Light Class 8th

WHAT IS LIGHT? 
    If we enter a dark room, the objects present in the room are not visible. However, if we switch on a bulb, everything in the room becomes visible. Why? 
    The bulb gives out an invisible energy called light. When this energy falls on the objects in the room, it bounces off from the surface of objects. When this energy enters our eyes, the eyes sense it and send a message to the brain. It is finally, the brain which really sees the objects. Eyes are only an aid in seeing the objects around us. 
    Why do we say that light is invisible ? Well, when light energy falls on the objects, we really do not see it. When energy bounces off from the surface of objects and enters our eyes, the sensation produced by this energy, helps our brain to see. Thus, to sum up we can say : 
    Light is an invisible energy, which causes in us the sensation of vision. When the light falls on any object, it bounces off from the surface of the object in all directions. This is called scattering of light. 

DEFINITION
    Light is form of energy which enables us to see objects which emit or reflect light.
    Light is a type of (form of) energy which can produce sensation in our eyes. So we can experience the sensation of vision.
    It travel in straight line in form of particles and waves. With the help of light we see all colours of nature.
    Our eyes are mostly sensitive for yellow colour and least sensitive for violet and red colour. Due to this reason commercial vehicles are painted with yellow colour, sodium lamps are used in road lights.

OPTICS
    It is a branch of physics which deals with the study of light. It is mainly divided into three parts :
(a)     Geometrical optics or ray optics: It deals with the reflection and refraction of light. 
(b)     Wave or physical optics : It is concerned with nature of light and deals with interference, diffraction and polarisation. 
(c)     Quantum optics: It deals with the interaction of light with the atomic entities of matter such as photo electric effect, atomic exitation etc.

NATURE OF LIGHT
    Theories about nature of light : 
(a)    Particle Nature of Light (Newton's corpuscular theory) : 
    According to Newton light travels in space with a great speed as a stream of very small particles called corpuscles. 
    According to this theory reflection and refraction of light are explained while this theory was failed to explain interference of light and diffraction of light. So wave theory of light was discovered. 
(b)     Wave Nature of Light: 
    Huygen consider the light remains in the form of mechanical rays and he consider a hypothetical medium like ether for propagation of light waves:
    So, light waves are declared electromagnetic waves so there is no need of medium for the propagation of these waves. They can travel in vacuum also. The speed of these waves in air or in vacuum is maximum i.e., 3 × 108 m/s.
    Photoelectric effect was not explained with the help of wave theory, so Planck gave a new theory which was known as quantum theory of light.
    This theory is failed to explain photo electric effect.
(c)     Quantum Theory of Light: 
    According to 'Planck' light travels in the form of energy packets or quanta's of energ
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