- Light is a form of energy.
- Light is needed to see things around us.
- We are able to see the world around us because of light.
- We detect light with our eyes.
- Various objects/things around us with our eyes but eyes alone cannot see any objects.
- We also need a source of light to make objects visible.
- Light enables us to see objects from which it comes / from which it is reflected.
For Example:
The sun gives out light.
We can see the sun because the light coming from the sun enters our eyes.
Types of objects:
(a) Luminous objects
(b) None Luminous objects
(a) Luminous Objects:-
The objects like the sum other stars, electric bulb, tube-light, torch, candle and fire etc. which emit their own light are called Luminous objects.
(b) Non-Luminous Objects:-
Those objects which do not emit light themselves but only reflect/ scatter the light which falls on them, are called non-luminous objects.
For Example- chain, table, book, trees, flower, fan etc.
- We can see the non-luminous objects because they reflect light into our eyes.
- Light travels in straight lines.
Nature of lights:
There are two theories about the nature of light :
(a) Wave theory of light of
(b) Particle theory of light
(a) Wave theory of light:-
- Light consists of electromagnetic waves which do not require a material medium (like solid, liquid, gas) for their propagation.
- Wavelength of visible light about 4×10-7m to 8×10-7m.
- speed of light waves 3×108 m/s in vacuum.
(b) Particle theory of light:
- Light is composed of particles which travel in a straight line at very high speed.
- The elementary particle that defines light is the photon.
- Light has a dual nature / double nature, its means light exhibits the properties of both waves and particles depending on the situation it is in.
- The modern theory of light called “Quantum Theory of light’ combines both the wave and particle models of light.
Reflection of light:
- The process of sending back the light rays which fall on the surface of an object is called reflection of light.
- For Example- If we throw a tennis ball at a wall, the ball bounces back. This means that the wall sends it back.
- The objects having polished, shining surfaces reflect more light than objects having unpolished, dull surfaces.
- Silver metal is one of the best reflectors of light.
- A ray, of light is the straight line along which light travels.
- A bundle of light ray is called a beam of light.
Reflection of light from plane surfaces:
Plane Mirror:
- The ray of light which falls on the mirror surface is called the incident ray.
- The point at which the incident ray falls on the mirror is called the point of incidence.
- The ray of light which is sent back by the mirror is called the reflection ray.
- The normal is a line at right angle to the mirror surface at the point of incidence.
- The angle of incidence is the angle made by the incident ray with the normal at the point of incidence.
- The angle of reflection is the angle made by the reflected ray with normal at the point of Incidence.
Laws of Reflection of light:
The laws of reflection of light.
(1) First law of Reflection:-
According to the first law of reflection of light – The incident ray, the reflected ray, and the normal all lie in the same plane.
(2) Second Law of Reflections:-
According to the second law of reflection of light – The angle of reflection is always equal to the angle of incidence.
∠i = ∠r
Where
∠i = Angle of incidence
∠r = Angle of reflection
Note:- When a ray of light which is incident normally on a mirror is reflected back along the same path because the angle of incidence, as well as the angle of reflection for such a ray of light, are zero.
Regular Reflection of light:-
- In regular reflection a parallel beam of incident light is reflected as a parallel beam in one direction.
- Regular reflection of light occurs from smooth surfaces like that of a plane Mirror.
- Thus, a plane mirror produces regular reflection of light.
- Since the angle of incidence and the angle of reflection are the same, a beam of parallel rays of light falling on a smooth surface is reflected as a beam of parallel light rays in one direction only.
Diffuse Reflection of light:-
- In diffuse reflection, a parallel beam of incident light is reflected in different directions.
- In this case, the parallel incident rays do not remain parallel after reflection, they are scattered in different directions.
- The diffuse reflection is also known as irregular reflection/scattering.
- Since the angles of incidence and the angles of reflection are different, the parallel rays of light falling on a rough surface go in different directions.
Object:-
Anything which gives out light rays (either it’s own/reflected by it) is called an object.
For Example- a bulb, a pin-head, our face, a tree, etc.
The objects can be of two types-
(1) very small objects/point objects
(2) large objects / extended Objects
Images:-
Image is an optical appearance produced when light rays coming from an object are reflected from a mirror.
OR
An image is formed when the light rays coming from an object meet at a point, after reflection from a mirror.
The images are of two types-
(1) Real Images
(2) Virtual Images
(1) Real Images
- The image which can be obtained on a screen is called a real image.
- For example- the images formed on a cinema screen is an example of real images.
- A real image can be formed on a screen because light rays actually pass through a real image.
(2) Vertical Images
- The image which cannot be obtained on a screen is called a virtual image
- A virtual image can be seen only by looking into a mirror/a lens.
- For example- The images of our face in a plane mirror is an example of virtual image.
- Virtual images are also called unreal images
- A virtual image is just an illusion.
- A virtual image cannot be formed on a screen because light rays do not actually pass through a virtual image.
- A plane mirror always forms virtual images.
- A convex mirror also form only virtual images.
- A concave mirror can form a virtual image only when the object is kept within its focus.
Formation of Image in a Plane Mirror:-
Diagram
fig: formation of Image in a plane mirror
Consider a small object O placed in front of a plane mirror MM’.
The Object O gives out light rays in all directions but we need only two rays.
Now, a ray of light OA coming from the object O is incident on the plane mirror
at point A.
It gets reflected in the direction AX, according to the laws of reflection of light.
Another ray of light OB coming from the object O strikes the mirror at point B and gets reflected in the direction BY.
Two reflected rays AX and BY are diverging, so they cannot meet on the left side.
Let us produce the reflected rays AX & BY backwards.
They meet at point I behind the mirror.
When the reflected rays (AX and BY) enter the eye at position E, the eye sees
the rays of light in the straight-line direction in which the reflected rays enter it.
The mirror from position E sees the reflected rays as if they are coming from the point I behind the mirror.
Thus, point I is the image of the object O formed by the plane mirror.
Note: The image formed by a plane mirror can be seen only by looking into the mirror.
An image of this type, which cannot be gets on a screen, is known as a virtual image.
