What is Light?

Hello friends, in this session, we are goingto talk about something that we have seen throughout the course of our life. Something,which we see a lot and of which we understand only little about.It is what separates the day from the night. Its what you want to see when you are in thedarkest hour and hope things are going to be alright. It comes in many colors, but collectivelyits white. When it comes to speed, nothing can be compared to the power of its flight.Believe it or not, its also responsible for the joy of sight. What is it ????........It'slight. 

Whether its the enjoyment of looking at ourself in the mirror or the beauty of nature or this colorful world we live in, at thevery base of our good fortune, is the phenomenon of light.For centuries, across many cultures, the sun has been considered as the giver of life onearth and has been worshipped as a source and symbol of light. So what is the relationbetween light and life?? Well that is easy.in our biology videos we have seen howsunlight is responsible for food production in plants, maintaining the balance of thewater cycle and for general health But, understanding what light is and what it does is a bit morechallenging.for that we will have to look beyond what we see. Dont worry we will coverthat gradually in this session. Depending on where it originates from, thesun, the moon, lightning, certain types of glowing insects, worms and fishes etc. areall examples of natural sources of light. While, a candle flame, various types of torches,lamps, and light fixtures are amongst the artificial light sources.Now lets understand this, our sun emits light .really strong intense light or radiationcalled electromagnetic radiation or EMR. The radiation is called electromagnetic becauseit contains both electric and magnetic properties. Electromagnetic radiation is a large familyconsisting of seven members known as The Electromagnetic Spectrum. One particular family member whichlies in the middle of the spectrum, falls on the earth and which our eyes detect iscalled visible light or the visible spectrum. 

Its so small, that if we consider the entireEM spectrum to be around say 2,500 miles, the visible fraction would be just one inch!!!!It is within this inch that we live in our world of natural and extra ordinarily highdefinition colour. We enjoy this privilege of colour due to the presence of certain specializedcells in our eyes called rods and cones which we have come across in our previous video.We have also seen what could happen if theres any problem with these cells.Now, the other types of electromagnetic radiations are also called light, but not visible light,.so automatically they are considered by us to be invisible.So now we know the light which we have naturally been introduced to since birth and that wesee throughout our life is actually radiation..and radiation of a good type thats an importantsource of energy for life on earth as we have seen earlier.

Now, just as members of a family differ from one another and have their own unique identity,so also, these fractions of radiation, although part of the same family of light, are slightlydifferent from one another. This difference is due to three factors that are interdependentand give each member, its individuality. These factors are Wavelength, Frequency and Energy.Lets look into what these are. CONCEPT OF WAVELENGTH, FREQUENCY AND ENERGYImagine the light that we know to be represented in the form of waves in an ocean like this.What differentiates a wave from calm waters is the presence of an area of elevation andan area of depression. In terms of our representation, the elevated area is called a crest, whilethe depression is called a trough. Wavelength is nothing but the distance between two successivecrests or two successive troughs. The smaller the distance between the crests or troughs,the shorter the wavelength and vice versa. 

Now lets consider the case of a rough seaand a relatively calm sea. When we look at any part of the water for say one minute,we notice, for that one minute where we look, the rough sea has more no. of waves passingus by, which are shortly spaced from each other while the calmer sea has fewer waveswhich are far apart. So, the no. of waves passing us, in the area we observe, for thetime of one minute is referred to as the frequency of waves. Thus for the rough sea the frequencyof waves is greater as compared to the calm sea. We just mentioned that for the roughsea the waves are closer to one another. Closeness means shorter space between waves which inturn means shorter wavelength. So, in all, we can conclude that the as wavelength shortens,the frequency increases and vice versa. The third factor is also easy. If you havestood near the sea when its wavy, you would have noticed the surrounding to be windy.Thus, for more no. of waves to be formed, the wind energy or just plain energy associated,is greater for the rough sea than that, for the calm sea. So now we will update our observation.Now lets join the dots as the wavelength shortens, the frequency increases and the energy increasestoo and vice versa] The three factors are different for each memberof the Electromagnetic spectrum. Now just because we cannot see the other typesof radiations there are doesnt mean that they arent there or that theyre not useful.  

Letsgo back to the EM spectrum. To the left we have radiations of the longestwavelength and thus the lowest frequency and energy. These radiations are called radiowaves. They are used for broadcasting TV and radio signals because they can cover a greaterdistance through the atmosphere due to their large wavelength.As we move towards the right of the spectrum, the wavelengths keep getting shorter and thefrequency and energy keep on increasing. Next to radio waves we have micro waves usedin appliances like the microwave . They are also used in channels of telecommunicationlike mobile networks, Bluetooth and Wireless LAN protocols.Next we move to Infrared. This region of the spectrum is more known for the aspect of heatassociated with it as they have higher energy as compared to radio waves and microwaves.They are used by appliances like grills and electric stoves for heating and cooking food.IR is also used as a part of heat therapy to relieve aches and pains. (Fun fact: Didyou know that reptiles like the snake view their surrounding through IR just like howwe do the same through visible light ??)

 We have earlier seen how we the visible lightmakes the world visible to us and adds colour to our world. So this part of the spectrumis associated with the various types of light fixtures the ones we enjoy (like disco lightsand room lights), and the ones that are really important (like traffic signals and airportrunway signals). Apart from us humans, animals like dogs, cats, mice , monkeys etc. alsosee in the visible spectrum, though their sight is not as good as ours.The part of the EM spectrum from radio waves to visible light have comparatively lowerenergy so these radiation are referred to as low energy radiations which are safe evenafter prolonged use. From ultraviolet to gamma rays, the energy associated is now higher,so these radiations are referred to as high energy radiations. These radiations are penetrativein nature and thus considered comparatively dangerous, where exposure to it, is kept toa bare minimum. If these radiations have to be handled over longer periods, then appropriateprecautions are taken. Moving further we have UV radiations. Theuses of ultraviolet radiations are also widespread in various aspects of life.

 They are usedas sterilizing agents by the medical industry and by water purifiers to get rid of harmfulbacteria. It is used by dentists to harden teeth fillings and by banks to detect fakecurrency notes by observing symbols or patterns on the notes that are only visible under UVlight. Another amazing fact is that certain organisms like insects form an image of theirentire surrounding using only UV radiations. X rays, as we are aware, are also used toa great extent by the medical industry to examine the state of bones or foreign objectsin the body. It is also used by airport security to scan luggage for potentially dangerousobjects. Gamma rays are a type of radiation with thelowest wavelength and the highest energy. They are the exact opposite of radio waves.They are emitted by radioactive material, given off during nuclear explosions and duringcertain types of thunder storms. But despite their fearful reputation, they are used tokill cancer cells, for medical and security scanning and also by space observatories forastronomical observations. So far weve discussed how all these types of radiations differ fromone another. But as they are all part of one big family, they have something in commontoo - Despite differences in their wavelength, frequency and energy, they all travel withthe same speed approximately 300 000 000 meters per second.

The fastest known speed in theuniverse. Now, as we conclude this session, lets havea quick recap of what we learnt today. 1) We learnt that light visible or otherwiseis a type of radiation called Electromagnetic radiation.2) Electromagnetic radiations are of seven types collectively belonging to a large familycalled the electromagnetic spectrum. 3) What differentiates them are factors ofwavelength, frequency and energy. 4) What unites them is their property of speed.But before we go away heres a little something interesting. In both these images we see thesame light behaving differently with different matter. Do you know what these behaviors are?Ok heres a hint they both begin with the letter R. You can write you answers in the commentsbox below and tune into our subsequent videos to know more about them in detail.Thats it for today. We hope that this session has been interesting. Like, subscribe, comment