What is Light? Definition, Properties, Speed, and Dual Nature of Light

The word "light" is frequently used to refer to radiant energy that gives us the feeling of vision. But today, the term "light" is used to refer to all kinds of electromagnetic radiation, both visible and invisible. Visible light is a small portion of the electromagnetic spectrum. There are many electromagnetic radiations that are not visible to the human eye such as radio waves, microwaves, infrared rays, ultraviolet rays, x-rays, and gamma rays. 

However, in this article, we are going to discuss various optical phenomena of light. In order to understand different optical phenomena we need to consider the nature and properties of light. The nature and properties of light are specifically studied in optics, which is a branch of physics

What is Light? Definition, Properties, Speed, and Dual Nature of Light

The subject of optics is conveniently divided into three distinct branches : (a) Geometrical Optics, (b) Physical Optics, and (c) Quantum Optics

Each of these branches requires a distinct method of theoretical treatment. 

In geometrical optics, many fundamental principles concerning light are studied by purely geometrical means without assuming anything regarding the nature of light. It assumes rectilinear propagation of light. So according to geometrical optics, if we consider light as a ray, then many physical phenomena like reflection, refraction, and absorption can easily be explained.

In physical optics, many experimental results are explained by considering primarily the wave nature of light. So according to physical optics, if we consider light as a wave, then many physical phenomena like diffraction, polarisation, and interference can easily be explained.

Where quantum optics deals with the interaction of light with atomic entities of matter. In quantum optics, we see that light is made up of tiny particles called photons. By considering light as a photon, phenomena such as the photoelectric effect can easily be explained. 

In the keyword links given below, we will discuss all these topics related to optics. For now, let's take a short note related to light that people usually ask.

What is Light?

Now we clearly know that "light is a sort of electromagnetic radiation that makes things visible or enables the human eye to see them". Basically, this light is produced when something burns. For example, when the wax burns, the light comes out from the candle. Similarly, there are many fuels whose burning produces light. In the solar system or universe, the only source of light is stars. But what burns inside a star that produces light? 

Actually, stars don't burn - in every star there is something called nuclear fusion happens which causes the release of energy (heat and light). In a star, a lot of energy is released when two hydrogen atoms combine to form helium.

These released energies that come out from the star contain all kinds of electromagnetic radiation. Among them, visible light is a small portion of it. Many of them are not visible to us but detectable to scientific instruments.

Typically, visible light is characterized as having wavelengths between 400-700 nanometers, corresponding to frequencies of 750–420 terahertz, that lie between the infrared and the ultraviolet radiation in the electromagnetic spectrum.

What is the Speed of Light?

The speed of light in a vacuum is fundamentally constant. The accepted value of the speed of light in a vacuum is 299,792,458 meters per second or around 186,282 miles per second. But as the density of the medium increases, the speed of light decreases. Such changes in the speed of light depend on the refractive index of the medium through which the light propagates.

The relationship between the speed of light and the refractive index can be determined through Snell's law or from the law of refraction.

What are the Properties of Light?

There are 7 basic properties of light - Reflection of light, Refraction of light, Diffraction of light, Interference of light, Polarization of light, Dispersion of light, and Scattering of light.

• Reflection of light: When light rays fall upon a smooth surface (or a reflector) and bounce back, it is called the reflection of light.

• Refraction of light: When light passes from one medium to another, its direction changes i.e. it bends, this is called refraction of light.

• Diffraction of light: When light passes around the edge of an object it slightly bends, this is known as diffraction.

• Interference of light: When two or more light waves combine or superpose on each other and form a pattern resulting in increased intensity at some points and decreased intensity at other points, it is called interference of light.

• Polarization of light: Light is an electromagnetic wave, it has both an electric field and a magnetic field. This electric field and magnetic field vibrate perpendicular to each other and light rays propagate perpendicular to the vibration of these electric fields and magnetic fields.

If in some way these vibrations of light waves vibrate only in a single direction then it is called polarized light waves and the phenomenon is called polarization.

Light sources, such as the sun, flames, and incandescent lamps produce unpolarized light. Polarized light can only be produced by passing unpolarized light through a polarizer.

• Dispersion of light: When white light passes through a prism it split into its constituent colors. This phenomenon is called the dispersion of light.

• Scattering of light: When light rays deviate from their original path in a different direction then it is called scattering of light.

Some facts about light that you must know-

1. Light is a form of energy.

2. Light always travels in a straight line.

3. Light doesn't need any medium to travel.

4. Different colors of light have different wavelengths and frequencies.

5. In our universe, nothing moves faster than light. The speed it has is  299,792,458 m/s.

Is Light a Particle or a Wave?

Light has been a mystery to many scientists since ancient times, including Isaac Newton. He believed that light was a particle, while Dutch physicist Christian Huygens thought it was a wave. But in 1801, before the discovery of photons, Young's famous double-slit experiment demonstrate the interference of light, which is only explainable if light being considers as a wave. 

After that in 1905 Albert Einstein explained his photoelectric effect in which he proposed the existence of discrete energy packets during the transmission of light. He said light is made up of photons, a type of particle, and these photons travel in waves. 

You may have heard when light strikes a metal surface, electrons are expelled from the metal. This phenomenon is called the photoelectric effect, which is only explainable if light being considers as particles.

Conclusion: Light shows dual nature - it has both a particle and a wave characteristic depending on the position. When we consider light as a wave, phenomena like interference, diffraction, and polarization can easily be explained. But considering light as tiny particles called photons, phenomena such as the photoelectric effect can be explained.

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