Class 10 - Physics - Chapter 15

Nature of Light and Electromagnetic Spectrum

Definitions:

1. Dual Nature of Light

Light has dual nature, it behaves not only as a particle (photon) but also as a wave. This is called dual nature of light.

2. Dispersion of Light

When a beam of sunlight falls on a prism, the light is split up in seven colours. This phenomenon is called Dispersion of Light.

3. Rainbow

The rainbow is an arc of spectral colours formed across the sky during or after rainfall in the morning or when the sun is behind us.

4. Photons (Quantum)

Photons are tiny packets of energy. They behave as particles but actually they are not particles.

==Newton's Corpuscular Theory of Light==

This theory which was proposed by Newton is as follows:

• Light is emitted from a luminous body in the form of tiny particles called corpuscles.

• The corpuscles travel with the velocity of light.

• When corpuscles strike the retina they make it sense light.

• Medium is necessary for the propagation of light.

• Velocity of light is greater in denser medium.

Wave Theory of Light:

In 1676, Huygen proposed this theory. According to this theory:

• Light propagates in space in the form of waves.

• It can travel in space as well as in a medium.

• Light does not travel in a straight line but in sine wave form.

• Velocity of light is greater in rarer medium.

• Medium is not necessary for propagation.

Quantum Theory of Light:

According to this theory of Max Plank:

• Light is emitted from a source discontinuously in the form of bundles of energy called Photons or Quantum.

• It travels in space as well as a medium.

• Speed of light is greatest in space or vacuum.

How A Rainbow is Formed?

As we know a prism disperses sunlight into a series of seven colours. When rain falls, raindrops behave like a prism and white light entering the raindrop splits up into seven colours on refraction. These are appeared as Rainbow.

Spectrum:

After the dispersion of light or any electromagnetic wave, a band of colours is formed, which is known as a spectrum.

Electromagnetic Spectrum

Electromagnetic spectrum is a result obtained when electromagnetic radiation is resolved into its constituent wavelength.

Waves of Electromagnetic Spectrum

Radio Waves

It has a large range of wavelengths from a few millimeters to several meters.

Microwaves

These radio waves have shorter wavelength between 1mm and 300 mm. Microwaves are used in radars and ovens.

Infrared Waves

It has a long range. Its mean wavelength is 10 micrometers.

Visible Waves

It has a range of 400 nm to 700 nm.

Ultraviolet Waves

Their wavelength ranges from 380nm onwards. These are emitted by hotter start (about 25000 C).