the

Why the Sky is blue and Sunsets red

why are sunsets red why is the sky blue

well it all starts with the composition

of our atmosphere and our atmosphere

scatters light proportional to the

frequency to the fourth power

well since blue has a large frequency

blue scatters a lot comparatively red

has a much smaller frequency than blue

and since it's to the fourth power this

scattering is a big effect red

therefore scatters a little and green is

in between

excellent so let's let's think about

this a bit let's start with the Sun so

here's our Sun and over here is earth

and here you are so it's High Noon

obviously things are not drawn to scale

here by the way how do I know it's high

noon for you at this position here well

I know that because if you look straight

up you see the Sun so therefore that's

the position on earth where the Sun is

high noon let me just redraw things here

because I'll need that ok now let's draw

atmosphere around you in fact to make

things look a little more realistic

let's draw you a little bit smaller so

here's you there that little dot there

so let's draw the atmosphere around

Earth

so this is the the atmosphere and then

this is the planet Earth right here with

the Big E now let's take them one at a

time

so first let's get let's talk about blue

light blue light as a high frequency and

therefore a small wavelength because it

has a high frequency it will start

scatters a lot in our atmosphere and

most of the blue light has already been

scattered out by the time it reaches us

now let's consider green light green

light has a slightly longer wavelength

than blue and it's in the middle so it

does scatter but you know not a lot and

now let's consider red light with its

long wavelength and due to its long

wavelength and small frequency it

doesn't scatter very much at all in our

atmosphere and therefore what reaches us

well we see we see red plus green which

equals yellow that's what reaches us

through the atmosphere and that's why

our Sun is yellow notice also that the

blue light scatters quite a lot recall

it so the fourth power so the the

difference in frequency has a big effect

well because the blue light is

scattering out when we look away from

the Sun we see blue because most of the

light that's scattered is blue so it

dominates so when you look away from the

Sun you see blue so that's why the sky

is blue now let's consider sunset so at

the time of sunset here's the Sun here's

Earth

the time of sunset you are here how do I

know that well if you look along the

horizon that is a long earth you'll see

the Sun and that's what you see at

sunset let's draw atmosphere and right

off the bat I want you to notice

something I want you to notice that the

distance between the surface of Earth

and the top of the atmosphere at high

noon is this but it's sunset you're

traveling farther it's like you're

taking the diagonal so notice that the

distance at sunset through the

atmosphere is much larger great let me

get rid of these so we can draw our

colors so let's first take blue so

here's our high frequency blue light

coming through and it scatters quite a

lot because it goes to the fourth power

and it's it's all scattered out in the

upper atmosphere

pretty much green light lower frequency

longer wave a longer wavelength it

doesn't scatter as much as blue but does

scatter more than red and due to the

extra distance that the light is going

well all the green light has been

scattered out as well red with the

longest wavelength is the one that

reaches it does scatter but not as much

as these guys this is the one that

reaches us and the one that still has

stuff left over to scatter in the lower

atmosphere well that's why at sunset the

sky is red I mean I should say red ish

right because there's still some green

and blue light that makes it through

especially with the green and that's why

you get that sort of orangish hue

all right so now we know why at sunset

the sky is red during the middle of the

day the sky is blue and the Sun yellow