Lesson Video: Separating White Light into Different Colors | Nagwa Lesson Video: Separating White Light into Different Colors | Nagwa

# Lesson Video: Separating White Light into Different Colors Science • Second Year of Preparatory School

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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.

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### 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.

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