Question

What is light? How fast does light travel?

Answer 1

In a general sense, we know that light helps us to see things around us. Our major source of natural light is our closest star, the Sun. We also know that we process light almost instantaneously, which means that light travels very fast. In this context, try and recall a few observational or experimental phenomena that are exhibited by light and how it behaves at classical and quantum scales. To this end, determine the nature of light and its inherent properties.

Complete Solution:
Let us begin by understanding what light means and the properties it entails.
Light, most often referring to visible light, is the region of the electromagnetic spectrum that is visible to the naked eye. It is a form of energy that is able to illuminate objects around us that enable us to see and forms a crucial part of our vision. Thus, light forms the visible part of the electromagnetic spectrum.

Light is usually associated with the property of possessing a dual nature. Heuristically, this means that light behaves either as a wave or a particle at any given time, and exhibits wave-like or particle-like properties accordingly.

Being a part of the electromagnetic spectrum, light exhibits wave-like properties when it travels. It behaves as a transverse electromagnetic wave (i.e., when the electric and magnetic field oscillations are in a direction perpendicular to the direction of wave propagation). The amplitude of the wave determines the energy carried by the light ray. The fact that light bends around objects (diffraction), superposes with coherent rays (interference) and travels freely through vacuum supports the claim that light is a wave.

However, light is said to be composed of discrete minute packets of energy called photons. The frequency of the photon determines the energy carried by it. The fact that light travels in a straight line (which is only possible for particles), and the exhibition of photoelectric effect (ejection of electrons from a metal surface when light above a certain frequency called the threshold frequency is incident on it) support the claim that light is composed of particles called photons.

Like all electromagnetic radiation, light travels at the speed of $299,792,458\;ms^{-1} \approx 3\times 10^{8}\;ms^{-1}$ in vacuum. Nothing in the physical world travels faster than the speed of light. This quantity is commonly denoted as c and is a universal physical constant. However, light may travel at lesser speeds through material media.

So, in summary, we can say that light is a particle with a wave-like behaviour that travels at the speed of $3\times 10^{8}\;ms^{-1}$ in vacuum.

Note:
It may be of importance to know that the disparity between the wave and particle nature of light was bridged by Young’s Double Slit Experiment, which involved the interference of two coherent light “waves” resulting in the formation of a distinct interference pattern producing bright and dark bands on a screen. This band pattern would not be expected if light consisted of classical entities or waves, since the light seemed to be differentially absorbed, hitting only specific regions of the screen at discrete points as individual “particles”, thus exhibiting a wave-particle duality.