Video Transcript
In this video, we’re talking about
colours of light. And the first thing to know about
these colours is that they exist. Not only do our eyes tell us this
everyday, but if we take white light — shown here — and send it into a bit of glass
— known as a prism — then on the other side of that prism, we’ll see the individual
colours that make up white light all spread out.
Colours of light come from the fact
that our eyes respond differently to different wavelengths of light. If we look up at the sun in the
middle of a clear summer day — don’t do this for long by the way — but if we do, the
sun will look white. Whenever we see white light, that
means that all the colours our eyes are sensitive to are present. But of course as we look out at a
natural scene, most of the light we see is not white; it has a particular colour to
it. For example, the sky looks blue to
our eyes, grass appears to be the colour green, and perhaps we see a flower that
looks yellow in colour. All these individual colours we’re
seeing are contained within the white light that comes from the sun. But clearly, there’s been some sort
of selection process so that we only see a particular colour when we look at a
particular object.
All this has to do with the fact
that whenever a light ray — say a ray of white light — is incident on an object,
there are a number of things that can happen to that light. One thing that can happen is that
the ray of light is reflected from the object. Another thing that can happen is
that it’s transmitted or goes through the object. And the third thing that can happen
is that the light goes into the object but never makes it through. In other words, it’s absorbed. In any given interaction like this,
how much light is reflected or transmitted or absorbed varies with the material. Interestingly, this series of
interactions is behind all the colours we perceive in the blue sky, the green grass,
and the yellow flower.
To see this a bit more clearly,
let’s look at the interaction of sunlight with grass in more detail. If we have a clump of grass that
sunlight is shining on, that is white light containing all the colours of the
rainbow, then we can represent that light based on the individual colours it’s made
up of. By the way, there are more colours
in visible light than the four we’ve shown here. But this is just a given idea. So anyway, all these different
colours of light that make up white light are running into this clump of grass. And for each individual colour, the
grass either reflects it, transmits it, or absorbs it.
Now, if we looked at this clump of
grass, what kind of light would our eyes see? Well, we wouldn’t see any of the
light that the grass absorbs because that stays within the grass. And we wouldn’t see any of the
light that it transmits that goes through the grass. Our eye would only see any colours
of light that the grass reflects. And of course, we know what that
colour predominantly is. It’s green, which tells us that in
the case of grass with all these other colours with the reds, with the blues, and
with the yellows, those get absorbed by this material or transmitted through it. It’s only green-coloured light that
bounces off the grass and comes to our eye. And since every time we look at
grass it looks green, we say the grass is green, even though actually this is the
colour that’s being reflected by that material.
This is the same thing that happens
by the way with that yellow flower we mentioned. When white light hits the flower,
that is light of all different colours, it’s the yellow light predominantly that
gets reflected or bounces off of the flower. And the other colours, the greens,
the blues, and the reds, are either absorbed or transmitted. This points to the interesting fact
that the colour of any particular object we see is actually only being reflected by
that object. So in some sense, the object is not
that colour.
But anyway, the important point
here is that whenever light interacts with an object, one of three things can happen
to the light. It will either be absorbed by the
object, transmitted through it, or reflected off from it. And it’s that reflected light that
our eyes are able to see when we look at the object. Building off that, if we look at an
object and the object appears white, say a white automobile or a white T-shirt, that
means that all the colours of the rainbow are being reflected by that object. It’s not absorbing any of them.
On the other hand, if an object is
coloured black or looks black to our eye, that means it’s absorbing all the
wavelengths of light incident on it and none of them are being reflected. By the way, this is related to the
fact that if you wanna stay cool on a hot summer day, it’s better to wear
light-colored clothing than dark. While a white material is one which
reflects all the colours of light which hit it, therefore absorbing none and heating
up very little, a black material is one that does the opposite. It doesn’t reflect any light, but
absorbs it all. Therefore, it heats up at a much
higher rate than a whiter material.
Now, we’ve said that white light
that comes from the sun can be spread out so that all the individual colours that
make up that light are visible. If we were to see all those colours
as a continuum, it would look something like this. This is known as the visible
spectrum because it contains all the types of light that our eyes are sensitive to
that our eyes can see. It’s not the only sort of light
that exists. But for all the light that’s
outside the spectrum, our eyes just don’t notice it. We can see in this spectrum that
there’s a whole range of colours, starting at red on the left side and getting to
violet on the right side.
Roughly speaking, the main colours
that our eyes pick out in this spectrum going from left to right are red, we’ll
abbreviate it R, orange abbreviated O, yellow abbreviated Y, green abbreviated G,
then blue B, indigo I, and violet V. So we have this somewhat
unmelodious acronym, ROYGBIV, that helps us remember the main colours in the visible
spectrum. One interesting fact about these
different colours in the visible spectrum is that there’s a mathematical way to tell
them apart. Now, this is actually really
helpful because if we pick out any particular spot on the visible spectrum, say that
spot there, and we try to identify the colour at that spot, well, we can get into
different names and different perceptions of colour and different descriptions of
colour if there wasn’t a quantitative way of saying where that part of the spectrum
is.
But thankfully, there is a
quantitative way to describe precisely any point on this visible spectrum. And that method is to use what’s
called the wavelength of a colour of light. This is a measure of the distance
along the wave’s direction that covers one complete wave cycle. So, for example, say we had some
yellow light. Well, one wavelength of that yellow
light would be equal to this distance, one complete wave cycle along the wave’s
direction. Often, for shorthand, the
wavelength of a wave is symbolized using the Greek letter lowercase 𝜆. This yellow light has a particular
wavelength. And we can call it 𝜆 sub 𝑦.
Now, if we were to compare this
wavelength of yellow light with a wavelength of green light, we would see that the
two are not the same; the wavelength of green light is shorter. And if we were looking at a
wavelength of blue light, then that would be shorter still than green or yellow. As we move across the visible
spectrum, there’s a trend. When we go from left to right, that
is when we go from red towards the violet end, the wavelengths of the colours of
light get shorter and shorter. And when we go the opposite
direction towards the red end of the spectrum, then the wavelengths get longer.
When we talk about the wavelengths
at the very ends of the spectrum, things get a bit interesting because remember this
is called the visible spectrum. This means that if our eyes can see
a colour, that is in the visible spectrum. But not all human eyes are the same
and not all sensitivities to light are quite the same. Despite those small differences,
there are accepted average values for the wavelengths of either end of the spectrum,
red light and violet light. White at the red end the spectrum
has a commonly accepted wavelength of 700 nanometres.
Now, a nanometre is equal to
one-one billionth of a metre. So what we’re saying is that red
light has a wavelength of 700 billionths of a metre. So that’s pretty short. But as it turns out, the wavelength
of violet light is even shorter. A commonly accepted average value
for violet light is 400 nanometres or 400 billionths of metre. And just as we can identify the red
and violet colours of the spectrum by specific wavelengths, so we can identify the
other colours. For example, green light is known
to have a wavelength of approximately 550 nanometres.
This wavelength measurement scale
that we’re talking about for the visible spectrum means that we can quantitatively
identify any particular spot on that spectrum. We could specify say a wavelength
of 476.325 nanometres and based on that number could find that exact spot and
therefore the corresponding colour on the visible spectrum. Now that we have a bit of a sense
for what the colours of light mean and how those colours interact with objects,
let’s get some practice through an example.
A light ray that strikes an object
is partly reflected and partly transmitted, as shown in the diagram. What percentage of the light ray’s
energy is absorbed by the object?
Taking a look at our diagram, we
see our object, this box here, and then light is coming in to the object from the
right. We see that some of the light 15
percent makes it all the way through and it’s transmitted and 35 percent is
reflected back. The question asks us how much of
the light ray’s energy is absorbed by the object. To start figuring this out, let’s
recall the ways that light can interact with an object that it’s incident on.
When light hits an object, there’re
only one of three things that can happen: the light can be transmitted, reflected,
or it’s absorbed by the object. This means that for any interaction
between light and an object, if we add up all the light that’s transmitted,
reflected, and absorbed, then we’ll get the total light that was incident on the
object in the first place. We could write it this way as an
equation. We could say that the total light
in incident on an object is equal to the light reflected plus the amount transmitted
plus the amount absorbed.
Now, in our scenario, we have some
amount of light in; we don’t know what that is. But we do know what percentage of
that light is transmitted and what percentage is reflected. Going to our equation then, we can
replace Trans, the amount transmitted, with 15 percent. And then we can replace reflected
with 35 percent of the amount of light that comes in. So we have that the total amount of
light incident on our object is equal to 35 percent of that amount plus 15 percent
of that amount plus the amount of light absorbed. If we add together 35 percent and
15 percent, we get 50 percent.
So 50 percent of the incident light
plus the amount that’s absorbed by the object is equal to the total incident
light. This tells us that 50 percent or
half of the light incident is being absorbed by this object. If we add that percentage to the
percentage that are reflected and transmitted, we get 100 percent. So we know that 50 percent of the
incident light ray’s energy is absorbed by this object.
Let’s look at a second example
involving the colours of light.
What colour is a beam of originally
white light after the beam is both transmitted through object B and then reflected
from object A?
Looking at our diagram for this
problem, we see these two objects, object B and object A. Both of these objects have all
these different colours red, orange, yellow, green, blue, indigo, and violet
incident on them. The fact that those specific seven
colours are being used tells us that white light is being shine on these two
objects; that is light that contains all the colours of the visible spectrum. But we see as we look at the
diagram that object A and object B respond differently to these colours.
For example, looking at object A,
we see that it reflects three of these seven colours. It reflects red light, yellow
light, and blue light. For object B on the other hand, we
don’t see what colours it reflects, but what colours it transmits. Object B transmits green light,
blue light, indigo light, and violet light. Based on the diagram, we can say
that all the other colours of light that reach object B are absorbed by it. And that brings us back to our
question. If white light, that is light with
these seven colours red, orange, yellow, green, blue, indigo, and violet is
transmitted through object B and then reflected from object A, what colour or
colours will remain?
Here’s how we could think of
it. Say that we have object B in front
of object A and we shine white light at an angle on object B. Some light will make it through
that has be transmitted through object B and then afterwards running to object A and
be reflected off of it. It’s that light in particular the
colour of that light that we want to solve for. To answer that question, we’ll need
to know what are the colours of light that are transmitted through object B and then
what are the colours of light reflected from object A.
Our diagram helps us answer this
question. We know that object B transmits
four colours of the original seven. It transmits green light, blue
light, indigo light, and violet light. So at this point in the light’s
journey after it’s made it through object B, there are four remaining colours, these
four we’ve just named. Then, these four colours run into
object A. And we want to see which colours if
any are reflected. Again, our diagram will help
us. We saw that object A reflects light
of three different colours. It reflects red light, yellow
light, and blue light. Now, in this case, the red and
yellow don’t matter much because no red or yellow light is actually reaching object
A in the first place. Those colours have been filtered
out by object B. But blue light is among the list of
colours that reach object A.
So of the green, blue, indigo, and
violet light that reach object A, only blue is gonna be reflected. Green, indigo, and violet will be
absorbed as we see in the diagram. Because blue light is the only
colour of light that’s both transmitted by object B and reflected by object A, we
know that that is the answer to our question. It’s blue light that will be
reflected from object A after this white light has been transmitted through object B.
Let’s summarize now what we’ve
learned about colours of light. Starting off, in this lesson, we
saw that our eyes are sensitive to a range of kinds of light, which we perceive as
different colours of light. We learned that all the colours of
light that we see are contained in white light. We saw that when light hits an
object, it can do one of three things: it can be absorbed, reflected, or transmitted
through the object. Furthermore, it’s the case that
light we can see is part of what’s called the “visible spectrum.” This spectrum ranges from 700
nanometres to 400 nanometres in wavelength. Lastly, we saw that the main
colours in this spectrum can be remembered using the acronym “ROYGBIV,” red, orange,
yellow, green, blue, indigo, violet and that the wavelengths of each of these
colours or any particular colour on the visible spectrum can be identified using a
specific numerical wavelength value.
