.

The regular reflection gives rise to image formation.

Regular reflection

Irregular reflection

When is reflecting surface is rough, the parallel rays falling on it are reflected in different directions. Such a reflection is known as irregular reflection or diffused reflection. No clear image is formed in case of irregular reflection.

Diffused or irregular reflection

Nature of Light

NATURE OF LIGHT

When we switch on a bulb/tube light, everything in the room becomes visible. When we switch off the bulb/tube light nothing can be seen. So we may conclude that it is the light which makes things visible when it falls on objects.

During the day, it is sunlight which makes things visible to us. The sunlight falling on objects is reflected or scattered and this reflected or scattered light when enter our eyes enable us to see those objects.

Thus,

Light is a form of energy which produces in us the sensation of sight.

What actually is light? or What is it made up of?
It is really a mysterious topic. It is an old topic of debate among scientists and science students. The explanation given by scientists appears to be against our common sense or the way we perceives the world. The theories are mind boggling for a student at the secondary level.

In junior classes, our teachers taught that light is a form of a ray which travels in straight line. A combination of rays forms a beam. A ray falling on a mirror is reflected in such a way that angle of incidence and angle of reflection both are equal. The concept of reflection and refraction can be explained completely on the ray theory of light. 

Interference and diffraction are the characteristics of a wave. It is observed that light also undergoes interference and diffraction supporting that light is also a wave. Scientists also explained reflection and refraction on the basis of wave theory. 

Scientists also found some phenomenon which cannot be explained on the basis of wave theory such as photoelectric effect and compton effect. Several experiments conducted on light proves that light is composed of particles known as photon. 

Now this is very confusing whether to call a light a wave or particle.  It is safer to assume that light has a dual character. It behaves like a wave as well as particle.


Properties of light 
(1)    Light travels in a straight line. This property of light is known as rectilinear propagation of light. This straight line path is usually indicated as a ray of light.
(2)   Nothing can travel faster than light. In vacuum the speed of light is 3 x 10⁸ m/s.
(3)  The speed of light in different medium is different. For example, speed of light in glass is 2 x 10⁸ m/s.
(4)   On entering from one transparent medium (say air) to another (say water) light changes its direction. The extent and way of bending depends upon the optical density of two media.
(5)   According to the modern theory of light, Light has a dual character. It is emitted or absorbed as a particle (called photon), but it propagates as a wave.
(6)     The particle character of light is called PHOTON. When light propagates in the form of wave it consist of electric and magnetic field hence light is also known as an electromagnetic wave.

Phenomena related to light:

(1)    Reflection of light
(2)    Refraction of light
(3)    Dispersion
(4)    Scattering
(5)    Interference
(6)    Difraction
(7)    Polarisation
(8)    Doppler effect