Determine the refractive index of glass

Hypothesis: The incident ray, the normal and the refracted ray all lie in the same plane. Also, according to Snell’s law, for 2 particular media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction should be a constant:

Sin i

Sin r


1. A rectangular prism

2. Four 1 inch pins

3. A protractor

4. A ruler

5. An A4 size cardboard

6. An A4 size sheet of plain paper

7. 4 bits of blue tack

8. A pencil


1. A table is drawn at the top left side of the A4 size paper with column titles: “Angle of Incidence” and “Angle of Refraction”. The table should contain 8 rows excluding the column titles.

2. The rectangular prism is placed at the middle of the top of the page and its perimeter is traced with the help of the ruler and pencil. The normal is soon after marked out in the approximate centre of the perimeter using the protractor at the 90 degrees point. The operation is then repeated at the middle of the bottom of the page and finally to the back of the page.

3. 4 perimeters with a normal on each should already be drawn at that stage. Next, an angle should be measured and traced from the point where the normal touches the perimeter line from any of the 4 sketch. The angle is noted at the table as an angle of incidence. This step is repeated five more times with different angles each time.

4. The page is placed on the piece of cardboard and stabilised using the blue tack bits.

5. 2 pins are stuck on any incident lines and the rectangular prism in placed fitting the perimeter marked previously.

6. The pins should be observed at a parallel and opposite side of the rectangular prism and the cardboard should be moved so that 1pin covers the other totally and 2 more pins should be stuck.

7. The pins are removed together with the page and a line is drawn from the 2 recent holes caused by the pins.

8. A new normal is outlined out of the new line and the angle is measured and noted as the emergent ray.

9. A line is drawn from the emergent ray to the incident ray and the angle from the normal is measured using the protractor.

10. Steps 4 to 8 are repeated 7 more times varying on the 4 perimeters.

11. The page should resemble the following:



The results seem satisfying enough to support the hypothesis, hence supporting the idea of the law of refraction.

Evaluation and precautions

* Blue tack was used so as to keep the paper stable while working to prevent errors.

* A double check with another protractor was performed to ensure its accuracy.

* Mistakes such as not perfectly vertical pins stuck or not exactly parallel positioning of the eye could have been performed relying on the results.

* Parallax errors or other mistakes such as too bumpy cardboard may have lead to not accurate enough results.

* Another method of performing the experiment yet roughly the same precision would be to use light beams which would be more accurate than our eyes’ positioning therefore reducing parallax error.