Refractive Index FHI
[Based on: http://webs.mn.catholic.edu.au/physics/emery/prelim_communication_worksheet.htm#refractive]
AIM: To determine the refractive index of Perspex using independent methods and to compare the results.
This investigation consists of two parts, using different methods to independently determine the refractive index of two
prisms. Part A involves passing light through a prism using a ray box, and recording the path. Part B involves
determining the apparent depth of a line drawn on paper, as seen through the prism.
Part A - Method
1. Work in conjunction with another group – you are required to swap results and equipment with this group, so this is
a part of your planning before you carry out the investigation. Each group is to use a different rectangular Perspex
prism, A and B (or a Perspex prism and a glass prism for comparison). Independently of the other group, each
group is to take 3 measurements of the angles of incidence and refraction for light passing into the prisms from air.
2. Using a standard school laboratory optics kit, set up a ray box with a single slit aperture. Connect it to a DC power
supply set to 12V.
3. Place a rectangular Perspex prism in the middle of a white sheet of A4 paper so that its long edge is parallel to the
long edge of the paper.
4. Trace carefully around the prism to mark its position on the paper.
5. Remove the prism and at ONE point somewhere near the centre of the left hand side of the prism draw the normal
to the side of the prism (using a protractor or set square).
6. From the point chosen in 4 above, draw three straight lines (representing rays of light) striking the side of the prism
at three different angles of incidence. Clearly label the angles of incidence. These lines are going to be used to
guide rays of light to strike the prism at those angles of incidence.
7. Shine a single ray of light striking the prism along one of the paths that you have marked on the paper and clearly
trace onto the paper the path of the refracted ray emerging on the other side of the prism. Repeat this for the other
two angles.
8. Measure and label the angles of refraction for each of the angles of incidence. Tabulate these results in the table
shown in the results section.
9. Complete all entries in the table and the other details required in the results section.
Results from your own group
Angle of Incidence (°)
Angle of Refraction (°)
Sin i
Sin r
Prism A
Prism A
Prism A
Average
Prism B
Prism B
Prism B
Average
Refractive Index
Results from the other group
Angle of Incidence (°)
Angle of Refraction (°)
Sin i
Sin r
Refractive Index
Prism A
Prism A
Prism A
Average
Prism B
Prism B
Prism B
Average
Analysis
1. Show how you calculated the refractive index of the prism using one angle of incidence data.
2. Determine an average value for the refractive index of each of the prisms by calculating an average of all three
refractive index measurements in the table.
3. Compare the values of the refractive index for the two prisms found by each of the two groups.
Questions
1. Identify the independent and dependent variables in this investigation.
2. State which physical factors you have tried to keep constant and explain why you have tried to do this.
3. Identify the physical factors that are most likely to have adversely affected the accuracy of your results and explain
how each factor affected the accuracy.
4. What aspect of the investigation was the focus of two groups independently determining the refractive index of
prisms A and B. Describe how you would improve this aspect of the investigation.
5. Comment on the validity of your investigation.
6. Identify the relationship between the independent and dependent variables in this investigation and provide evidence
to support your view.
Part B – Establishing Validity of the Procedure Used in Part A
Evidence supporting the validity of a procedure can be obtained by conducting the investigation using an alternative and
independent method and then comparing the results of the two methods. If the results are similar, this suggests that the
procedure is valid.
It is a necessary condition for a procedure to be valid for the measurements taken in the procedure to be valid.
Measurements may be invalidated in different ways. e.g. by using incorrectly calibrated equipment, by errors such as
parallax error or
Follow these steps
1. Draw a straight line on two pieces of plain paper.
2. Place one of the sheets under the prism.
3. Sticky tape a piece of paper to the side of the prism so that the
position of the piece of paper used to determine the apparent depth
held at the side of the prism can be marked easily and measured.
4. Looking down into the prism from above, raise the second sheet of
paper until the two lines appear to coincide with no parallax.
5. Record the real and apparent depths. The apparent depth is the
distance from the top of the prism to the line on the raised sheet of
paper – this is where the line under the prism appears to be
Real depth
________
Apparent depth ________
Calculate the refractive index from the formula:
6. Compare the refractive index using this method to that obtained using the method in part A.
Conclusion: Now write a simple conclusion for this investigation. [Look at the aim first]