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Basic Physics lesson-16: Light

Umesh is a freelance writer contributing his creative writings on varied subjects in various knowledge and educational sites in internet.



Light is a form of energy. It makes us to see things. Sunlight is the source of all life and vegetation on the Earth. It is obvious that we cannot imagine life without sunlight on the Earth. It is one of the most natural entities like air, water, and fire and is essential for our survival. The most interesting and amazing thing about light is, which is generally used to define it, that with its presence we can see the world around us. Sun is the primary source of light for us. It is actually a star, like so many stars in the universe, in which due to high temperature and nuclear reactions between the gaseous matter present, a lot of energy is generated and propagates out from it in all the direction.


Light as an energy

Light is in the form of the Electromagnetic waves. These waves have a wide spectrum of frequencies. Out of those frequencies there is a small band of frequencies, known as visible band, for which our eyes are susceptible and can see and perceive that through a combination of eye and brain. Eye works like a camera and brain perceives the image as it is formed in the retinal wall of the eye. Human brain is capable of recognising everything about an item like its colour, size, shape etc. For example using the eye we are able to see the Sun as a bright yellowish disc in the day sky, the Moon as a white disc in night sky, the stars twinkling in the night, the hills, the rivers, the landscapes, and what not. Light makes it possible for us to see things all around us.

Slightly out of the visible band of light a thin band of ultraviolet light is present in one side while a thin band of infrared light is there in other side but our eyes are not able to perceive them and we cannot see them though they are used for various purposes in the industry as well as in some specific medical and general gadgets.

There are other sources of light also like when we burn a material or light a candle then due to its burning and high temperature created, it also emits some electromagnetic radiations and we then see the fire and its colour. Another example is that in a cloudy sky when it rains associated with the thundering then also we see the white yellowish light associated with the collision of the clouds with each other. We have electric bulb and other such devices in our houses which give us light by conversion of the electrical energy in the light energy.

Light has many properties but the most obvious one is the reflection or absorption of light from any surface or material. This is very interesting because we are able to see a body or object when either it is emitting light itself or some light is falling on it and reflecting back from it and reaching our eyes enabling us to see it. For example Moon has no light of its own. It is just like another heavenly body like Earth in our planetary system and is actually the natural satellite of Earth revolving around it and we see it because the Sun light falls on it and the reflected light reaches our eyes. Depending upon the position of Sun, Moon, and Earth we see this reflection which is nothing but the different phases of Moon from new Moon to full Moon. We can also appreciate and understand the tremendous light energy that is produced by the mighty Sun so that a small part of it reaching Moon and then reflecting back is sufficient to make the moon so conspicuously visible from an observer or person standing on Earth. Let us some day plan to glimpse the night sky and note what we observe. We find that there are so many stars twinkling in the sky. What are they? They are also like our Sun, some even mightier than it and that is the reason why light from them, so far from us, is reaching Earth and visible to the naked eyes. There are many stars which are very far and the light which reaches us is too feeble to see them and then we have to use a telescope which can collect more light and help us in seeing them more clearly. But the universe is so big that even with the help of a telescope we cannot see all the stars clearly and then more advanced methods of collecting light from them are required to locate them. We have to note that some of the small light dots seen in the night sky are not stars though they might be reflecting the light falling on them from the Sun. These are actually the planets in our planetary system. In the night sky, we can distinctly see these planets of our planetary system revolving around the Sun. One can see them as fixed dots here and there. They do not twinkle as they are relatively nearer to us than the far off stars. They do not have their own light but they are visible to us because of the Sun light just the same situation we saw with the Moon.

Nature of light

Light propagation and its nature is explained in two ways one is through the wave motion where it is understood that it is an electromagnetic form of wave propagation and moves ahead like a wave having a frequency and wavelength. It was subsequently found that the light consists of so many different wavelengths and frequencies and that can be seen with the help of some method which can disperse them in its components. Scientist found one method to do that by using a prism (a triangular shaped solid made up of glass) in which light is passed through it and its components are separated giving rise to the different colours that our eyes are able to distinguish. It has a great analogy with the rainbow that is formed during the rainy season when the light rays pass through the tiny water droplets present in the atmosphere and each of that tiny droplet which act like a prism to break the main sun light in its component in different colours having different frequencies and wavelengths of course within a narrow visible light band and a beautiful rainbow is seen in the atmospheric space in front of us.

Another way to explain light is that it consists of small weightless particles called photons and these photons carry that energy and fall on the materials and then they are reflected or absorbed etc and the result is perceived by our eyes. Both of these theories are good in their own place and are used by the scientists to explain the character of light energy and researchers also use these basic postulates to understand more about it and then find new things about it. So, it is believed that light energy is having a dual character which sometimes can be explained with wave theory while at times certain properties are understood with the assumption that light consists of photons and that is the particle theory.

Electromagnetic spectrum

Electromagnetic waves have a broad spectrum ranging from low energy radio waves to high energy gamma rays. There is a small band somewhere in this large range having wavelengths in the range 10-4 to 10-7 meters. This narrow bond is known as visible spectrum.The energy of the Electromagnetic waves is proportional to their frequency. Waves of any kind (like sound waves, mechanical waves in a string etc) have two inter-related parameters - frequency and wavelength. When the frequency increases, then the wavelength decreases and when the frequency decreases the wavelength increases. This is expressed in a very precise way relating to the velocity of the wave and is presented and expressed as -

v = fλ where v is the velocity of the wave, f is its frequency, and λ (lambda) is the wavelength.

In MKS system of units v will be in m/s, f will be in Hertz (c/s), and λ will be in meters.

Radio waves have wavelengths in the order of meters (from 0.1 meters to 1000 meters) while gamma rays have very small wavelengths of the order of a billionth of a meter(from 10-11 to 10-15 meters). It is also interesting to note that the radio waves have a frequency of the order of megahertz while gamma rays will have a very high frequency of the order of millions of tera Hertz (1 tera Hertz is equal to 1012 Hertz). This is really a very high frequency range and as the waves have energies proportional to their frequency, it is natural that gamma rays will be more powerful and that is the reason why they harm the human body also much and people dealing with gamma rays, especially in radioactive material handling, take special precautions.

The visible light spectrum (detectable by human eye) which is in a narrow range considering the wide range of Electromagnetic radiations, contains various coloured light waves corresponding to different colours starting from red light at the lower frequency to the violet light at the relatively higher frequency. In numerical terms this red to violet band has frequencies in the range 380 to 750 tera Hertz (1 tera Hertz is equal to 1012 Hertz) corresponding to wavelengths from 780 to 400 nano meters (1 nano meter is equal to one billionth of a meter).

Speed of light

Light energy, whether it is understood as a wave or a photon beam, moves with a very very fast speed. It is beyond our comprehension. Have you observed during the thundering of clouds in the rainy season that we see the light a few seconds before we hear the thundering sound? Actually sounds travels at a much lower speed (about 330 m/s) than the light and that is the reason why we see the lightening much before the thunder sound. The speed of light as calculated by scientific experiments is about 3 x 10^8 m/second. For example if some clouds are colliding with each other and thundering in the sky say about a height of 800 meters from our rooftop then their thundering light will reach us in no time almost very near to zero second while the sound will reach us after 800/330 = 2.4 second. That is the delay between lightening and thundering sound which we often observe during those rainy days.

Speed of light is really something very amazing. It is so big that to understand it we have to view it in relation to the cosmic bodies in this universe which are present in the universe at great distances. As stars and galaxies are located very far from each other, their distance can be understood in terms of the speed of light. The nearest heavenly body to Earth is Moon and it takes only about 1.3 second to reach light from moon to Earth. From that perspective, Moon is very near to Earth. On the other hand the sun which is the centre of our planetary system is at a bigger distance from Earth and light takes about 8 minutes to reach Earth from Sun. Let us now go to bigger distances in the universe. Light takes about 4.3 years from Earth to reach the nearest star system called Alpha Centauri. Universe is very big but we only know its perceived size as per the space exploration technology available today and it is very interesting to note that the universe is so big that light can travel across the presently known universe in a whopping 93 billion years time. Unbelievable? But it is true. In fact light year is now a unit of distance when we talk matters related to universe. So, Alpha Centauri is 4.3 light years away from Earth. The size of universe as known today is approximately 93 billion light years.

Speed of light is maximum in the vaccum and decreases slightly in a medium depending upon the refractive index of the medium. For example glass has a refractive index of 1.52 while water has a refractive index of 1.33 and depending on that the velocity of light decreases in these mediums proportionately. The general formula governing to calculate speed of light in a medium is given by -

Speed of light in a medium = speed of light in vaccum / refractive index of the medium

Please note that air has a very small value for refractive index almost equal to 1 and that shows that speed of light in vaccum and air are almost same.

Reflection and refraction

Some materials reflect a lot of light and absorb a little while some absorb them more and reflect less. A plane mirror reflects a lot of light. We can see ourselves in it. Then there are materials like glass panels which pass the light falling on them which is scientifically known as refraction. These materials come under the category of transparent materials. The materials which do not allow light to pass through them are known as opaque materials.

Mirrors are so powerful in reflecting light that using certain types of curved mirrors one can focus the light at a point for some significant time so that if we keep a wood or some wool at that point it will burn with that sharp focussed beam of light. In this case the light energy is converted to heat energy.

The transparent materials like glass are used to make lenses for eye which are specially shaped items for focussing the light correctly on the retina of the eye. Generally, people with weak eye power have a difficulty in seeing far things or near things and in some cases it could be both. In such cases by using a concave lens the far vision of the person can be corrected and by using a convex lens the near vision can be corrected. Spectacles which we use so commonly are based on this property of light.

A plane glass sheet will not focus or disperse the light but a curved glass, shaped like a convex or concave lens, especially that we see in optical lenses (spectacle lenses) will be able to do that. These lenses help in focussing a clear image on the retina of the human eye and a person, who has some eye defect like myopia or hypermetropia, starts seeing things clearly. This, I think is the great gift of scientific knowledge to the large number of people having imperfect vision.


Light interacts with matter in various ways like reflection, absorption, refraction etc and in this process the resulting light gives a lot of information about the material as the spectrum of the reflected or refracted light is actually a characteristics of that material. This is studied under the branch of Physics known as spectroscopy and is used in material identification, testing, and other related scientific works.


Light is a form of energy and in Physics we learn about its properties and characteristics. Light in natural form is available to us from the sun, the centre of our planetary system. There had been many scientific inventions for creating artificial light by burning the natural occurring fuel or converting electric power to light energy. Understanding about the light helps us in learning various physical processes related to light in this world.


1. https://andor.oxinst.com/learning

2. http://www.bbc.com/earth/story

3. https://en.wikipedia.org/wiki/Observable_universe

4. https://en.wikipedia.org/wiki/Unit_prefix

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 Umesh Chandra Bhatt


Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on May 29, 2021:

Vidsagster, thanks for visiting and also for sharing your college time memories of Physics. Blessings,

VIDYA D SAGAR on May 28, 2021:

Very interesting and informative, truly a refresher course Umesh. You have explained everything in such simple terms and made it easy to understand. When I studied science at graduate level, I found Physics difficult. I loved Biology and Chemistry more. But the chapter on Light fascinated me even then. Thanks for sharing.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on May 28, 2021:

Flourish, thanks a lot for your visit and nice sweet words. Blessings.

FlourishAnyway from USA on May 28, 2021:

This is very detailed and interesting. It's clear that you truly love this subject!

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on May 27, 2021:

Brenda, thanks for your interest in this science article. Appreciate. Have a nice day.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on May 27, 2021:

Moondot, thank you for your visit. Blessings.

BRENDA ARLEDGE from Washington Court House on May 26, 2021:

This is very informative.

I must say that I didn't know the moon had no light of it's own.

I always look at it at night innall it's beauty, but now I will think about your article.

I don't see how its possible that the sun can help in this, but alas I'm not too savy with science.

It's still amazing to me.

EK Jadoon from Abbottabad Pakistan on May 26, 2021:

Well-written and detailed article, Bhattuc. Thanks for sharing.

Stay safe and healthy.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on May 26, 2021:

Alexadry, so kind of you to spare your time snd going tgrough this. Thanks and blessings.

Adrienne Farricelli on May 26, 2021:

Thank you for explaining in easy terminology light and its various properties. We are so used to seeing light, but rarely question how it all works.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on May 26, 2021:

Thank you Audrey for visiting. Appreciate much. Blessings.

Audrey Hunt from Idyllwild Ca. on May 26, 2021:

Thank you for this informative and interesting lesson on basic Physics. It makes me want to go back to school and study this topic.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on May 25, 2021:

Thanks Chitrangda, for going through this. Appreciate much. Good day.

Chitrangada Sharan from New Delhi, India on May 25, 2021:

A very well explained Physics lesson. You are right about this being a refresher lesson. We generally become out of touch, of what we have learnt theoretically at our schools.

Thank you for sharing.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on May 25, 2021:

RoadMonkey, thanks for your observation. Yes, refraction is an important aspect and due to bending of the light rays at the boundary we perceive wrong water depths especially in clean shallow places where we are able to observe it.

I would elaborate it more when I update this article.

Good day.

RoadMonkey on May 25, 2021:

Very clear explanation if many aspects of light. Maybe under refraction, you could add that when the light waves pass through another medium, eg water, they change their speed and direction. This is why it can be hard to tell the depth of water just by looking at it and also why it can be hard to catch a fish by spearing it.

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on May 25, 2021:

Laxmi, thanks for your visit.

Laxmi Patil from Dist:- Satara, State:- Maharashtra, country:- India on May 25, 2021:

Very nice information...

Umesh Chandra Bhatt (author) from Kharghar, Navi Mumbai, India on May 24, 2021:

This Physics lesson is meant for students as well as the adults who had studied Physics in their school or college time but are not in touch now. Would act like a refresher for them.

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