Hello friends, in this session, we are going to 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 the darkest hour and hope things are going to
be alright.
It comes in many colors, but collectively its 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 ????........Its light.
Whether its the enjoyment of looking at our self in the mirror or the beauty of nature
or this colorful world we live in, at the very 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 on
earth and has been worshipped as a source and symbol of light.
So what is the relation between light and life??
Well that�s easy �.in our biology videos we have seen how sunlight is responsible for
food production in plants, maintaining the balance of the water cycle and for general
health But, understanding what light is and what it does is a bit more challenging.for
that we will have to look beyond what we see.
Dont worry we will cover that gradually in this session.
Depending on where it originates from, the sun, the moon, lightning, certain types of
glowing insects, worms and fishes etc. are all 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 radiation
called electromagnetic radiation or EMR.
The radiation is called electromagnetic because it contains both electric and magnetic properties.
Electromagnetic radiation is a large family consisting of seven members known as The Electromagnetic
Spectrum.
One particular family member which lies in the middle of the spectrum, falls on the earth
and which our eyes detect is called visible light or the visible spectrum.
Its so small, that if we consider the entire EM 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 high
definition colour.
We enjoy this privilege of colour due to the presence of certain specialized cells 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 we
see throughout our life is actually radiation..and radiation of a good type thats an important
source 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 slightly
different from one another.
This difference is due to three factors that are interdependent and give each member, its
individuality.
These factors are Wavelength, Frequency and Energy.
Lets look into what these are.
CONCEPT OF WAVELENGTH, FREQUENCY AND ENERGY Imagine 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 and
an area of depression.
In terms of our representation, the elevated area is called a crest, while the depression
is called a trough.
Wavelength is nothing but the distance between two successive crests 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 sea and 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 passing us by, which are shortly
spaced from each other while the calmer sea has fewer waves which are far apart.
So, the no. of waves passing us, in the area we observe, for the time of one minute is
referred to as the frequency of waves.
Thus for the rough sea the frequency of waves is greater as compared to the calm sea.
We just mentioned that for the rough sea the waves are closer to one another.
Closeness means shorter space between waves which in turn 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 have stood 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 increases
too and vice versa] The three factors are different for each member
of the Electromagnetic spectrum.
Now just because we cannot see the other types of radiations there are doesnt mean that they
arent there or that theyre not useful.
. Lets go back to the EM spectrum.
To the left we have radiations of the longest wavelength and thus the lowest frequency and
energy.
These radiations are called radio waves.
They are used for broadcasting TV and radio signals because they can cover a greater distance
through the atmosphere due to their large wavelength.
As we move towards the right of the spectrum, the wavelengths keep getting shorter and the
frequency and energy keep on increasing.
Next to radio waves we have micro waves used in appliances like the microwave . They are
also used in channels of telecommunication like mobile networks, Bluetooth and Wireless
LAN protocols.
Next we move to Infrared.
This region of the spectrum is more known for the aspect of heat associated 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: Did you know that reptiles like the snake view their surrounding through IR
just like how we do the same through visible light ??)
We have earlier seen how we the visible light makes the world visible to us and adds colour
to our world.
So this part of the spectrum is associated with the various types of light fixtures the
ones we enjoy (like disco lights and room lights), and the ones that are really important
(like traffic signals and airport runway signals).
Apart from us humans, animals like dogs, cats, mice , monkeys etc. also see 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 lower
energy so these radiation are referred to as low energy radiations which are safe even
after 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 penetrative in nature and thus considered comparatively dangerous,
where exposure to it, is kept to a bare minimum.
If these radiations have to be handled over longer periods, then appropriate precautions
are taken.
Moving further we have UV radiations.
The uses of ultraviolet radiations are also widespread in various aspects of life.
They are used as sterilizing agents by the medical industry and by water purifiers to
get rid of harmful bacteria.
It is used by dentists to harden teeth fillings and by banks to detect fake currency notes
by observing symbols or patterns on the notes that are only visible under UV light.
Another amazing fact is that certain organisms like insects form an image of their entire
surrounding using only UV radiations.
X rays, as we are aware, are also used to a great extent by the medical industry to
examine the state of bones or foreign objects in the body.
It is also used by airport security to scan luggage for potentially dangerous objects.
Gamma rays are a type of radiation with the lowest 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 during
certain types of thunder storms.
But despite their fearful reputation, they are used to kill cancer cells, for medical
and security scanning and also by space observatories for astronomical observations.
So far weve discussed how all these types of radiations differ from one another.
But as they are all part of one big family, they have something in common too - Despite
differences in their wavelength, frequency and energy, they all travel with the same
speed approximately 300 000 000 meters per second.the fastest known speed in the universe.
Now, as we conclude this session, lets have a quick recap of what we learnt today.
1) We learnt that light visible or otherwise is a type of radiation called Electromagnetic
radiation.
2) Electromagnetic radiations are of seven types collectively belonging to a large family
called the electromagnetic spectrum.
3) What differentiates them are factors of wavelength, 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 the same 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 comments
box 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