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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 in the range between 450nm and 750nm.

## What is Light Made of?

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 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 which depends on the frequency of light.

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 travels in the form of straight line. Light travels with constant speed in the vacuum and this constant speed is equal to the c = 2.9979245$\times$10 m s-1. This is approximately taken as 3$\times$108 m s“1.
2. Reflection: The basic is meaning of a reflection is something returned in response". When the light reflected from the medium or the surface, the angle of reflection depends only on the angle of incidence to the surface. According to the law of reflection, the angle of reflected wave reflected from the surface is always equal to the angle of incidence.
3. Superposition: If in the space multiple light waves are present then at a particular point in the space 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 magnetic and electric fields at that point.
4. Refraction: When light passes through a substance or medium, light gets bend on the basis of the wavelength or frequency. This phenomenon is termed as the Refraction. It follows Snells Law of Refraction.
5. Light Transmittance : When light falls on a substance, some of the light energy gets refracted, some gets refracted, and some gets absorbed by the substance. Light transmittance is defined as the fraction of incident light on the substance which passes through the substance at a particular frequency or wavelength. Light transmittance is defined as the ratio of the intensity of the Incident light on the substance to the intensity of light which passes through the substance or comes out from the substance.
6. Wavelength: Wavelength of Light is defined as the distance between the two consecutive crests or between two consecutive troughs. Wavelength of the light ranges from 380nm to 750nm. In the electromagnetic spectrum, Infrared rays has higher wavelength than the visible rays and the ultraviolet rays has smaller wavelength as compared to the light.
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 proportional 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 consist of different types of colors. 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.
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 a energy is also used to heat the substances : When an electron in the 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, one 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, when electricity is given to them. Here in case of incandescent lamps also 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.

## Photometry

Photometry is the astronomical measurement of brightness or intensity and color. It measured as the amount of light energy that strikes a certain surface area on the earth over a certain period of time.

## Monochromatic light

Monochromatic light is the light made of only one color having a single wavelength. An emission lamp is a common source of monochromatic light. For example a mercury or low-pressure sodium streetlight, filtered to remove unwanted emission lines.
 More topics in Light Visible Light Photometry Diffraction Applications of Lenses White Light Transmission of Light Photography and Video Dispersion and Scattering of Light Wave Properties of Light Geometric Optics Light Sources
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