Quick answer: Refraction is the bending of light as it changes speed between materials, and its size is given by the refractive index n = sin i ÷ sin r. This Physics 9-10 lab on refractive index gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How can you find the refractive index of a glass block using only pins, paper and a protractor? In this lab you trace rays through a block, measure the angles of incidence and refraction, and use the results to calculate n.
Level: Physics grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: IGCSE 3.2
Aim
To measure the refractive index of a rectangular glass or acrylic block by tracing a ray through it and to test that sin i ÷ sin r stays constant.
Equipment
- Rectangular glass or acrylic block
- Four pins
- Sheet of plain A4 paper on a corkboard or thick cardboard
- Protractor and sharp pencil
- Ruler
- Calculator
Safety
Pins are sharp. Push them in carefully into the board and never hold a pin pointing at your hand or face. Put the block down away from the table edge.
Method
- Place the block on the paper and draw round it. Remove it, mark point O on the top edge and draw the normal there.
- Use the protractor to draw a line from O at i = 15°, then replace the block in its outline.
- Push pins P1 and P2 upright on the line, about 8 cm apart.
- Look through the block from the opposite side and place P3 and P4 so that all four pins appear in one straight line.
- Remove the block and pins. Draw a line through P3 and P4 to the bottom edge, meeting it at O′, and join O to O′.
- Measure the angle r between OO′ and the normal inside the block.
- Repeat for i = 20°, 30°, 40°, 50°, 60° and 70°.
Results
Sample measurements from a glass block (angles to the nearest degree):
| i (°) | r (°) | sin i | sin r | sin i ÷ sin r |
|---|---|---|---|---|
| 15 | 10 | 0.259 | 0.174 | 1.49 |
| 20 | 13 | 0.342 | 0.225 | 1.52 |
| 30 | 19 | 0.500 | 0.326 | 1.54 |
| 40 | 25 | 0.643 | 0.423 | 1.52 |
| 50 | 31 | 0.766 | 0.515 | 1.49 |
| 60 | 35 | 0.866 | 0.574 | 1.51 |
| 70 | 39 | 0.940 | 0.629 | 1.49 |
Analysis questions
Question 1 Plot the graph
Plot sin i (y-axis) against sin r (x-axis) for the sample data and draw a line of best fit through the origin.Question 2 Gradient
Determine the gradient of your line and state what it represents.Question 3 Mean value
Calculate the mean of the last column of the table and compare it with the gradient.Question 4 Critical angle
Use your refractive index to predict the critical angle of the block.Question 5 Sources of error
Suggest two sources of error in locating the pins and in reading r.Question 6 Why a wide spacing
Explain why P1 and P2 should be far apart rather than close together.Question 7 Improvement
Suggest one improvement that would give a more precise value of n.
