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What is Light?

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Light rays, also known as visible light, is electromagnetic radiation that can be detected by the human eye. The electromagnetic spectrum as a whole is extremely broad, ranging from low energy radio waves with wavelengths measured in metres to high energy gamma rays with wavelengths less than 1x10-11 metres.

Light can also be described as a stream of photons, which are massless packets of energy that travel at the speed of light with wavelike properties. A photon is the smallest packet (quantum) of energy that can be transported, and the discovery that light travels in discrete quanta was the impetus for Quantum Theory.

Light Definition

Light, also known as visible light, is electromagnetic radiation that falls within the portion of the electromagnetic spectrum that the human eye can perceive. Visible light is typically defined as having wavelengths in the 400–700 nm range, which lies between infrared (which has longer wavelengths) and ultraviolet (which has shorter wavelengths) (with shorter wavelengths).

Basic Properties of Light

  • Light radiation travels at a speed of approximately 299 792 458 metres per second (m/s) in a vacuum (a container with no air). This is referred to as the speed of light.

The speed of light is the same for all electromagnetic waves.

The wavelengths and frequencies of electromagnetic waves distinguish them from one another (wavelength is inversely related to frequency).

                   с = f    

                   c = speed of light =3108 in a vacuum = 300,000 km/s = 186,000 miles/s 

  • The electromagnetic spectrum is a continuous representation of all electromagnetic waves organized by frequency and wavelength. Visible light constitutes only a small portion of the electromagnetic spectrum.

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  • The intensity or brightness of light decreases as the square of the distance from the source increases (inverse square law).

  • Ray Model of Light- 

To predict the path that waves will take, they are frequently approximated as rays. The ray model of light assumes that light moves in a straight line in a vacuum or uniform medium. A ray is a straight line that depicts the path of a very narrow light beam. Ray diagrams are diagrams that show the path of light rays. Despite ignoring the wave nature of light, ray diagrams are useful in describing how light behaves at boundaries (reflection or refraction) and are frequently used to locate the image formed by a mirror or a lens.

  • The Behaviour of Light at a Boundary-

When a light ray (or any wave) incident on a medium, one of three things can happen. Light can be absorbed by the new medium and converted into internal energy and/or heat, transmitted through the new medium, or reflected into the original medium. In reality, a combination of the three fates is more likely.

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The Three Possible Results of Light Energy when it interacts with Matter/Substances:

  1. Reflected- 

Light reflection returns energy to the same medium from which it originated.

  1. Transmitted Through- 

Light can pass through the new material with varying degrees of interaction with the molecules of the substance.

  1. Absorbed- 

The light energy can be completely absorbed by the substance's molecules and converted to heat.

  • We Can Classify Substances Based on How Light Interacts With Their Molecules.

  1. Light cannot pass through opaque objects because they absorb and/or reflect all light.

  2. Transparent objects allow light to travel in straight lines through them. Objects can be transparent to certain colours or frequencies of light while being opaque to others. Normal glass is transparent to visible light but opaque to UV and IR light.

  3. As light passes through translucent substances or objects, it scatters in all directions. Visible light can pass through our atmosphere.

Propagation of Light

The process by which an electromagnetic wave transfers energy from one point to another is referred to as light propagation. When light passes between boundaries from one medium to another, three major processes occur; Transmission, Reflection and Refraction.


Polarisation of Light Definition:

Polarization of transverse waves is possible. 

The alignment of a transverse wave along a single plane is known as linear polarisation. Light can be linearly polarised by using polarising film or filters to allow only waves vibrating along the axis of the filter to pass through.

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Light's linear polarisation provides experimental evidence that light is a transverse wave. Polarized light can be created by nature through reflection or scattering. The glaring light emitted by roads and bodies of water is horizontally polarised due to reflection.

Polarizing sunglasses eliminate glare by vertically orienting the transmission axis. Polarized light and polarising lenses are used in a variety of situations, such as creating a 3-D image from a specially created flat image, analysing metallic and plastic materials for stress points and weak spots in structures, eliminating glare from water or other vehicles while boating, fishing, or driving, and creating colour in films that either reflects or transmit light.

Optical Nature of Light

Ray Optics-

Reflection of light: The turning back of an electromagnetic wave at the surface of a substance is referred to as reflection. According to the Law of Reflection, the angle of incidence equals the angle of reflection. Angles are typically measured with respect to the normal. At the point of incidence, the normal is a line drawn perpendicular to the surface.

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Types of Reflection-

The size of the irregularities on the surface in relation to the wavelength of the light incident on the surface determines whether rays are reflected in a regular or irregular pattern. 

  1. When light strikes a "smooth" surface, the rays are reflected parallel to each other, resulting in specular (regular) reflection.

  2. When light strikes a "rough" surface, the rays are reflected in a variety of directions, resulting in diffuse reflection.

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Laws of Reflection-

  1. First Law: According to the first law of reflection; when a ray of light strikes a mirror and gets reflected back then the angle of incidence is equal to the angle of reflection

I=R

Where,

          I: Angle of incidence

          R: Angle of reflection

  1. Second Law: According to the second law of reflection the incident ray, reflected ray, and the normal lie on the same plane on the surface of reflection.

Refraction of Light:

Refraction of light is the phenomenon of bending a wave when it enters a medium with a different speed. When light passes from a fast medium to a slow medium, the light ray bends toward the normal to the boundary between the two media. Snell's Law describes the amount of bending as a function of the indices of refraction of the two media.

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The Amount of Bending is Determined by Two Factors:

  1. Change in Speed – If a substance causes light to refract (bend) more, it will refract (bend) more.

  2. The Angle of the Incident Ray – the amount of refraction will be more noticeable if the light enters the substance at a greater angle. If, on the other hand, the light enters the new substance from the side (at 90° to the surface), it will slow down but not change direction.

Index of Refraction-  

The index of refraction (refractive index)  is defined as the difference between the speed of light in a vacuum and the speed of light in the medium.

n = cv

n = Index of refraction
c=velocity of light in vacuum

v= velocity of light in the medium

The Refractive Index of Some Transparent Substances are-

Substance

Refractive Index

Speed of Light in Substance

(x 1,000,000 M/s)

The Angle of Refraction If Incident Ray Enters Substance at 20º

Air

1.00

300

20

Water

1.33

226

14.9

Glass

1.5

200

13.2

Diamond

2.4

125

8.2


  • Light slows down when it enters a substance with a higher refractive index (for example, from air into glass). The light bends in the direction of the normal line.

  • When light enters a substance with a lower refractive index (such as water into the air), it accelerates. The light deviates from the straight line.

Light will slow down and change direction more as it enters the substance with a higher refractive index.

Snell’s Law

Snell's Law describes the relationship between the indices of refraction ‘n’ of two media and the propagation directions in terms of angles to the normal. Fermat's Principle or the Fresnel Equations can be used to derive Snell's law.

                                        \[\frac{n_{1}}{n_{2}}\] =  \[\frac{Sinθ_{2}}{Sinθ_{1}}\]

Application of Light Waves

Light is used in various industries such as medical, automobile, manufacturing and scientific research. It is mainly used in telescopic equipment used for research purposes.

Uses of Light-

  • Cleaning robots detect obstacles.

  • Atmospheric observations are made using laser beams

  • Early detection of cancer and dementia.

  • Multifunction sensors for smartphones.

  • Exploring the interior of objects without destruction

  • Product manufacturing.

  • Eyesight correction

FAQ (Frequently Asked Questions)

Q1. What is an Incident Ray?

Ans) The incident ray is the ray that strikes the surface separating the two optical media.

Q2. What Do We Mean by Unpolarized Light?

Ans) Unpolarized light is defined as a light wave that vibrates in more than one plane. Polarized light waves are light waves with vibrations that occur in a single plane. Polarization is the process of converting unpolarized light into polarised light.

Q3. State the difference between Emission and Absorption Spectra.

Ans) When an atom's electrons absorb energy, they move from lower energy levels to higher energy levels. These excited electrons must radiate energy in order to return to ground states from the unstable excited state. The frequencies of the emitted light from the emission spectrum.

An absorption spectrum, on the other hand, is made up of the frequencies of light transmitted with dark bands when energy is absorbed by electrons in the ground state to reach higher energy states.


Emission Spectra

Absorption Spectra

It is produced when atoms release energy.

It is produced when atoms absorb energy.

Consists of coloured lines

Consists of dark lines or gaps

Helps in identifying the composition of a matter.

Helps is deciphering the ability of matter to retain heat and its absorption level.

Photons emitted help in identifying the kind of element the substance is composed of as each element radiates a different amount of energy and has a unique emission level.

The wavelength of light absorbed helps in figuring out the quantity of the substance.

Q4. Define Reflection of Light.

Ans) Reflection of light is the phenomenon of bouncing back of light in the same medium on striking the surface of any object.