Refraction

What is Refraction

Refraction occurs when light moves from one medium (material) to another

Because of density differences light will change speed as it moves from one material to another. This change in speed bends the light as it crosses the boundary separating the two mediums

High Density

Low Density

Refraction

Refraction is what makes a spoon in a glass appear to be disjointed

This is because light willTravel faster in air thanin water and appear to bendAs it enters the water

The greater the differencein density between the two mediums the greater the angle of the bending

The angle that light bends as it moves from one medium to another is called the angle of refraction

Refraction Index (n)

Each material has its own refraction index that is used to calculate the amount of refraction that occurs as light moves into a new optical medium

The refraction index is a physical property of any optical material and is equal to the velocity c of light of a given wavelength in empty space divided by its velocity v in a substance,

or n = c/v

It can be also calculated mathematically using the following geometric relationship: the refractive index n is defined as the ratio of the sine of the angle of incidence (sin i) to the sine of the angle of refraction (sin r),

n = sin i / sin r (AIR)

Common Refraction Indices

Optical Medium State Refraction Index

Air Gas 1.000293

Ice Solid 1.31

Water Liquid 1.33

Ethyl Alcohol Liquid 1.36

Quartz Solid 1.54

Salt Solid 1.54

Tourmaline Solid 1.62

Garnet Solid 1.73-1.89

Cubic Zirconia Solid 2.14-2.20

Diamond Solid 2.41

Light Direction and Refraction

When going from a slower to a faster medium, light bends away from the normal. When going from a faster to a slower medium, light bends toward the normal

Total Internal Reflection We have learned that when light goes from a slow medium to a fast medium that the light bends away from the normal.

We can make the angle in the slow medium as close to 90 degrees as we like -- which means that eventually we will "run out of room" to bend the light! The series of images below illustrate this point:

Total Internal Reflection Where does the light go after we no longer have any more room in the air for it to bend into?

The answer is that it is all reflected.

In all cases some light is reflected. A more realistic series of diagrams is shown below, with thicker lines corresponding to brighter light

Only in the last picture does Total Internal Reflection Occur

Critical Angle

Total Internal Reflection

The Angle at which the refracted rays become parallel to the boundary between two optical mediums is called the “Critical Angle

Once the Critical Angle is exceeded Total internal reflection will occur.

Total internal reflection can only occur when light is travelling from a denser medium into a less dense medium

Snell’s Law

ni sin i = nr sin r

Where ("theta i") = angle of incidence

("theta r") = angle of refraction

ni = index of refraction of the incident medium

nr = index of refraction of the refractive medium

Practice Question

In the following two examples, use Snell's law, the sine button on your calculator, a protractor, and the index of refraction values to complete the following diagrams. Measure , calculate , and draw in the refracted ray with the calculated angle of refraction.