The method and the results are on the refractive index lab. Do the analysis yourself first, then check the answers below.
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. These answers go with the Physics 9-10 lab on refractive index: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
Here are the refractive index lab answers for physics grades 9-10: graph, calculations, uncertainties and conclusion. First do the lab yourself, then compare, and go back to the lesson if needed.
These model answers use the sample results in the table. Your own values will differ a little, but sin i against sin r should still give a straight line through the origin with a gradient equal to the refractive index.
Level: Physics grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: IGCSE 3.2
Refractive index lab answers: the analysis explained
Question 1 Plot the graph
The points lie close to a straight line through the origin, showing that sin i is directly proportional to sin r.
Question 2 Gradient
Using the first and last points: gradient = (0.940 − 0.259) ÷ (0.629 − 0.174) = 0.681 ÷ 0.456 = 1.49. The gradient equals the refractive index n.
Question 3 Mean value
Mean of the last column = 1.51. It agrees with the graph gradient within experimental error.
Question 4 Critical angle
sin c = 1 ÷ 1.51 = 0.663, so c = 41.5°.
Question 5 Sources of error
Pins are thick, so the line through them is uncertain by about 1°; the protractor is read to 1°. Both cause random error in r, which is about ±1°.
Question 6 Why a wide spacing
A long line through widely spaced pins fixes the direction more precisely, so the angle drawn has a smaller percentage error than with pins 1 cm apart.
Question 7 Improvement
Use a ray box or laser pointer with a thin beam, or a larger protractor and thinner pins, and repeat each angle three times to average the results.
