Fun everyday optic facts: why is the sky blue? How does a rainbow form? - Fatti
divertenti di ottica quotidiana: perché il cielo è blu? Come si forma un arcobaleno?
Each day of our life we wake up, we wash ourselves, have a little breakfast and we commute
to our job. If it’s a sunny day a wonderful blue sky will make us feel ready to go out and start to
work. Otherwise if it’s raining, but the sun is visible it’s possible to notice a breath-taking
rainbow that will fill us with joy and amusement. Later on, after an entire day in our office we
return home and, if it’s a special occasion, we might have a nice aperitif with our loved one
in one of those romantic places with a stunning view on the city. We’ll rest from the hard day
and enjoy the time by seeing a wonderful sunset. Some of us will probably prefer to go out after
dinner and reach the top of a hill away from the city in order to have a magical night watching
stars with their partner in the night dark sky. But do we ever ask ourselves why the sky changes
color throughout the day starting blue, becoming red and getting dark at night?
And why a rainbow forms only if it’s raining? If you want to know more about this, follow
me in this video: a fantastic journey through some ordinary but surprising optic fun facts.
Before diving in the explanation of these common phenomena we need to take a step back and try
to answer a fundamental question: what is light? The main science that studies this topic is optic,
which was firstly developed by Isaac Newton in the sixteenth century. At that time everyone
thought that light was a wave, characterized by its own wavelength (the distance between
two successive peaks), frequency and intensity (the height of the peaks).
The main behaviors of light, which were studied in that period, were two: reflection and refraction.
The first one happens when a ray goes against a reflective material like a mirror, that doesn’t
let anything pass through and “reflects” the ray with the same angle of incidence.
The second phenomenon is typical of those surfaces that are not totally reflective. When
light passes from a material to another it changes trajectory, the deviation is proportional to the
difference in density between the two areas. Have you ever seen what happens if you point
a laser on a glass of water? The beam bends in a different direction when it enters in the fluid.
Later on, in 19th century with the contribution of the famous James Clerk Maxwell,
physicists discovered that light is just a type of electromagnetic radiation with a particular range
of wavelengths that is visible by our eyes, between 300 and 700 nm. As a matter of fact,
light just physically differs from an x ray, gamma ray or infrared, in the value
of wavelength. These types of radiation undergo the same effects of reflection and refraction.
As an example, Sun rays appear white because they’re equally composed of each possible visible
wavelength, whereas special lamps alimented with sodium are characterized by an orange light due
to the fact that emission is mainly made up of some types of wavelength around 600 nm.
Moreover, thanks to the development of advanced mathematical tools scientists
were able in the last 50 years of the 19th century to explain in a more
rigorous way a wider range of luminous phenomena. They understood that light has a different angle
of refraction depending on its wavelength. By knowing this, scientists were able to explain some
effects that were observed by older physicists like Newton. One of them is particularly famous:
do you know what is the world-wide known symbol of Pink Floyd? The prism.
And why is it important? The prism has the ability of separating different wavelengths
of a luminous ray. How does it manage to do that? The beam pointed on the prism undergoes
a first refraction when it enters in the glass and a second one when it returns in the air.
Due to this double refraction all the different colors take slightly different paths in space:
the components of the original light are separated in what we call as spectrum.
Did you know that a similar phenomenon is responsible
for those marvelous rainbows you see after a rainy afternoon? Do you know
how that events can be explained? The answer will may be surprising…
In order to solve this problem, we must ask ourselves this question:
What are the two major requirements for the appearance of a rainbow? Sun and Rain.
This is our starting point. As a matter of fact, the solution
is already contained in those two words. Let’s start from the first one. What is
rain made of? Water. And how can we sketch the rain water? We can imagine it as a
collection of spheres with different sizes. Now we can consider the second feature that
characterizes the formation of a rainbow: sunlight. When a beam of sun rays hits a
sphere of water it undergoes a first refraction, a second reflection on the back of the droplet and
a final refraction when getting out. As a result of this process light is diffused like in a prism.
Since the droplets are in different positions, the rays will come out in various directions.
So when we consider rain as composed by a huge amount of water droplets,
the final effect is the separation of sunlight in its different colours: the rainbow.
In addition, a similar effect can be observed thanks to another different material. Have you
ever seen something strange while filling up your car at the refuelling station? If some
spots of oil are hit by a sunray, they produce a little rainbow due to the same diffusion.
To go on with other everyday fun facts, we must define a new phenomenon: scattering.
With the beginning of twentieth century scientists discovered that light was quantized in the form of
small particles called photons. Suddenly they started to doubt about the wave appearance
of light, approaching a more corpuscular description. Therefore, with the development
of quantum mechanics, a model that is able to describe particles better than any other theory,
physicists found out that each particle has an undulating behaviour. In conclusion, we can say
that light can be treated both as a particle or as a wave depending on the different situations.
Thanks to this theory, scientists were able to describe the interaction of single atoms
with radiation. This particular phenomenon is also known like scattering. The outcomes
of the interaction depend on the size of the atom compared to the characteristics
of the wave. Shortly, the atom absorbs light and re-irradiates it emitting new photons.
Firstly, if we have a particle much smaller than the wavelength of the
radiation we call it “Rayleigh Scattering”, which is named after the nineteenth-century
British physicist Lord Rayleigh. It is possible to find that the power
of light diffusion is proportional to the inverse of (λ4). As a consequence of that,
blue (which has a smaller wavelength of 500-520 nanometers) is reflected much more strongly than
red (which has a greater wavelength of 700 nm). This is the main reason why our sky happens to
be blue as the particles in our atmosphere interact according to Rayleigh scattering.
This demonstration might have satisfied your answer, but an accurate viewer won’t be totally ok
with these statements and will remain skeptical. As a matter of fact, a single question seems
to destroy this description. Do you know it? Here it is: why is sunset red? Air is always
the same and behaves according to Rayleigh laws, but there
isn’t any trace of blue. Why is it like that? The answer is really simple. When sun is setting
at the end of the day, it appears in front of us and its light will have to pass through a greater
layer of atmosphere than at midday. Since blue and green components are emitted with a higher power,
they are consumed after a certain distance traveled in air, and only the other colors
like red, purple and orange remain visible. Going on with this topic, further questions
might rise about the sky. For example, why is night sky dark? Where has our atmosphere gone?
Atmosphere is transparent when the sun doesn’t lighten it up. Let me try to give you an easy
example. Have you ever driven with fog? What does it happen when you open your headlights?
Light interacts with water particles of the fog and undergoes a certain scattering. The beams are
diffused and the fog obstructs our vision more than before. On the contrary, if our headlights
are turned down the fog seems to be less thick. The same happens on Earth at night. When sun
goes down atmosphere is no more enlightened and becomes almost transparent, allowing the view
of stars and of the dark background of space. The last, but not the least, effect that will
may you feel totally shocked is suntan. Have you ever desired a beautiful tanned
body in hot summer? Well, a lot of optics lays under this phenomenon. Let’s go through it.
We have previously said that solar light is white because is equally composed of
all colors. Therefore, the Sun is not just a source of electromagnetic visible radiation.
As a matter of fact, it irradiates in other wavelengths such as infrared.
Being hit by visible light has positive effects on our body, but the same can’t be said for other
frequencies of radiation that happen to be harmful for our organisms. Although the majority of them
is totally absorbed by our atmosphere (that acts as a shield which allows life on Earth),
some dangerous rays might reach the ground. The most famous ones are UV rays. They can
seriously damage our skin causing sunburn. Fortunately, our body has a great protection
against sunlight which is called melanin. It is a natural pigment that has the ability
to absorb a certain amount of UV rays and it can have two different colorations:
one is red (pheomelanin) and the other one is brown (eumelanin).
When sun hits our skin, the organism starts to product melanin at a higher rate,
reducing the risk of a sunburn. But how melanin does manage to give us
that beautiful tan we love to show at the beach? There are two main processes, the first one is the
oxidation of melanin that makes its color turn darker and darker. In this case we don’t have a
greater protection from future UV rays even if we are totally tanned, we should still use sun cream.
On the other hand, a different way of getting tanned is possible. If the majority of the rays
is UVB, our body creates an excess of melanin that causes delayed tanning which is far more
protective than the oxidation one. A skin tanned due to this process may be able to sustain UV
rays three times better than a normal one. In conclusion, we are surrounded by several
phenomena that might appear ordinary and common, but secretly hide a lot of amazing
physics effects. The majority of us has probably never thought about the origin
of a rainbow or a breath-taking sunset, but trying to find a possible explanation
for these events is what science is there for and it will continue to make you feel
surprised and astonished every single time. "This video ends here! Thanks for watching
everyone! Did you find this topic exciting? Are you amazed by the description of this fun
everyday facts? Did you already know why sky is blue or how a rainbow can form? Have you ever
heard about scattering before? Is there anything more you want to hear about Optics? Let me know
in the comments below, be sure to subscribe, and I'll see you next time on the channel!"