Introduction

We all are familiar with light and its importance. Light is such an important source that without it all colourful things would have been black for us. So let us see how we can define such an important source. Light is a form of energy that enables us to see around us or we can say that it is an electromagnetic wave that can travel through any medium as well as through vacuum.

Types of objects

There are two types of objects as follows -

Luminous objects

Non Luminous objects


Luminous Objects: They are those which have their own light. We can see them as they have their own light. When their light reaches our eyes that is the time when we can see them. For example we have sun, tubelight, etc. It is not necessary that it should possess light naturally. If any substance glows on passing current, that also falls under the same category.


Non Luminous Objects: They are those which do not have their own light. So, if there is no light, we can’t see them. To see, we need a luminous object around it. For example: furniture, walls, moon, etc.
If we talk about the tendency of light that how it gets affected when it falls on any surface, then we conclude that when light falls on any surface, either of these phenomena takes place:

Light falling on a surface: It may get absorbed. The surface that absorbs all the light falling on it appears to be black in colour, or in other words we say that black is a good absorber and emitter of light.

When light falls on the surface it may pass through it. But it is not that all light rays would pass. Certain substances allow all rays to pass and few substances allow only certain rays to pass. Those substances which allow all rays to pass appear to be transparent. The process of allowing rays to pass through is called transmit process. For example: glass is transparent as it allows all rays to pass through it. In this figure we have tinted a plane glass. You can easily notice that tinted glass allows only a few rays to pass and on the other hand, plane glass allows all rays to pass through it.

Light falling on a surface may strike the surface and bounce back. This striking and bouncing back of light rays is called reflection. For example: we all often look in the mirror at home when we dress up. We can look at ourselves just because of the phenomenon of reflection.

In this unit we are going to consider the important phenomenon of light that is reflection.

Reflection

As we have discussed about reflection so now we are going to define it. “Reflection is the bouncing back of light rays on striking the surface”. As you can see the picture given below, that in it the ray is striking the surface and then it bounces back.


There are many surfaces that show reflection but not all surfaces show reflection. To show reflection, there are certain characteristics that must be fulfilled as given below.

Characteristics of Best Reflector

It should have a shiny surface

It should have a polished surface

 

It should have a smooth surface

Out of all, silver metal is the best reflector as it fulfills all the above discussed features.

Terminology involved in case of reflection

Look at the figure to understand this in a better way.

Incident Ray: It is the ray that strikes the surface.

Point of Incidence: It is the point at which the incident ray strikes the surface.



Reflected Ray: The ray  that bounces  back  on striking the surface.

Normal Ray: The  perpendicular drawn to surface.

Angle of Incidence: The angle between  the incident ray and  the normal ray.

Angle of Reflection: The angle between the reflected ray and the normal ray.

Plane of Reflection: Plane where incident ray reflected ray and normal ray lies.

 

Laws Of Reflection:These are certain sets of laws that are obeyed by all surfaces that show reflection.
1st law of reflection: Incident ray, reflected ray and normal ray all lie on the same plane.
2nd law of reflection: Angle of incidence is always equal to the angle of reflection.

Types Of Sources Of Light

As we know, there are different objects that emit light so, depending upon the size of object, we have two types of sources of light:

Point Sources

Finite Sources

Point Sources: These are those sources that are too small in size as shown in the figure.

Finite Sources: These are the sources that have certain height as shown in figure. The given diagram candle has certain length and is regarded as a finite source.
Types Of Mirror

Mirrors show the phenomenon of reflection so, depending upon the type of reflecting surface of mirrors, we have two types of mirrors, as follows:

Plane mirror

Spherical mirror

Plane Mirror: Plane mirrors are those that have plane reflecting 


Spherical Mirror: Spherical mirrors are the other types that have a curved reflecting surface. They are generally of two types i.e., one with a bulge and other with a depression.

 

The bulged one is convex mirror and the one with depression is concave mirror.

Types of spherical mirrors

Concave mirror

Convex mirror

Concave mirror: It is that in which the reflection occurs through inner surface of mirror.
Convex mirror: It is that in which the reflection occurs through outer surface of mirror.


An image is formed when the reflected rays actually meet or appear to meet at a certain point. Accordingly, two types of images are formed depending upon whether the reflected rays meet or not. We have two types of images

Real Image:

It is that which is formed when reflected rays actually meet at a certain point.

It is always inverted.

It can be obtained on screen.

 

 

Virtual Image:

It is that which is formed when reflected rays do not meet actually but appear to meet at a certain point.

It is always erect.

It can’t be obtained on a screen.


Law of Refraction

These are the laws obeyed by the surface from wherever the refraction takes place :

 

Incident ray, normal ray and the refracted ray all lie in the same plane.

The ratio of sine of angle of incidence to the sine of the angle of refraction is always constant for a particular pair of media.

i.e.: sin i/sin r = constant (Snell’s law)


When light passes from a denser to a rarer medium, it gets deflected away from the normal and in this case the angle of refraction is greater than the angle of incidence.

Refractive Index

This is another physical quantity that is related to refraction. It is  defined as the speed of light in vacuum /speed of light in a given medium or it is the ratio of the sine of the angle of incidence to the sine of the angle of refraction is always constant for a particular pair of media.

Note: The refractive index is denoted by ‘n’ and it has no units. Also, value of refractive index for one medium with respect to another is the reciprocal of the value of refractive index of the second to the first medium.


We have noticed that the path of light is reversible. For example: If light enters as shown in the figure, it undergoes refraction as shown and also, if the light enters in the opposite manner, it will follow the same sequence. The ray that moves out to its original medium after passing through certain medium is emergent ray and it can be defined as:

 

Emergent Ray: The ray that passes through a certain media and then returns to its original media is known as the emergent ray.
Angle of Emergence: The angle between a normal ray and an emergent ray is known as angle of emergence.

Lateral Displacement

An interesting phenomenon that we notice while studying reversibility of light is lateral displacement. It can be defined as the perpendicular shift in the path of incident ray when it travels through a certain medium and then returns to its original medium.

And images formed by the use of concave and convex lens and concave mirror and convex mirror is important important.

important questions

1. What is the magnification of a plane mirror

2. What is the radius of curvature of plane mirror?

3. Why paper catches fire when a convex lens is used to ficus sunlight?

4. What is silvering of mirror?

5. What is refractive mirror?


6. State the formula, lens formula and power of lens

7. The refractive index of water is 1.33 and kerosene is 1.44. Calculate refractive index of the kerosene with respect to water.

8. What kind of mirrors are used in big shopping stores to watch activities of customers?

9. Give mirror image of word
ambulance,


10. The magnification produced by a plane mirror is +1. What does it mean

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