## 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 10m/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