Video Transcript
In this video, we will learn how to
describe the changes that occur to the color of white light when it passes through
different objects. And we’ll also see why this
customer is partly right.
We’re talking here about white
light, which is the most common color of light in the world. This is because light from the Sun
is white. Even though we often draw the Sun
as though it’s yellow or orange, really it gives off light that is white.
There’s something special about
white light. The color white is a mixture of all
the colors of the rainbow: yellow, green, red, blue, and so on. Anytime we see white light, all of
those colors are actually there already. We might wonder though, how do we
know that?
There is an object made of glass or
plastic called a prism. Often a prism is shaped like a
triangle. When a ray of white light, like
light from the Sun, passes through a prism, the ray is separated into all the colors
of the visible spectrum. The prism doesn’t add any light to
the ray, so we know that any colors we see were already there in the incoming
ray. This is how we know that white
light is a combination of every color of light.
If we look closely at this ray as
it reaches and passes through the prism, we notice that the colors in it change
direction by different amounts. For example, the yellow light
follows this path while the blue light goes in this direction. When the rays of these colors leave
the prism, they are going in different directions because they have changed
direction by different amounts. This is how a prism separates white
light into its many colors.
Notice that in this example, red
light is deflected the least and violet light is deflected the most. We can even measure the angle by
which a ray of a certain color is deflected. That angle is the difference
between the original direction of the white light ray before it reaches the prism
and the final direction of a ray of a given color. So the angle that red light is
deflected is this angle. And the angle that violet light is
deflected is shown here. It’s always the case that a prism
deflects red light the least and violet light the most.
One reason prisms are able to
separate light by color is that they are transparent. This means almost all of the light
entering a prism is able to pass through. Windows also allow light to pass
through, meaning they are transparent. And lenses used for eyeglasses are
similar. Nearly all the light that reaches
these objects is able to pass through. Of course, not every object is
transparent. Anything we cannot see through, and
this is most objects, is blocking the light landing on it from passing through. Books, desks, walls, and so on do
this. These objects are called
opaque.
So some objects are transparent,
and some are opaque. And in fact, some are neither. There is a third type of object
that lets some of the light falling on it pass through but not all. Sunglasses do this, so does wax
paper and vegetable oil. Objects like these are called
translucent.
Knowing all this about light as it
encounters different objects and can be separated into different colors, let’s look
at a few examples.
The diagram shows a prism
separating a beam of white light into rays of various colors. The light beam changes direction
when it is separated, and some rays change direction more than others. Which changes direction more, the
red ray or the yellow ray?
Looking at our diagram, we see that
red and yellow are at or near the top of the colored rays. We want to know which of these two
rays changes direction more. We can see that rays of each color
change direction two times. First, they change direction when
entering the prism, and then they change direction while leaving it. To find the total change in a ray’s
direction, we measure the difference between its final direction and its starting
direction. The starting direction is the
direction of the white light before it reaches the prism. The final direction of the red ray
is along this line, while the yellow ray’s final direction is along this one.
The red ray’s change in direction
is shown by this angle. And the yellow ray’s direction
change is shown by this one. Since the angle to the yellow ray
is greater, it must change direction more than the red ray. For our answer, we say that of the
two rays, the yellow ray changes direction more.
Let’s now look at an example
involving two prisms.
A white light beam enters a prism
and leaves the prism as a beam of various colors. Only the yellow light from the beam
of various colors passes through a narrow gap. The yellow light enters a second
prism that is identical to the first. Which of the following is the color
of light that leaves the second prism? (A) All the colors seen in the beam
that came from the first prism. (B) All the colors seen in the beam
that came from the first prism except yellow. (C) Yellow. (D) White.
Since the light that enters the
first prism is white, we know it contains all colors. We can see that these colors spread
out after the light leaves the first prism. Of all these colors, only yellow
light is allowed to pass on to the second prism. When this light enters the second
prism, it will change direction. But it won’t separate into other
colors. It can’t, because it’s only yellow
light. When the yellow ray leaves the
second prism, it will again change direction but will always remain just one color,
yellow. The correct answer to this question
is (C). The color of light that leaves the
second prism is yellow.
Let’s look at one last example
about how an object is seen by the eye.
The picture shows an apple viewed
by an observer in white light. The apple is red and has a green
leaf. An object that is transparent to
red light but opaque to green light is placed between the apple and the
observer. Which of the following pictures
best shows what the observer would see? And here we see answer options (A),
(B), (C), and (D).
We begin with the apple shown in
the question statement, which is red with a green leaf. The reason it looks this way to an
observer is that red light reflects from the fruit and green light reflects off the
leaf. An object is placed between the
apple and our eyes. We are told this object is
transparent to red light and opaque to green light. When an object is transparent to
light, that means it allows light landing on it to pass through. We could see the light coming out
the other side. However, if the object is opaque to
light, that means light cannot pass through it. The light is not visible on the
object’s other side.
The red color of the apple means
that red light is coming from this part of the fruit. Likewise, the green leaf and stem
tells us that green light is coming from these parts. Rays of these colors of light reach
the object. Since the object is opaque to green
light, it blocks all the green rays that reach it. But it lets the red rays pass
through, since the object is transparent to red light. The only light from the apple that
reaches the observer is red. No light from the leaf or stem is
visible.
Among our answer options, we see
that either option (B) or option (D) will be correct. Both of these show the red part of
the apple that we know the observer can see. The question is, will the leaf and
stem look black, like they do in option (B)? Or will they not appear at all, as
in choice (D)? Let’s remember that white light
includes all colors. The color black is the absence of
any color. So option (B) tells us that no
light from the leaf and stem reaches the observer. And option (D) shows background
white light where the leaf and stem should be.
Since the only light that could
reach the observer from the leaf and stem is green and we know the object blocks
that light, it must be the case that no light from these parts is seen. Yet the leaf and stem are still
there, which means they will block any white light in the background. Therefore, the stem and leaf would
appear black to the observer. This is how the apple is shown in
option (B). This is the correct option.
Let’s now finish this lesson by
remembering a few key points about separating white light into different colors. White light includes light of every
color. A prism separates light by color,
redirecting rays of different colors by different amounts. Prisms deflect red light the least
and violet light the most. And lastly, objects can be either
transparent, translucent, or opaque to colors of light. Transparent objects let nearly all
light through. Translucent objects allow some
light to pass. And opaque objects allow no light
to travel through.
