Nature of light
Light is a form of energy (optical energy) which helps us in seeing objects by its presence.
Theories about nature of light
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.
This theory was failed to explain interference of light and diffraction of light. So wave theory of light was discovered.
Wave nature of light
Light waves are 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 Plank gave a new theory which was known as quantum theory of light.
Quantum theory of light
When light falls on the surface of metals like caesium, potassium etc., electrons are given out. These electrons are called ‘photo-electrons’ and phenomenon is called ‘photo-electric effect’.
This was explained by Einstein. According to plank light consisted of packets or quanta’s of energy called photons. The rest mass of photon is zero. Each quanta carries energy
E = hv.
h → Planck’s constant = 6.6 × 10–34 J-s.
n → Frequency of light
Some phenomenons like interference of light, diffraction of light are explained with the help of wave theory but wave theory was failed to explain the photo electric effect of light. It was explained with the help of quantum theory. So, light has dual nature.
(i) Wave nature (ii) Particle nature
Sources of light.
The objects which emit (give) light are called luminous objects. It may be natural or man-made. Sun is a natural source of light and electric lamp, and oil lamp, etc. are man-made source of light.
The Non-luminous objects do not emit light. However, such objects become visible due to the reflection of the light falling on them. Moon does not emit light. It becomes visible due to the reflection of the sunlight falling on it.
Propagation of light
Light travels along straight lines in a medium or in vacuum. The path of light changes only when there is an object in its path or where the medium changes. Apart from vacuum and gases, light can travel through some liquids and solids as well.
Transparent medium : A medium in which light can travel freely over large distances is called a transparent medium.
Examples: Water, glycerin, glass and clear plastics are transparent.
Opaque : A medium in which light cannot travel is called opaque.
Examples : Wood, metals, bricks, etc., are opaque.
Translucent : A medium in which light can travel some distance, but its intensity reduces rapidly. Such materials are called translucent.
Examples : Oil
The Characteristics of light
⇒ Light is an electromagnetic wave.
⇒ Light travels in a straight line.
⇒ Light is a transverse wave, and does not need any medium to travel. Light can travel through vaccum. Its speed through vaccum is 3 × 108 m/s.
⇒ The velocity of light changes when it travels from one medium to another.
⇒ The wavelength (λ) of light changes when it goes from one medium to another.
⇒ The frequency (f) of the light wave remains the same in all medium.
⇒ Light gets reflected back from polished surfaces, such as mirrors, polished metal surfaces, etc.
⇒ Light undergoes refraction (bending) when it travels from one transparent medium to another.
⇒ Light does not need a material medium to travel, that is, it can travel through a vacuum too. Scientists have assigned a value of 299, 792, 458 m/s to the speed of light in vacuum.
⇒ According to current scientific theories, no material particle can travel at a speed greater than that of light in vacuum.