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The index if refraction is defined as a ratio of the average speed of light through a medium divided by the speed of light through a vacuum.

“n” is the index of refraction, “v” is the average speed of light through a medium, and vc is the speed of light through a vacuum. The speed of light is 3.00x 108 m/s in a vacuum.

All angles in optics are measured with respect to the line normal to the surface.

Because of the duality nature of light, it can be said that it travels in rays or waves. For this unit we are looking at the ray properties of light. The ray of light that hits a surface is called the “incident” ray. The angle the incident ray makes between the normal line and the ray called the incident angle. The Ray that bounces off a surface is called the “reflected” ray. The ray that travels to the other side of the surface is called the “refracted” ray. "To refract" means "to bend." The line separating the two mediums is called the “boundary.” “Medium” is another name for the material light travels through.

Snell’s Law mathematically states:

Where  is on one side of the boundary and  is for value on the other side on the boundary


Refraction of Light in Glass

Purpose: To use Snell's Law to determine the index of refraction (n) of a piece of glass and to use it to find the speed of light in that glass.

Materials: Laser level, protractor, plate glass, plain paper, pencil, data sheet.

Procedure: Check off each procedure upon completion.


The video instructions above can be found here,

1. Place your glass in the middle of a piece of paper and sketch its outline with a pencil. Some glass pieces have chipped edges and may cut you. Be careful !
2. Use a sharp pencil to draw an outline of the glass.
3. Remove the glass from the paper.
4. Use the protractor to draw line that is normal to the long edge of the rectangle. This line should be near the middle of the long side of the rectangle.
5. Turn the Laser level on. Place it on the paper such that the red line it projects is aimed at the intersection between the normal line you drew and the edge of the rectangle.
6. Place the glass on the rectangle. Carefully line it up so that is it is perpendicular to the normal line.
7.  Make a dot where the laser’s light first hits the paper as it comes from the line level.

You should see two red lines on the side of the glass opposite the laser level. One line is projected above the glass. The second line is shorter and not as bright. Only the second line went through the glass. This is the refracted light ray and it the one you are interested in. Use the protractor to draw a DOTTED line along this refracted red line.

9. Remove the glass from the paper. Draw a spline consisting of two lines. The first is the incident ray, the second is the refracted ray as it travels through the glass. Draw the refracted ray about 0.12 cm or longer.
10. Extend the refracted ray about 0.12 m.
11. Measure the angle between the incident line and normal line.

Measure the angle between the refracted line and the normal line.

13. Repeat this procedure for 5 different angles between 15 and 70 degrees.FIll out the data table on the handout shown below in the "Data and Questions" section.
pdf icon Click here for pdf document of the data page shown below.






DRAW A GRAPH of the sin(θincident) vs sin(θrefracted) on the grid below.

The slope is the ratio of . The index of for the incident ray is air, n=1.00.

Use the graph to determine the index of the glass the light travels through and the average speed of the light through this glass. Show your work.

by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)