so now brady if you put those on your
camera if you look through a white light
source
you're going to see a really dramatic
effect
today we're talking about why i can't
tell my rubbish bins apart when i go out
the night before to put them to the curb
you may be like me you have multiple
bins that are due to be put out on
different dates you have to keep track
of when the recycling goes out when the
normal rubbish goes out and here in
nottingham we have several bins we have
to put out in my case one is green one
is black one is brown and often when you
go out at night you can't actually tell
them apart and that's because much of
the illumination that we get now from
street lights comes from a certain kind
of lamp called a sodium lamp
that is changing however certainly here
in nottingham around the country and
probably in places around the world
there's a big drive towards replacing
those sodium bulbs by led light light
emitting diodes which give a very
different kind of light so today we're
going to talk about why under sodium
lamps we can't distinguish colors very
well this is a light source that is
emitting light at only a single
particular wavelength 589 nanometers and
that corresponds to a very
yellow orange light as you can see
because that's the wavelength of of
yellow in the optical spectrum so
there's a tube in here
in which atoms of sodium are being
excited the electrons are jumping up to
high energy levels when they fall down
they emit a certain wavelength of light
corresponding to the energy difference
between those energy levels well the
question is why do we see color at all
if we're outside in the daytime and our
only source of illumination is the sun
then the sun we know emits light at all
wavelengths and together that makes a
very whitish light it doesn't emit light
at exactly the same amount at each
wavelength but overall it covers the
whole spectrum however when we have an
artificial source of light we're not
always going to get that full spectrum
wavelength coverage and here's a really
extreme case of that so we don't have
white light at all we only have one
specific wavelength this very very
yellow light and so when we have white
light it may hit objects uh some of that
light will be absorbed some will be
transmitted and some will be reflected
it's only the light that's reflected
back to us that gives an object a color
so in the case of white light
all of those wavelengths are provided
and only some of them you know bounce
back to us if you don't have that full
spectrum to begin with if you're only
bouncing light of a certain color off
your object
you're not going to be able to
distinguish what its true color is so
these are really fun these are
diffraction glasses and so etched in
very very fine fine much more fine than
you could actually see
are little grooves etched into the
plastic and what happens when the light
passes through those is it's diffracted
so it spreads out like a prism will
spread out white light and it lets you
see the whole rainbow of color so brady
if you put the glasses over your camera
and you look at a white light source you
should see a pretty spectacular effect
with that white light being spread out
across all wavelengths of the spectrum
so now you're seeing what is making up
all that white light
but if you then look at our sodium lamp
you're not going to get that same effect
of course there isn't that wide
wavelength range to begin with to spread
out you should only see another image
of the same lamp because that's all you
have to diffract so for comparison
here's the white light
and then you can have a look at the
sodium lamp
and see the difference
okay so let's go into the white light
here so here we have a fluorescent light
emitting wavelengths
from the blue to the red which gives it
a white appearance overall
and so we've got our
our peanut as well yeah
that was what was available in the
vending machine right so we've got the
full rainbow here under the white light
you know the the white light is bouncing
off uh the blue light is bouncing off
these ones so we see them as blue the
yellow lights bouncing off these ones
everything else is being absorbed
however we take them back to our sodium
lamp we're going to see something very
different all right i'm following the
bow
and mix these up now
yeah i mean you can still tell which
ones are light and which ones are dark
but you've lost the hue you've lost the
ability to distinguish what color it is
what wavelength it is you can probably
take a pretty good educated guess that
that is yellow or orange because it's
matching the color of that light
but beyond there i think you'd be
hard-pressed to determine which one is
blue or green or purple so we've got
some crayons here but i don't think
similarly you would be able to tell
apart which ones these are do you want
to do do you want to be camera camera
woman yeah sure you hold the camera
i'll put you out of work this is your
first time ever yeah i'm behind the lens
i want you to take those crayons and
write down what you think what color you
think each one is using that crayon i
mean i think they're all brown but but i
won't write brown for all of them i
think that is bru i think they're all
brown
that might be orange that looks like a
like purple see i think that looks
yellow too red maybe okay there's the
great experiment
you happy with that i don't know let's
find out
okay so there's my there's my guesses
all right so we've got your colors down
here let's take it back into the white
light
okay so you got the yellow right but
then you did it again and thought it was
orange
uh and then you got the green right
but i don't think you got anything else
right
red was purple orange was purple
brown was red i'm i'm pleased i got that
one right
yeah well that's not surprising because
it's yellow light
so imagine a city where this is the only
light source available at night where
these are what the street lights look
like
in reality around here that's not quite
the case because these sodium lights we
have are high pressure sodium lights so
while they're intensely orange they're
not single wavelengths like this there's
a bit of mercury in the tube it gives it
a bit of coverage in the in the blue and
the violet but there are cities around
the world san jose in california is one
of them that uses or has used low
pressure sodium lights completely to
illuminate the city the reason san jose
did this is there's actually an
astronomical observatory lick
observatory on mount hamilton not too
far away
so in the interest of
helping the astronomers maintain the
purity of the night sky they legislated
that they would use low pressure sodium
lights
and so they produce a light that's
easily blocked out by filters for the
astronomers it makes everyday life or
every night life
very challenging because you can't
distinguish colors you can't tell the
colors of cars apart you pull up to a
curb that's painted red for no parking
you can't see it you get a ticket from
the observatory's point of view if
you've got a city that's nearby and
growing and encroaching and having a lot
of light pollution if you can keep that
light at a single wavelength then it's
better than nothing because you can have
a filter in place that would remove only
that amount of light
if the light is very broad spectrum and
very white
there's no filter that you can use to
remove all that so just with one filter
you can make the city almost invisible
to the telescope well things are
changing now they're changing on a
household level i've changed all my
interior lights to leds because they're
energy efficient they cost a little more
to put in
but they're going to last for 10 or 15
years and they draw much much less
current so
the running costs are much lower
the same thing is happening on city and
county-wide levels so street lights are
being replaced with leds the advantages
are there's potentially better ability
to distinguish color you get a
you know more true representation of
color they are more energy efficient so
they cost less to run they're more
adaptable so you can dim them and you
can have them on on switches they don't
take so long to light up
and if you get the casing right you can
direct them a little better as well but
it's not it's not a cut and dry case
just because one bulb is more energy
efficient doesn't mean your overall
savings are going to be greater if you
go and put lots more light bulbs out
there some people don't like the glare
there's a lot more reflection
some of that reflection may actually go
up into the night sky so it's not at all
clear that that's going to be better for
light pollution
this is a transformation that as we make
this video is still very much in
progress
and it's going to take some adaptation
on everyone's part to get used to it why
don't you miss that yellow light too
from your childhood like do you have a
fondness for it are you glad to see the
back of it i don't go out at night
anymore so it doesn't matter
actually from the from the space station
you can actually see changes in cities
as these transformations take place you
can see how city centers you know the
very color of them changes as led lights
are put in
possibly really emitted as heat and so
by the time they fall back to their
ground state they don't have as much
energy to get rid of and so the energy
they emit
is lower energy than the energy they
absorbed