AP Physics 2 : Index of Refraction

Study concepts, example questions & explanations for AP Physics 2

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Example Questions

Example Question #1 : Index Of Refraction

The speed of light in a vacuum, , is calculated to be . The speed of light in a diamond is calculated to be . What is the refractive index of diamond?

Possible Answers:

Correct answer:

Explanation:

The definition of refractive index of a medium  is the speed of light in a vacuum divided by the speed of light in the medium:

We have values for  and , so we can plug in our numbers into the equation.

Because we're dividing two values with the same units, our answer is unitless. 

Example Question #2 : Index Of Refraction

What is the speed at which light travels through the water?

  and 

Possible Answers:

Correct answer:

Explanation:

Write the formula relating the speed of light, velocity of light in specific medium, and index of refraction of the medium.

Rewrite the equation so that we are solving for the velocity of light in water. Substitute given values.

Example Question #3 : Index Of Refraction

A beam of light travelling through air enters a transparent material at an angle of incidence at , with the refracted beam's angle being . What is the index of refraction of the material? Assume the index of refraction of air is 1.

Possible Answers:

Correct answer:

Explanation:

To find the index of refraction given two angles and another index of refraction, we use Snell's law.

We can plug in the numbers we have to find the answer.

Simplifying, we get:

Example Question #4 : Index Of Refraction

A beam of light travelling through air enters a medium with an index of refraction of  and goes through it at an angle of . What is the angle of incidence?

Assume the index of refraction of air is 1.

Possible Answers:

Correct answer:

Explanation:

To find the angle of incidence, we use Snell's law.

We're given both indices of refraction, and the second angle, so we can plug in our numbers.

Therefore, the angle of incidence is .

Example Question #5 : Index Of Refraction

An argon/krypton laser puts out green light at . It takes  to travel through  of honey. What is the index of refraction of the honey?

Possible Answers:

Correct answer:

Explanation:

First off we can find the velocity of the laser beam since we know the distance of honey it travels through and the transit time.

However, this doesn't need to be explicitly solved for. The velocity of light inside a material is inversely proportional to the index of refraction of that material where

where  is the speed of light in vacuum. Setting these equal, we can solve for the index of refraction of the honey.

We expect that the index of refraction will be greater than that of air and a velocity less than .

Example Question #6 : Index Of Refraction

The speed of light through a particular material is measured to be , calculate the index of refraction of the material.

Possible Answers:

Correct answer:

Explanation:

The speed of light passing through a medium is given by:

Rearrange to solve for the index of refraction and plug in numbers.

Example Question #7 : Index Of Refraction

You are passing a ray of light through clear alcohol to determine properties. You shine the light ray exactly  to the surface of alcohol. 

Determine the index of refraction required in the alcohol to have total internal reflection? 

Possible Answers:

Correct answer:

Explanation:

To have total internal reflection, our equation will become:

Example Question #8 : Index Of Refraction

You are passing a ray of light through clear alcohol to determine properties. You shine the light ray exactly  to the surface of alcohol. 

Determine the index of refraction of alcohol if the light ray bends to  to the normal. Assume index of refraction of air is 

Possible Answers:

Correct answer:

Explanation:

We can use our knowledge about the indices of refraction to come up with our equation:

, where  is the index of refraction of air and  is the index of refraction for our alcohol. 

Since 

Example Question #9 : Index Of Refraction

Suppose that a ray of light traveling through air strikes a new medium. Upon doing so, the light bends away from the normal. Which of the following could this new medium be?

Possible Answers:

Vacuum

Glass

Carbon dioxide

Water

Diamond

Correct answer:

Vacuum

Explanation:

For this question, we're told that light is passing from air into another medium. In doing so, the light is refracted such that it bends away from the normal. We're asked to identify a possible medium.

The most important thing to understand about refraction is that when light passes into a new medium at an angle with respect to the normal, that light will be refracted, either away from or toward the normal. This is because light will travel through different media at different speeds. The faster light travels, the more it will bend away from the normal.

Generally, the angle at which light is bent can be predicted by Snell's law. In doing so, this equation takes use of the refractive index, a value unique to each medium. The expression for refractive index is as follows.

Where  refers to the refractive index,  refers to the speed of light in a vacuum, and  refers to the speed of light in a given medium.

Since the speed of light is fastest when in a vacuum, the refractive index can never be less than . Only when the light is in a vacuum is the refractive index equal to . In any other medium, the refractive index will be greater than , even if it is slight.

Light will always refract away from the normal when it passes into a medium with a lower refractive index (indicating the light is traveling faster). Starting from air, the only way this can happen is for the new medium to have an index of refraction that is less than air. Of the answer choices shown, the only one that fits that criteria is the vacuum.

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