What is Light?

In the seventeenth century, there were two theories about the nature of light: One theory, championed by Sir Isaac Newton, explained that light consisted of tiny particles called corpuscles. Another theory proposed that light is a wave moving through space in the same way that ripples move across a pond. The advent of quantum physics in the twentieth century reconciled these two theories: Light can assume the characteristics of either a particle (called photon) or as an electromagnetic wave.

What is Electromagnetic Wave (EM Wave)?

Light is just one of the many Electromagnetic Waves that the universe radiates. Electromagnetic waves or EM waves are waves that are created as a result of vibrations between an electric field and a magnetic field. Electromagnetic waves are formed when an electric field comes in contact with a magnetic field. Hence we call these waves ‘electromagnetic’ waves.

What is the Electromagnetic Spectrum?

In recent decades, astronomers have developed telescopes that allow them to see radio waves, microwaves and infrared radiation. Satellites above the Earth’s obscuring atmosphere have also detected electromagnetic waves such as x-rays, gamma rays and ultraviolet radiation, emitted from celestial objects. The entire distribution of electromagnetic radiation is called the Electromagnetic Spectrum.

Illustration of Electromagnetic Spectrum (image from NASA, available at Wikipedia Commons.


What is the Speed of Light?

All Electromagnetic waves travel at  3.0 x 10 ms-1 in vacuum. They will slow down when travelling through glass or water. Hence, the speed of light is 3.0 x 10 ms-1 in vacuum. Although all electromagnetic waves travel at the speed of light in a vacuum, they do so at a wide range of frequencies, wavelengths, and photon energies.

What is a Photon?

Albert Einstein discovered that when light falls on a metal plate, electrically charged particles are released (or ejection of electrons) from the metal plate. This is known as the photoelectric effect. Hence, he proposed  that discrete energy packets exist during the transmission of light. A Photon is therefore described as a minute energy packet of electromagnetic radiation. It is also called light quantum. The energy of a photon depends on radiation frequency; there are photons of all energies from high-energy gamma- and X-rays, through visible light, to low-energy infrared and radio waves. All photons travel at the speed of light. Photons are have no electric charge or mass. They are field particles that are thought to be the carriers of the electromagnetic field.

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