Why is the sky red at night sometimes
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
