IGCSE · 3 · practical
Waves · Question 10
Waves · Original GioPhysics question with a detailed, mark-by-mark answer guide.
- Demand
- demanding
- Marks
- 8
- Topics
- 1
- Answer
- Complete
A student determines the refractive index of a glass block using optical pins. She places the block on a sheet of paper, draws round it, and shines a ray at the top face at an angle of incidence i, marking the ray in and the ray out with pins. She repeats this for several values of i and measures the angle of refraction r inside the block.
| i / degrees | 20 | 30 | 40 | 50 | 60 |
|---|---|---|---|---|---|
| r / degrees | 13.2 | 19.5 | 25.4 | 30.7 | 35.3 |
| sin i | 0.342 | 0.500 | 0.643 | 0.766 | 0.866 |
| sin r | 0.228 | 0.334 | 0.429 | 0.511 | 0.578 |
- (a)
Describe Describe how the student should place and use the optical pins so that the path of the ray is marked accurately.
3 marks - (b)
Explain Explain why plotting a graph of sin i against sin r gives a more reliable value for the refractive index than calculating sin i / sin r for a single pair of readings.
3 marks - (c)
Determine Use the first and last results to determine a value for the refractive index of the glass.
2 marks
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(a)
Describe Describe how the student should place and use the optical pins so that the path of the ray is marked accurately.
Turn the task into a measurable method: name the independent, dependent and controlled quantities; describe apparatus and repeats; then explain how the evidence will be processed and how uncertainty or safety will be managed.
- 1
place two pins in the incident ray, as far apart as possible
- 2
view from the far side of the block and place two more pins so that all four appear in line, one behind another
- 3
remove the block and draw a straight line through each pair to find the ray in and the ray out
(b)
Explain Explain why plotting a graph of sin i against sin r gives a more reliable value for the refractive index than calculating sin i / sin r for a single pair of readings.
Turn the task into a measurable method: name the independent, dependent and controlled quantities; describe apparatus and repeats; then explain how the evidence will be processed and how uncertainty or safety will be managed.
- 1
n = sin i / sin r, so a graph of sin i against sin r should be a straight line through the origin of gradient n
- 2
the gradient uses all of the readings rather than one, so random errors in individual measurements average out
- 3
an anomalous result is visible as a point off the line and can be identified rather than silently included
(c)
Determine Use the first and last results to determine a value for the refractive index of the glass.
Turn the task into a measurable method: name the independent, dependent and controlled quantities; describe apparatus and repeats; then explain how the evidence will be processed and how uncertainty or safety will be managed.
- 1
gradient = (0.866 − 0.342) / (0.578 − 0.228) = 0.524 / 0.350
- 2
n = 1.50 (accept 1.5)
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