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.
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