Why is the Sky Blue? | Scattering of Light
Summary
TLDRThis educational video delves into the fascinating phenomenon of light scattering, explaining why the sky appears blue, clouds are white, and the sun looks red at dawn and dusk. It explores the Tyndall effect, which distinguishes colloids from solutions and suspensions, and discusses how particles of varying sizes scatter light differently. The video simplifies complex concepts, using the example of air molecules scattering blue light more than other colors to explain the sky's hue, and why larger particles like dust and water droplets scatter light evenly. It also touches on why red is used for danger signals due to its minimal scattering, making it more visible over long distances.
Takeaways
- 🌞 The sky appears blue due to the scattering of light by air molecules, which preferentially scatter shorter wavelengths like blue light more than longer wavelengths like red.
- ☁️ Clouds appear white because they are made up of water droplets and dust particles that are larger than the wavelength of visible light, scattering all colors equally.
- 🌅 The sun appears red at sunrise and sunset because when the sun is low in the sky, sunlight passes through more of the atmosphere, scattering shorter wavelengths like blue and green, leaving the longer wavelength red light to reach our eyes.
- 🏙️ Light beams are visible in a dusty room because the dust particles scatter the light, making the beam visible to us.
- 🔴 Danger signal lights are red because red light, having the longest wavelength, is scattered the least by air molecules, allowing it to travel the farthest and be most visible.
- 🌈 White light is composed of a spectrum of colors, but for simplicity, the script considers the three primary colors: red, green, and blue.
- 🔬 Scattering of light can be explained by considering light as both a wave and a particle (photon), leading to different interpretations of how particles scatter light.
- 🌬️ The size of particles in relation to the wavelength of light determines how they scatter light; smaller particles scatter shorter wavelengths more, while larger particles scatter all colors equally.
- 🌟 The Tyndall effect, visible when a beam of light passes through a colloid, is an example of light scattering and can be used to differentiate colloids from solutions and suspensions.
- 👨🏫 The script aims to simplify the concept of light scattering, making it accessible to viewers by using everyday examples and breaking down complex phenomena into understandable parts.
Q & A
What phenomenon causes the sky to appear blue?
-The sky appears blue due to the scattering of light by the air molecules in the atmosphere. These molecules scatter the shorter wavelength blue light more than the longer wavelengths, creating the blue color we perceive.
Why do clouds appear white?
-Clouds appear white because they are made up of water droplets and dust particles, which are larger than the wavelength of visible light. These larger particles scatter all colors of light equally, resulting in the white appearance.
Why does the sun appear red at sunrise and sunset?
-During sunrise and sunset, the sun appears red because the sunlight has to pass through a greater distance in the atmosphere. This increased scattering reduces the amount of shorter wavelength light (blue and green) reaching our eyes, while the longer wavelength red light is scattered less and thus appears more prominent.
What is the Tyndall effect, and how is it related to scattering of light?
-The Tyndall effect is the scattering of light by particles in a colloid or in very fine suspensions. It is related to the scattering of light because it demonstrates how light is scattered by particles in a medium, making a beam of light visible when it passes through such a medium.
How does the size of particles affect the scattering of light?
-The size of particles affects the scattering of light by determining whether the scattering is more or less pronounced. Smaller particles, like air molecules, scatter shorter wavelengths (like blue light) more than larger particles, which scatter all colors equally, like dust particles and water droplets in clouds.
Why are danger signal lights often red?
-Danger signal lights are often red because red light has the longest wavelength and is scattered the least by air molecules. This allows red light to travel the farthest and be visible over greater distances, making it an effective color for signaling caution or danger.
What is meant by preferential scattering of light?
-Preferential scattering refers to the phenomenon where particles scatter different colors of light at different rates based on their wavelengths. Shorter wavelengths (like blue) are scattered more than longer wavelengths (like red), which is why the sky appears blue and the sun red during sunrise and sunset.
How does the wavelength of light affect its scattering?
-The wavelength of light directly affects its scattering. Shorter wavelengths, such as blue light, are scattered more by particles in the atmosphere, while longer wavelengths, like red light, are scattered less. This is why the sky appears blue and why the sun can appear red at certain times.
What would the sky look like without an atmosphere?
-Without an atmosphere, the sky would appear black because there would be no air molecules to scatter the sunlight, creating the blue color we see. The absence of scattering particles would result in no light being scattered into our eyes, making the sky appear black.
Why does the sun appear white at noon?
-At noon, the sun appears white because the sunlight has to travel a shorter distance through the atmosphere. This reduced scattering allows all colors of light to reach our eyes with minimal loss, resulting in the perception of white light.
Outlines
🌞 Introduction to Light Scattering
This paragraph introduces the concept of light scattering, explaining that the color phenomena we observe in the sky, clouds, and during sunrise or sunset are not due to reflection, refraction, or dispersion of light, but rather due to scattering. The script uses the example of a torch and dust particles to illustrate how scattering allows us to see a beam of light. It also explains the Tyndall effect, which is a visible demonstration of light scattering in colloids, distinguishing them from solutions and suspensions.
🌈 Scattering by Particles of Different Sizes
The paragraph discusses how particles of different sizes scatter light differently. It categorizes particles based on their size relative to the wavelength of visible light, explaining that particles smaller than the wavelength, like air molecules, scatter light in a way that favors shorter wavelengths, such as blue. In contrast, larger particles like water droplets and dust scatter all colors equally, resulting in the white appearance of clouds. The paragraph also addresses the Tyndall effect and how it can be used to test for colloids.
🌅 Why the Sky is Blue and the Sun is Red at Sunset
This paragraph delves into the reasons behind the blue color of the sky and the red appearance of the sun during sunrise and sunset. It explains that the blue color of the sky is due to preferential scattering of shorter wavelength light (blue) by air molecules, which are smaller than the wavelength of visible light. Conversely, the sun appears red at sunrise and sunset because longer wavelength light (red) is scattered less by the atmosphere, allowing it to reach our eyes after traveling through a greater distance at the horizon.
🚨 The Significance of Red in Danger Signal Lights
The final paragraph explains why danger signal lights are red. It relates this choice to the scattering properties of light, where red light, having the longest wavelength, is scattered the least by air molecules and can therefore travel the farthest. This makes red light the most visible color for signaling purposes, such as on tall buildings or aircraft, ensuring that pilots can see the signals from a distance.
Mindmap
Keywords
💡Scattering of Light
💡Tyndall Effect
💡Wavelength
💡Preferential Scattering
💡Red Sunset
💡White Clouds
💡Danger Signal Lights
💡Sunlight
💡Air Molecules
💡Photon
Highlights
The sky appears blue due to the scattering of light by air molecules, which scatter shorter wavelengths more than longer ones.
Clouds appear white because they are made of water droplets and dust particles that scatter all colors of light equally.
The sun appears red at sunrise and sunset because longer wavelengths of light, like red, scatter less and can travel longer distances through the atmosphere.
The color of light is influenced by the size of the particles it interacts with; smaller particles scatter shorter wavelengths more.
The Tyndall effect, visible when a beam of light passes through a colloid, is evidence of light scattering.
Light scattering can be explained by both wave and particle theories of light, resulting in different interpretations.
The color of the sky would be black without an atmosphere, as there would be no particles to scatter sunlight.
Danger signal lights are red because red light has the longest wavelength and is scattered the least by air molecules, making it more visible over long distances.
The sun appears white at noon because the atmosphere's scattering effect is less pronounced, allowing all colors of light to reach our eyes.
White light is composed of a spectrum of colors, but for the purpose of this explanation, it is simplified to the three primary colors: red, green, and blue.
Light scattering by larger particles, such as dust, results in the light appearing white because all colors are scattered equally.
The preferential scattering of blue light by air molecules is responsible for the blue color of the sky.
The visibility of different colors in sunlight is affected by the distance the light travels through the atmosphere.
The Tyndall effect is a test for colloids, as colloids scatter light differently than solutions and suspensions.
The color of the sun and the sky are optical illusions created by the scattering of light in the atmosphere.
The choice of red for danger signal lights is based on the physics of light scattering, not just tradition or symbolism.
Understanding light scattering helps explain various natural phenomena, such as the colors of the sky and the sun at different times of the day.
Transcripts
hi friends have you wondered why the sky
is blue
but the clouds are white why the sun
appears
red at sunrise or sunset
why we can see a beam of light in a room
filled with dust
why danger signal lights are always red
in color
is it due to reflection of light
refraction of light
or dispersion of light the answers
none of these it's due to a completely
different phenomena
known as scattering of light and that's
going to be the topic
of this video i'm going to make the
concept of scattering of light
really easy for you first let's
understand
what is scattering of light if i switch
on this torch
can you see the beam of light no
because light is invisible but when i
place my hand
or if you place a screen then we can see
the light
because we are seeing the reflection of
the light
by the hand now let's imagine
this room is filled with dust or like
you see in the movies
when the detective enters the dark and
dusty room
with his torch yes then you can clearly
see the
beam of light why is that
because the large number of dust
particles are scattering the light
so we can see the beam of light
similarly we can see the beam of light
when light is passed through a
colloid but the beam is not visible
when light is passed through a solution
or a suspension
now do you remember what is this known
as
that's right it's called tinder effect
tyndall effect is due to scattering of
light
colloids show tinder effect but not
solutions
and suspensions so it can be used as a
test for colloids as we discussed
light is scattered by dust but light can
be scattered by
other particles as well such as air
molecules
and water droplets now let's take a
closer look
how these particles are scattering the
light
well it depends on which theory of light
you're considering
if we consider light as a wave then we
can say
that these particles are reflecting the
light waves
in different directions so it's like
scattered reflections and we say that
the particles are
scattering the light in random
directions
now if we consider light as a particle
made up of
photons then we say that these particles
are absorbing the photons and
re-emitting them
in different directions in different
random directions
and so we say the light is scattered
now do all these particles scatter the
light in the same way
the answer is no let's go ahead and
compare their scattering before we
analyze scattering
let me ask you what is the color of
white light
is it really white that's right
white light is made up of millions and
millions of colors
but for simplicity we take it as the
seven rainbow colors
or the seven whip gear colors
in this video we'll simplify it even
further
we'll just take the three primary colors
red green and blue
now are the different colors scattered
in the same way
the answer is it depends on the size of
the scattering particles
so let's take a look how the different
size particles
scatter light the atmosphere is a
mixture of
many different particles such as air
molecules
water droplets dust particles and so on
to study the scattering of light by
these different size particles
we are going to divide them based on the
wavelength of
visible light wavelength means
the length of one wave so do you know
what is the wavelength of visible light
the correct answer is the range is 400
nanometers to
700 so we are going to divide these
particles
into two categories are they smaller
than the wavelength of visible light
or larger than that the air molecules
have a very tiny size they are smaller
than
0.4 nanometers or even less
because the air molecules are made up of
nitrogen and oxygen molecules
and they are really really tiny
so these particles are smaller than the
wavelength of
visible light and the water droplets
and dust particles these are much larger
than the wavelength of visible light
to give you some rough numbers the size
of these particles
is larger than 1000 nanometer or 10 000
nanometer or
even more so clearly they are larger
than the wavelength of
visible light we'll first analyze
the scattering of the light by these
larger particles
that is the water droplets and the dust
particles
let's say white light from the sun is
falling on larger particles
such as dust particles for simplicity
let's consider the white light to be
made of the three primary colors only
red green and blue
let's focus our attention on only one
dust particle
the dust particle scatters all the
colors
equally so as you can see red
green and blue are scattered equally in
all the directions so the white light
when scattered by this dust particle
appears
white in color an important point to
note
is that most of the light passes
straight through
so most of the sunlight just goes
straight through
and only a small percentage is scattered
by the
dust particles now when you
shine a torch in a room full of dust
what do you think will be the color of
the beam of light
that's right it's going to be white in
color
because the dust particles scatter all
colors
equally in all the directions so the
beam will appear
white now let's look at the smaller
particles
the air molecules for example
nitrogen and oxygen molecules
these particles are smaller than the
wavelength of visible light
so let's say white light from the sun is
falling on the
air molecules again we are taking the
white light to be composed of
red green and blue now
let's focus our attention on a single
air molecule
the air molecule scatters the different
colors
differently and as you can see
the blue color is scattered more than
the green color
and that's scattered more than the red
color
why because different colors have
different
wavelengths and shorter wavelengths
are more scattered than larger
wavelengths
blue color has the smallest wavelength
so it is scattered
the most and red color has the largest
wavelength
so it's scattered the least in fact blue
color is scattered
10 times more than red color
this is known as preferential scattering
and it happens in particles that are
smaller than the wavelength of visible
light
such as air molecules so are you getting
a clue
why the sky appears blue to us
we know that sunlight is white sunlight
is made up of
all the seven colors now when sunlight
enters the atmosphere
the light is scattered by the atmosphere
the atmosphere is mainly made up of air
molecules
and they are trillions and trillions of
air molecules
as we discussed each air molecule is
doing
preferential scattering it scatters
blue color more than the red and green
color
so blue color is scattered the most
it's like the air molecules are playing
with the blue colored powder
and scattering it all around
so whenever we look up at the sky we see
blue color and that's why the sky
appears
blue to us what do you think the color
of the sky would be
if there was no atmosphere
that's right black just like it appears
to an
astronaut because if there are no
particles to scatter the light
the sky would appear black to us
the sun would still appear white
so the blue color of the sky is an
optical illusion
it is due to the preferential scattering
of light by the air molecules
we know that the sky is blue but why are
the clouds white
think about what the clouds are made up
of
that's right water droplets and dust
particles
as we discussed these particles are
larger in size
larger than the wavelength of visible
light
so these particles scatter all colors
equally so when the white light from the
sun
falls on the clouds all the colors are
scattered equally
and so the cloud appears white to us
now let's look at why the sun appears
red at sunrise
and sunset but white at noon
let's consider the sunset case
obviously i have seen more sunsets than
sunrises
you need to wake up really early to see
the sunrise to analyze
we need to zoom out quite a bit so
imagine you're standing on the earth
like me
and looking at the sunset during sunset
the sun is near the horizon as you can
see
the sunlight has to travel the greatest
distance through the atmosphere
to reach us since sunlight has to travel
through such a large distance
in the atmosphere a lot of scattering of
light
will happen we know that sunlight is
made of white light
the seven colors but for simplicity
let's consider the three colors
red green and blue light
now blue light has the shortest
wavelength
so it gets scattered the most let's say
it gets scattered so much
from our line of sight that no blue
light
reaches our eyes similarly
green light is also scattered a lot and
it does not
reach our eyes red light
which has the largest wavelength reaches
our eyes
because it is scattered the least so
what will be the color of the sun
that's right the sun appears red to us
of course this is an exaggeration small
amount of blue and green light
also reaches our eyes but red light
reaches us the most
since it is scattered the least and
that's why the sun appears red during
sunrise
and sunset but the amount of red light
that reaches our eyes is much more than
the other colors
so the sun appears reddish at sunrise
and sunset now why do you think the sun
appears
white to us when it is above us
let's say at noon once again
let's zoom out and take a look as you
can see
at noon the sunlight has to travel
through a much smaller distance
in the atmosphere to reach our eyes
again blue color is scattered the most
green color
is scattered less and red color is
scattered the least
since sunlight has to travel through a
much smaller distance
the percentage of blue and green color
that scattered is very small so all the
colors of the sunlight
reach our eyes the difference in
intensity of the different colors
is small and that's why the sun appears
white to us at noon now let's talk about
danger signal lights have you seen these
lights
at the end of aeroplanes or on the top
of a tall building
so that the planes know that there's a
tall building here
what is the color of these lights that's
right
they're always red in color
why is it because red is the color of
danger or caution no
let's discuss the physics behind the
choice of color here
imagine there are three danger signal
lights at the top of a
tall building red green
and blue light and they are of equal
intensity
let's say a plane is flying at a far
distance
which light do you think would be most
visible to the pilot
that's right it's going to be the red
light
because red color has the largest
wavelength
so it's scattered the least by the air
molecules
red light can travel the furthest
distance
so that's why danger signal lights are
always
red in color because it's due to the
preferential scattering of light
by the air molecules as we discussed in
this video
the blue color of the sky and the red
color of the sun
at sunrise and sunset these are all
optical illusions optical illusions
due to the scattering of light and next
time you see the
red danger signal lights at the end of
an aeroplane
or at the top of a building do remember
why they are red in color because red
color is scattered the least
which reminds me do hit the red
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