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# Light

Light is radiant energy, an electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. It is the most popular form of energy, the wavelengths visible to humans range between 380nm (nanometers) to 740nm. It takes about seven minutes for light from the Sun to reach Earth. There are many sources of light a natural source of light is the sun as it plays an important role in our daily lives. Light behaves as a wave - it undergoes reflection, refraction, and diffraction just like any wave would. The details about this energy form are discussed in detail.

## What is Light?

Light is a form of electromagnetic radiation. Human eye has the capability to detect or sense some part of the complete electromagnetic spectrum. The portion of the electromagnetic spectrum which is sensed by our human eye having wavelength ranges between 450nm and 750nm. Light energy comes from many different sources, both natural and artificial. The Sun is the most important natural light source. Light is a form of energy which our sense of sight can detect. Examples: Sunlight, Candle, Fire flash light, electric bulb, kerosene lamp, stars and other luminous bodies etc.

## What is Light Made of?

According to the particle theory of light, light is composed of particles and these particles are termed as photons. Light consists of group of particles and each particle is termed as a photon and each photon has some energy associated with it depending on the frequency of light.
Light shows two types of nature. It shows particle nature and it also shows the wave nature. According to the theory of electromagnetic radiation, each wave consists of two types of field, i.e. electric field and the magnetic field. According to wave theory of light it is basically a wave, composed of perpendicular magnetic and electric fields. Each wave has its associated wavelength, frequency and energy.

## Properties of Light

1) Speed of Light: Light energy is energy that travels in the form of straight line. The speed of light in a vacuum is travels constantly in vacuum which is exactly 299,792,458 meters per second (approximately 186,282 miles per second). This is approximately taken as 3×108 m s“1. The fixed value of the speed of light in SI units results from the fact that the meter is now defined in terms of the speed of light.
Speed of light = wavelength x frequency
2) Reflection: reflection is something returned in response or the bouncing off of an obstacle. When the light reflected from the medium or the surface, the angle of reflection depends only on the angle of incidence to the surface i.e. Angle at which the wave approaches a flat reflecting surface is equal to the angle at which the wave leaves the surface. The angle between the incident ray and normal ray is known as the angle of incidence. The angle between the reflected ray and the normal ray is known as the angle of reflection.
3) Superposition: Multiple light waves are present in space, and then at a particular point the resultant electric field would be the vector sum of the all the electric fields present at that point. This is the law of Superposition. As light consists of electric and magnetic field, the law of superposition is followed by both electric and magnetic fields and we get the resultant at that point. When two waves interact, the principle of superposition says that the resulting wave function is the sum of the two individual wave functions. This phenomenon is called interference.
4) Refraction:  Refraction is the change in direction of a wave due to a change in its transmission medium. When light passes through a substance or medium, light gets bent on the basis of the wavelength or frequency. The refraction of light when passed from a fast medium to a slow medium bends the light ray toward the normal, to the boundary between the two media. This phenomenon is termed as Refraction and is described by Snell’s Law of Refraction.
5) Light Transmittance: When light falls on a substance, some of the light energy gets refracted, and some gets absorbed by the substance. Light transmittance is therefore defined as the fraction of incident light on the substance which passes through the substance at a particular frequency or wavelength. Light transmittance is the ratio of the intensity of the Incident light on the substance to the intensity of light which passes through the substance or revealed from the substance.
6) Wavelength: Wavelength of Light is defined as the distance between two consecutive crests or between two consecutive troughs. Light waves or infrared (heat) waves make characteristic patterns as they travel through space each having a shape and length, the distance between the peaks is called Wavelength. In the electromagnetic spectrum, Infrared rays have higher wavelength than the visible rays and the ultraviolet rays have smaller wavelength as compared to light. Visible light is defined as having a Wavelength in the range of 380nanometers to 750nm.
7) Frequency: Frequency of light is defined as the number of crests which passes through a particular point in a second. It is represented in Hertz. Frequency of waves is inversely proportionate to their wavelength, which means that higher the wavelength, lower is the frequency and vice versa. The frequency range of the light is between 400 to 790 THz.
8) Colors in light: Light is made up of wavelengths of light, and each wavelength is a particular color. The colors we see are the wavelengths that are reflected or transmitted. These colors are differentiated on the basis of their wavelengths in the visible spectrum. For example, when visible white light is made to pass through the prism, the different colors present in the visible white light bend at different angles depending on their wavelengths and as a result different colors of light are observed. The colors present in the light are red, orange, yellow, green, and cyan, blue, violet. White objects appear white because they reflect all colors. Black objects absorb all colors so no light is reflected.

Different properties of light like reflection, refraction, scattering, absorption are shown in the Figure below,

## Light as Energy

Light shows both types of behaviors, particle nature as well as the wave nature. According to the particle nature of the light, light consists of particles termed as the photons
Energy of the light is given by the formula,
$E$ = $hf$

Above formulae can also be given as:
We know that the speed of the light can be given as $f$ = $\frac{c}{\lambda}$

Therefore,

$E$ = $\frac{hc}{\lambda}$

Where,
$E$ = Energy of the light wave,
$h$ = Planck’s constant,
$\lambda$ = wavelength of the wave
$c$ = speed of the light

As light consist of photons, it exerts pressure or force on the particles which comes in its path. For example ultraviolet rays are used in the photoelectric effect to eject the electrons from the substances. When rays fall on the substances, photons transfer their energy to the electrons; electrons gain energy and come out from their orbits. Pressure exerted by the light is given as the Power of light divided by the speed of the light.

Light as an energy which is used to heat the substances: When an electron in an excited state in the higher orbit comes to the lower orbit energy is emitted in the form of photons. Due to the transition of electron from the higher orbit to the lower orbit, single photon energy is emitted.

We use light energy in our daily lives to see the world around us. For example incandescent lamps emit light energy, heating the filament present in the bulb by an electric current passing through it. In case of incandescent lamps due to electric energy the electrons of incandescent lamps jumps from the lower energy state to the higher energy state and when they come back to the lower orbit they emit light energy or photons.

## Visible Light

Visible Light which is also known as visible spectrum or simply light, the visible spectrum is a portion of the electromagnetic spectrum that the human eye can perceive.