Why is the Sky Blue? | Scattering of Light

Summary

This 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

00:00

🌞 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.

05:01

🌈 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.

10:04

🌅 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.

15:06

🚨 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.

Keywords

💡Scattering of Light

Scattering of light is the phenomenon where light particles (photons) are deflected in various directions upon interacting with small particles or molecules. In the video, scattering explains why we see certain colors in the sky, why the clouds appear white, and why we can see a beam of light in a dusty room. The speaker emphasizes that scattering is responsible for various optical effects in nature.

💡Tyndall Effect

The Tyndall effect refers to the scattering of light by particles in a colloid or very fine suspension, making the beam of light visible. In the video, it is explained using an example of dust in a room, where a beam of light becomes visible due to the scattering of light by the dust particles. This effect distinguishes colloids from solutions and suspensions.

💡Wavelength

Wavelength is the distance between consecutive peaks of a wave. The video discusses how the wavelength of visible light ranges from 400 to 700 nanometers and how different wavelengths of light are scattered differently. For example, shorter wavelengths like blue are scattered more than longer wavelengths like red, which plays a crucial role in explaining the color of the sky and sunsets.

💡Preferential Scattering

Preferential scattering is the selective scattering of shorter wavelengths of light (such as blue) more than longer wavelengths (such as red). This concept is key in explaining why the sky appears blue during the day. Air molecules, which are smaller than the wavelength of visible light, scatter blue light more than other colors, leading to the blue appearance of the sky.

💡Red Sunset

A red sunset occurs because, at sunset, sunlight has to travel a greater distance through the atmosphere. The shorter wavelengths (blue and green) are scattered out of view, leaving predominantly red light to reach the observer. This is an example of how the scattering of light depends on the distance the light travels through the atmosphere.

💡White Clouds

Clouds appear white because the water droplets and dust particles in them are much larger than the wavelength of visible light. These larger particles scatter all wavelengths of light equally, resulting in the white color of clouds. This contrasts with the blue sky, where smaller air molecules cause preferential scattering of blue light.

💡Danger Signal Lights

Danger signal lights are red because red light has the longest wavelength and is scattered the least by air molecules, allowing it to travel longer distances. In the video, this is used to explain why red is chosen for warning lights, as it is more visible from a distance, ensuring safety for pilots and drivers.

💡Sunlight

Sunlight is described as white light, which is a mixture of all visible colors. The video simplifies this by breaking white light into the primary colors of red, green, and blue. The scattering of these different colors in the atmosphere is used to explain the blue sky and red sunsets, as well as why the sun appears white at noon.

💡Air Molecules

Air molecules, primarily nitrogen and oxygen, are much smaller than the wavelength of visible light. They play a significant role in the scattering of light, especially in preferentially scattering shorter wavelengths like blue. This scattering is what causes the blue appearance of the sky, as these molecules scatter blue light more than other colors.

💡Photon

A photon is a particle of light that can be absorbed and re-emitted by molecules, causing the scattering of light. In the video, the explanation of scattering alternates between the wave theory of light and the particle theory, where photons are absorbed and then scattered in different directions by particles like air molecules or dust.

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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