Lesson Explainer: The Dispersive Power of a Prism Physics

In this explainer, we will learn how to calculate the dispersive power of a prism, given the refractive indices of the different colors of light passing through it.

Dispersion is the word we use to describe the process of distributing, or spreading out, something over a large area. In particular, we will discuss the dispersion of light as it travels through a prism. Light is deviated from its original path when it enters the prism and again when it leaves the prism.

A sketch of this kind of dispersion is shown below. We have a beam of light, which contains both red and violet light, entering a prism. Inside the prism, the light separates into its component beams of pure red and pure violet light.

In this explainer, we will explore a quantitative way of explaining how prisms do this separating. We will also see how the index of refraction affects how much each color of light is spread out. For example, violet light refracts at a greater angle than red light, as shown here.

We call any material that affects different wavelengths of light differently “dispersive.” A prism is an example of a dispersive material because different wavelengths of light are refracted by different amounts.

Why does a prism cause light of different wavelengths to act differently?

To understand this, we can recall Snell’s law:

How does Snell’s law tell us that the angle of refraction is different for different wavelengths of light in a prism?

Imagine a light ray, made up of all wavelengths of light, entering a prism, as shown in the following diagram.

The light ray enters from the left of the prism. Since the ray contains all wavelengths of light, we see that all the colors of light will enter the prism with the same angle of incidence.

However, if the index of refraction on the outside of the prism is different to the index of refraction on the inside of the prism, then we see an interesting effect.

From Snell’s law, we have the relationship .

Recall that the refractive index of a material is a measure of how fast light travels through that material. If the refractive index of a material is equal to 1, then light travels through the material at the same (very high) speed as it travels through a vacuum. As the refractive index increases from 1, this tells us that light travels slower through the material.

Let’s assume that the refractive index on the outside of the prism is the same for all wavelengths of light. An example of a material in which this is true is air. In the language of Snell’s law, this means is the same for all wavelengths of light.

However, we know that prisms refract different wavelengths of light by different amounts. For example, red light is refracted less than violet light. This means that the angle of deviation for red light, which we will call , is smaller than the angle of deviation for violet light, which we will call .

The reason that this happens is that the prism has a different index of refraction for different wavelengths of light. We can see this by comparing the cases of red light and violet light. The following diagram shows how different colors of light are refracted by different amounts. We have labeled the refractive index outside the prism and the angle of incidence . We have also labeled the angles of refraction for red and violet light as and respectively.

Before each of the red and violet wavelengths enters the prism, they both have the same refractive index and the same angle of incidence. This means that the left-hand side of Snell’s law, , is the same for both wavelengths.

For the right-hand side of Snell’s law, we know that the angle of refraction is different for the two wavelengths. In order to satisfy and , must not be the same for the two different wavelengths!

This means we have an index of refraction for red light, , and a different index of refraction for violet light, . Snell’s law for the two wavelengths should therefore read and .

In fact, since each different wavelength of light has a different angle of refraction in a prism, we know that each wavelength has a different index of refraction.

We can use this property as a way of defining a dispersive material.

We will now explore a way of quantifying how much dispersion happens in a given prism.

Let’s return to our prism, but just consider the paths of the red and violet light. We choose to consider these two wavelengths of light because red is the longest wavelength of visible light and violet is the shortest wavelength of visible light. This means we see the two most extreme cases, and we study the largest range of wavelengths that we can.

In the diagram below, we can see that both of these wavelengths of light have deviated from the path they are following due to the prism. If they had not changed path, they would have continued to travel along the black arrow shown here.

We can call the angle that the red beam deviates from the black path , since it is the smallest angle that any visible light will be deviated from. This label is the Greek letter “alpha.”

Similarly, we can call the angle of deviation for violet light , since it is the largest angle of deviation that any visible light will experience.

We can quantify the amount of dispersion that a particular prism causes using these two angles of deviation: and .

We do this by defining a quantity called the “dispersive power” of the prism, which we denote by , where is the Greek letter “omega.” This dispersive power is given by the equation

This equation looks a bit complicated, but if we break it down, it is fairly simple to see what is happening.

On the top of the fraction, we have the difference between the maximum angle of deviation and the minimum angle of deviation, which is . This tells us the range of angles of deviation for the light going into the prism.

On the bottom of the fraction, we have the average value of and , i.e., the average angular deviation caused by the prism. This is what we get when we calculate the quantity . This also corresponds to the angle of deviation of the “average” or “mean” wavelength of light to enter the prism. In terms of colors of light, if it is white light that enters the prism, this will be a color in between yellow and green.

To get the dispersive power of a prism, we need to find these two things and then work out the fraction given above. Let’s also notice that the dispersive power of a prism does not have any units. This means we call it a “dimensionless number.” Each angle and has units of degrees, but in the equation for dispersive power, all of these units cancel out and leave us with no units.

Note that if and are the same, then . This means that the largest and smallest angles of deviation are the same and the prism is not dispersive at all. So, the smallest value of is therefore zero (for nondispersive materials), and it increases to larger values for prisms that disperse light in a large amount. However, typical values for most glass prisms, or similar, are between 0.01 and 0.1.

It is important to remember that, even though the name “dispersive power” uses the word “power,” this does not have the usual meaning it has in physics of an amount of energy over an amount of time. This is just a name we use that happens to reuse the word “power.” Dispersive power is just a quantitative description of how much a prism spreads out light.

The larger the value of , the larger the range of dispersion that light experiences through the prism. The smaller the value of , the smaller the range of dispersion that light experiences through the prism.

This is shown in the diagram below. The prism on the left has a small dispersive power and hence the different colors of light do not spread out very much. The prism on the right has a large dispersive power and hence spreads out the different colors to a greater extent.

This means the prisms must be made of different materials because they have different indices of refraction. This is shown here by the different colors of the two prisms.

Prisms with more dispersive power spread out light more, while prisms with less dispersive power spread out light less.

Recall that we have also said that we can define a dispersive material as a material whose refractive index varies with the wavelength of light. We will now see that we can rewrite the equation for the dispersive power of a prism in terms of the refractive indices of the prism.

We can do this because there is a certain refractive index that corresponds to the wavelength of light that is deviated by the angle and a certain different refractive index that corresponds to the wavelength of light that is deviated by the angle .

The reason that violet light experiences the greatest angle of deviation is because violet light experiences the greatest difference in the refractive index as it enters the prism. This means that the index of refraction for violet light is the largest of all the possible values for the index of refraction that visible light can experience in the prism. To denote this, we call the index of refraction for violet light . This means that the largest angle of deviation corresponds to the largest refractive index.

Similarly, red light experiences the smallest angle of deviation because red light experiences the smallest change in the refractive index as it enters the prism. This means that the index of refraction for red light is the smallest of all the possible values for the index of refraction that visible light can experience in the prism. We will call the index of refraction for red light , so the smallest angle of deviation corresponds to the smallest refractive index. The following diagram is an illustration of this, where different colors of light are shown to have different refractive indices.

In order to rewrite our equation for the dispersive power in terms of the refractive index instead of in terms of the angle of deviation, it is almost as simple as replacing with and replacing with . However, there is one small extra step we must remember, too.

In terms of the minimum and maximum refractive indices, the dispersive power of a prism can be written as

We can see that this equation is very similar to the previous expression we had for the dispersive power. The biggest difference is that we need to include a in the denominator. This is a convention that is used to account for the fact that prisms are usually surrounded by air, which has a refractive index very close to 1.

This means that, whether we know the maximum and minimum angles of deviation of a prism or we know the maximum and minimum indices of refraction of a prism, we can work out the dispersive power of that prism. This tells us how much that prisms spreads out light as it passes through the prism.

Example 2: Identifying the Relationship between Wavelength and Angle of Deviation

The diagram represents red, green, and blue light being deviated through an angle by a prism. Which of the following correctly describes how , the wavelength of the light, relates to the angle ?

1. As increases, decreases.
2. As increases, increases.
3. and are unrelated.

Answer

We know that red light has the longest wavelength of any color of visible light. Out of the colors shown on the diagram, the color that has the shortest wavelength is blue.

This means that is larger for red light, and is smaller for blue light. So, as we go from red to green to blue light, decreases.

We can also see that red light is deviated the least out of all of the colors in the diagram, while green light is deviated a bit more and blue light is deviated the most. This means that , the angle of deviation, is smallest for red light. This angle is larger for green light, and increases even more for blue light.

So we have seen that, as we go from red to green to blue light, the wavelength decreases, but the angle of deviation gets larger. This means that, if we go the other way, from blue light to green light to red light, the wavelength gets larger but the angle of deviation gets smaller.

We can summarize what we have said as the following statement, which we give as our answer: As increases, decreases.

Let’s note here that we also know that the index of refraction increases (or decreases) in the same way the does. So, we also know that as increases, decreases.

Let’s now give symbols to name the average angle of deviation of a prism and the average refractive index of a prism:

Recall that the “apex angle” of a prism is the angle at the top, or tip, of the prism. In the following diagram, we see two prisms with two different apex angles, which are labeled and . Both of these prisms have the same refractive indices.

The left-hand prism has a small apex angle , while the right-hand prism has a large apex angle . The larger the apex angle of a prism, the larger the angle of deviation is for each wavelength of light. Another way to say this is that if the apex angle increases, then the average angle of deviation, , increases too.

In the following diagram, we now show two prisms with the same apex angle, labeled , but different indices of refraction. This is demonstrated by the different colors of the two prisms.

We can see that if the refractive index of the prism increases, then the angle of deviation of a light beam increases too. If the prism is made of a dispersive material and if we then increase the average refractive index, , then the average angle of deviation, , increases.

From now on, let’s consider “thin prisms,” which are prisms whose apex angle is small. In this context, an apex angle of approximately or less is considered small.

Let’s consider the quantity for a thin prism. If we want to increase this quantity, what would we need to change in the prism to make this quantity get larger?

We know that if we increase , then this increases . However, we are considering the fraction , not just . This means that if we increase , this does make get bigger, but the overall fraction does not get bigger because increasing the denominator of a fraction makes the whole fraction smaller. This means that increasing cannot be the answer.

Instead, the answer is the apex angle, . If the increase , then the quantity will increase too. In fact, for a thin prism, we can actually set these two things equal to each other, so we have

Let’s check if the units of this equation are consistent. On the left-hand side, is an angle, so it has the unit of degrees. On the right-hand side, we have a fraction. The numerator of this fraction is the average angle of deviation, which is an angle, so this also has the unit of degrees. The denominator of the right-hand side is the average refractive index, which is a dimensionless, or unitless, number. This means that, overall, the right-hand side of the equation also has the unit of degrees, so our equation has consistent units.

We have also seen two different expressions for the dispersive power of a prism, one in terms of angles of deflection and one in terms of the refractive index. Since these expressions are equivalent, we can combine them to find an expression that relates angles of deviation to the refractive index.

That is, we have the following:

We can rewrite this with our new notation for the average angle of deviation and the average refractive index. Doing this gives us

Let’s now take just the second equality here, and multiply both sides of the equation by . This gives us

We can rewrite the right-hand side of this in the following way:

In the second line here, we have used our equation for the apex angle in terms of and , which is only true for thin prisms, so here we are assuming that is small.

Let’s think about how we might use this new equation for thin prisms: .

Suppose we have a prism that has an apex angle of , which is a small enough angle for us to consider this a thin prism. This prism has different refractive indices for different wavelengths of light that are transmitted though the prism. This refractive index for the longest wavelength of light is 1.512, and the refractive index for the shortest wavelength of light is 1.535. We call the difference between the deviation angle for the most deviated and least deviated rays passing through the prism its “angular dispersion.” We can calculate this as follows.

The following diagram shows the refraction of light in this thin prism.

Angular dispersion is the difference between the maximum angle of deflection and the minimum angle of deflection for the prism. We call the angular dispersion , where is the Greek capital letter delta. Note that should be read as one quantity, and it is not a multiplication of times . This means that .

Let’s use our equation relating refractive index to angle of dispersion, which is . We can replace the left-hand side of this equation with the angular dispersion, so we have

We can now input the values we are told in the question, which are , , and . This gives us

This means that our answer for the angular dispersion of the prism is .

Let’s now summarize what has been learned in this explainer.