Light

1 Plan and risk assessment

A smooth surface reflects light at the same angle it arrives at, measured from the normal (a line at 90° to the surface). When light goes from air into glass or Perspex it slows down, so it bends towards the normal: the angle of refraction is smaller than the angle of incidence.

angle of incidence = angle of reflection

Prediction

2 Apparatus

• ✓Ray box with a single-slit card and lens, and a 12 V power supply Makes a narrow ray of light.
• ✓Sheet of plain A3 paper and tape You draw the lines and mark the rays on it.
• ✓Sharp pencil Draws thin, exact lines and crosses.
• ✓30 cm ruler (resolution 1 mm) Draws the surface line and joins the crosses.
• ✓Protractor (180°) (resolution 1°) Draws the normal and the incident lines, and measures the angles.
• ✓Plane mirror in a holder Reflects the ray (specular reflection).
• ✓Rectangular glass block Refracts the ray.
• ✓Rectangular Perspex block, the same size Refracts the ray: compared with glass.

3 Method: your choices

• Which version are you doing?: AQA required practical 9: reflection and refractionA plane mirror, then glass and Perspex blocks, on one sheet.
• Make a narrow ray: DoneYou mark the centre of the ray with a cross. The narrower the ray, the more exactly you can find its centre.
• Draw the normal: With the protractor, at 90°Your normal is within about half a degree of 90° to the surface: no effect you could measure.
• Draw the incident rays: 10° to 70°, every 10°Seven points spread across the whole range: the curve of your graph is clear, and one point that does not fit stands out.
• Clear the gangwaysSafety: DoneIn a darkened room it is easy to trip over things you cannot see.
• Darken the room: DoneThe reflected and refracted rays are faint. In the dark their edges, and so their centres, are much easier to see.
• Mark the crosses: Far apart: one near the block, one about 9 cm awayA small error in placing a cross hardly changes the direction of the line you draw through it.
• Measure the angles: From the normal (the line N)Angles of incidence, reflection and refraction are all measured the way the laws are written.

Anomaly: No anomalous reading.

6 Analysis

1. The law of reflection

   Your mean angles of reflection were 31° at 30° and 61° at 60°. What do your results show?

   The angle of reflection equals the angle of incidence, within experimental accuracy. Right first time

2. Uncertainty in the mean

   uncertainty = range ÷ 2

   Work out the uncertainty in your mean angle of reflection at 60°.

   1.0° Right first time

3. Shape of the graph

   What does your graph show?

   As the angle of incidence increases, the recorded angle increases, but less and less steeply: a curve. Right first time

4. Glass or Perspex?

   Compare your angles of refraction for the two blocks. Do your results show that one material bends light more than the other?

   No: the differences are no bigger than the protractor’s resolution (1°) and the scatter in my results, so I cannot tell. Right first time

7 Conclusion and evaluation

Conclusion

Sources of error

Improvement

Repeatable or reproducible

9 Exam questions on your data

On your own readings, self-marked against the mark scheme, with error carried forward where it applies. Total 32/32.

1. 01.1[1 mark]

   A student shone a ray of light at a plane mirror. The mirror has a smooth, shiny surface.

   What type of reflection happens at the mirror?

   Tick (✓) one box.

   • Diffuse reflection
   • ✓ Specular reflection (your choice)
   • Refraction
   • Absorption

   Your answer

   Specular reflection

   1/1

   • Specular reflection

   Full marks. Your answer meets all the points for this question.

   Model answer: Specular reflection: a smooth surface reflects the ray in a single direction.

2. 01.2[1 mark]

   Table 1 shows the angles of reflection you measured for an angle of incidence of 60°.

   Table 1

   Angle of incidence in degrees	Angle of reflection in degrees, trial 1	Angle of reflection in degrees, trial 2	Angle of reflection in degrees, trial 3
   60	62	60	61

   Trials 1 to 3 are the angle of reflection in degrees.

   Calculate the mean angle of reflection.

   Your answer

   61

   Working: mean = (62 + 60 + 61) ÷ 3 = 61°

   1/1

   • 61°

   Full marks. That is exactly what the mark scheme is looking for.

   Model answer: mean = (62 + 60 + 61) ÷ 3 = 61°

3. 01.3[2 marks]

   Determine the uncertainty in the mean angle of reflection.

   Use Table 1.

   Your answer

   1

   Working: range = 62 − 60 = 2° uncertainty = 2 ÷ 2 = ±1°

   2/2

   • Range = 62 − 60 = 2°
   • Uncertainty = ±1°

   Full marks. Well done.

   Model answer: range = 62 − 60 = 2° uncertainty = 2 ÷ 2 = ±1°

4. 01.4[1 mark]

   The three angles of reflection in Table 1 are different.

   Name the type of error that caused this.

   Your answer

   random error

   1/1 self-marked

   • Random (error); do not accept "human error"

   Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

   Model answer: Random error.

5. 01.5[1 mark]

   A student shone a ray of light from a ray box at a plane mirror. They marked crosses along the incident and reflected rays and used a protractor to measure the angle of reflection. They repeated this three times and the three angles of reflection were different.

   Suggest one practical reason why the student’s angles of reflection were different.

   Your answer

   It was hard to judge where the centre of the ray was when marking the cross.

   1/1 self-marked

   • Misjudging the centre of the ray; or the ray box (or mirror) not put back in exactly the same position; or the eye not in the same position; allow protractor not in the same position. Ignore "human error"

   Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

   Model answer: It is hard to judge exactly where the centre of the ray is when marking the cross, and the ray box may not have been put back in exactly the same position each time.

6. 01.6[2 marks]

   Does the data in Table 1 support the law of reflection?

   Use data from Table 1 in your answer.

   Your answer

   Yes. The mean angle of reflection is 61 degrees, which equals the angle of incidence of 60 degrees within experimental accuracy, because 60 is within the range 60 to 62.

   2/2 self-marked

   • Yes: within experimental accuracy the angle of incidence and the angle of reflection are the same
   • Relevant use of data, e.g. the mean (61°) is within 1° of 60°, or 60° lies within the range 60° to 62°

   Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

   Model answer: Yes. The mean angle of reflection is 61°, which is the same as the angle of incidence (60°) within experimental accuracy: 60° lies within the range of the results, 60° to 62°.

7. 01.7[1 mark]

   Suggest how the student could get more evidence that the angle of reflection always equals the angle of incidence.

   Your answer

   Measure the angle of reflection for other angles of incidence that have not been measured yet.

   1/1 self-marked

   • Collect results for angles of incidence not yet measured (a wider range of angles)

   Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

   Model answer: Collect results for other angles of incidence that have not been measured yet, such as 10°, 20°, 40° and 50°.

8. 01.8[1 mark]

   A ray of light hits a plane mirror. The angle between the ray and the surface of the mirror is 25°.

   What is the angle of incidence?

   Your answer

   65

   1/1

   • 90 − 25 = 65°

   Full marks. Clear and precise.

   Model answer: The normal is at 90° to the mirror, so the angle of incidence = 90 − 25 = 65°.

9. 01.9[1 mark]

   What is the angle between the incident ray and the reflected ray?

   Your answer

   130

   Working: angle of reflection = angle of incidence = 65° angle between the rays = 65 + 65 = 130°

   1/1

   • 65 + 65 = 130° (allow 2 × their previous answer)

   Full marks. Clear and precise.

   Model answer: angle of reflection = angle of incidence = 65° angle between the rays = 65 + 65 = 130°

10. 01.10[2 marks]

    Figure 1 shows a ray of light meeting a plane mirror at point P. The angle of incidence is 35°.

    Complete Figure 1. Draw the normal at P, then draw the reflected ray.

    Put an arrowhead on the reflected ray. Each line is right if it points within 6° of the correct direction and starts or passes through P.

    Your answer

    2 items drawn on the figure

    2/2

    • the normal at P
    • the reflected ray

    Every item is in the right place and the right direction.

    Model answer: The normal is a straight line at 90° to the mirror through P. The reflected ray leaves P on the other side of the normal, 35° from it (the angle of reflection equals the angle of incidence), and has an arrowhead pointing away from the mirror.

11. 02.1[1 mark]

    A student investigated how the angle of refraction in a glass block depends on the angle of incidence.

    What is the independent variable?

    Tick (✓) one box.

    • Angle of refraction
    • ✓ Angle of incidence (your choice)
    • Size of the glass block
    • Width of the ray

    Your answer

    Angle of incidence

    1/1

    • Angle of incidence

    Full marks. That is exactly what the mark scheme is looking for.

    Model answer: Angle of incidence: the student chooses and changes it.

12. 02.2[1 mark]

    The lamp inside a ray box gets very hot when it is switched on.

    Give the harm this hazard could cause.

    Your answer

    It could burn your fingers.

    1/1 self-marked

    • Burns (to the skin or fingers); ignore "it is hot"

    Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

    Model answer: It could burn your fingers if you touch it.

13. 02.3[6 marks]

    A student has a ray box, a rectangular glass block, a protractor, a ruler, a sharp pencil and a sheet of paper.

    Describe a method the student could use to investigate how the angle of refraction depends on the angle of incidence.

    Methods that only use a mirror and reflection score no marks.

    Your answer

    1. Place the glass block on a sheet of paper and draw around it. 2. Remove the block and draw a normal at 90° to one long side using a protractor. 3. Put the block back on its outline and use the ray box to shine a narrow ray of light at the point where the normal meets the block. 4. Mark the ray of light entering the block with crosses, and mark the ray emerging from the other side with two crosses far apart. 5. Remove the block and join the crosses to show the complete path of the ray through the block. 6. Use the protractor to measure the angle of incidence and the angle of refraction from the normal. 7. Repeat for a range of angles of incidence, increasing the angle in 10° intervals from 10° to 70°.

    6/6 self-marked

    • Place the glass block on the paper and draw around it
    • Draw a normal at 90° to the surface
    • Use the ray box to shine a ray of light at the block (where the normal meets it)
    • Mark the ray of light entering the block
    • Mark the ray of light emerging from the block
    • Remove the block and join the points to show the complete path of the ray
    • Use a protractor to measure the angle of incidence
    • Use a protractor to measure the angle of refraction
    • Use a range of different angles of incidence
    • Increase the angle of incidence in 10° intervals from 10° to 70° (allow 60° or 80°)
    • Allow: optical pins instead of a ray box

    Self-marked: Level 3, 6 of 6 marks. You ticked 11 of 11 indicative points. This is your own judgement against the mark scheme.

    Model answer: 1. Place the glass block on a sheet of paper and draw around it. 2. Remove the block and draw a normal at 90° to one long side using a protractor. 3. Put the block back on its outline and use the ray box to shine a narrow ray of light at the point where the normal meets the block. 4. Mark the ray of light entering the block with crosses, and mark the ray emerging from the other side with two crosses far apart. 5. Remove the block and join the crosses to show the complete path of the ray through the block. 6. Use the protractor to measure the angle of incidence and the angle of refraction from the normal. 7. Repeat for a range of angles of incidence, increasing the angle in 10° intervals from 10° to 70°.

14. 02.4[2 marks]

    Another student used the ray box without its lens, so the ray of light was much wider.

    Explain why a wider ray increases the uncertainty in the measured angles.

    Your answer

    It is harder to judge where the centre of a wider ray is, so the crosses can be in the wrong place and there is a larger uncertainty in the angles.

    2/2 self-marked

    • It is harder to judge where the centre of a wider ray is
    • So there is a larger uncertainty in the position of the crosses and the angles (allow more random error)

    Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

    Model answer: It is harder to judge where the centre of a wider ray is, so each cross could be further from the true centre and the angles measured have a larger uncertainty.

15. 03.1[1 mark]

    The protractor used in the investigation has a scale marked in single degrees.

    What is the resolution of the protractor?

    Your answer

    1

    1/1

    • 1°

    Full marks. Well done.

    Model answer: 1°

16. 03.2[1 mark]

    Another student did the same investigation with a glass block. In one trace the block was knocked while the crosses were being marked.

    Table 2 shows the student’s results.

    Which result is anomalous?

    Select one row.

    Table 2

    Angle of incidence in degrees	Angle of refraction in degrees
    10	6
    20	13
    30	19
    40	25
    50	24
    60	35
    70	39

    Your answer

    the result at 50°

    1/1

    • The result at 50°: it does not fit the smooth pattern of the others

    Full marks. That is exactly what the mark scheme is looking for. At 50° the angle of refraction (24°) does not fit the smooth pattern of the others.

    Model answer: The result at 50°: 24° does not fit the smooth curve through the other results.

17. 03.3[1 mark]

    Look at the results in Table 2. Ignore the anomalous result. Which statement describes the relationship between the angle of incidence and the angle of refraction?

    Tick (✓) one box.

    • The angle of refraction is directly proportional to the angle of incidence.
    • ✓ As the angle of incidence increases, the angle of refraction increases by less and less: the graph is a curve. (your choice)
    • The angle of refraction is equal to the angle of incidence.
    • As the angle of incidence increases, the angle of refraction decreases.

    Your answer

    As the angle of incidence increases, the angle of refraction increases by less and less: the graph is a curve.

    1/1

    • As the angle of incidence increases, the angle of refraction increases by less and less: the graph is a curve.

    Full marks. Your answer meets all the points for this question.

    Model answer: As the angle of incidence increases, the angle of refraction increases by less and less, so a graph of the results is a curve, not a straight line.

18. 03.4[2 marks]

    The student repeated the investigation with a Perspex block of the same size.

    For an angle of incidence of 70°, the angle of refraction should be 38.2° in the glass and 39.1° in the Perspex.

    Explain why the student could not use the protractor to show this difference.

    Your answer

    The resolution of the protractor is 1 degree, so it could not be used to measure the difference because it is less than 1 degree.

    2/2 self-marked

    • The resolution of the protractor is 1°
    • So it could not measure the difference (0.9°, less than 1°); allow could not measure to 1 decimal place

    Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

    Model answer: The resolution of the protractor is 1°. The difference between the angles is only 0.9°, which is less than 1°, so the protractor could not be used to measure it.

19. 03.5[1 mark]

    Another student drew the normal by eye. It was 2° away from a right angle to the surface.

    Name the type of error this causes in every angle the student measured.

    Your answer

    systematic error

    1/1 self-marked

    • Systematic (error)

    Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

    Model answer: A systematic error: every angle is shifted by the same amount.

20. 03.6[3 marks]

    A ray of light enters a glass block at an angle to the normal.

    Explain why the ray changes direction as it enters the glass.

    You should refer to wavefronts in your answer.

    Your answer

    Different parts of the wavefront enter the glass at different times. The speed of light is less in glass, so the part of the wavefront that enters first slows down before the other parts, which changes the direction.

    3/3 self-marked

    • Different parts of the wavefront enter the glass at different times
    • The speed (velocity) of light is less in glass
    • So one part of the wavefront changes speed before the other parts

    Self-marked: 3 of 3 marks. This is your own judgement against the mark scheme.

    Model answer: Different parts of the wavefront enter the glass at different times. Light travels more slowly in glass, so the part of the wavefront that enters first slows down before the other parts. This makes the wavefront, and so the ray, change direction.

Unseen data: the same skills, new apparatus

1. 01[2 marks]

   A student shines a ray of light into a thin-walled clear plastic box filled with water. They trace the ray four times at the same angle of incidence, putting the ray box back on the incident line each time, and measure the angle of refraction from the normal with a protractor.

   Angle of incidence = 60°

   Trial	Angle of refraction in degrees
   1	40
   2	40
   3	41
   4	35

   Calculate the mean angle of refraction. Do not include the anomalous result. Give your answer to 1 decimal place.

   Your answer

   40.3

   Working: 40 + 40 + 41 = 121 121 / 3

   2/2

   • 40 + 40 + 41 = 121, leaving out 35.
   • Divided by 3, in degrees, to 1 decimal place.

   Full marks. Clear and precise.

   Model answer: Trial 4 (35°) is anomalous: leave it out. Mean = (40 + 40 + 41) ÷ 3 = 121 ÷ 3 = 40.3°.

2. 02[2 marks]

   A student does the refraction practical with a glass block and then with a thin-walled clear plastic box of water the same size. They use the same incident lines, ray box and protractor (1° divisions).

   Angles measured from the normal

   Angle of incidence in degrees	Angle of refraction in glass in degrees	Angle of refraction in water in degrees
   30	19	22
   40	25	29
   50	31	35
   60	35	41

   Which TWO conclusions do these results support? Tick (✓) two boxes.

   • ✓ Light changes direction less going into water than into glass: at every angle of incidence the angle of refraction in water is bigger. (your choice)
   • ✓ The differences (3° to 6°) are several times the protractor’s 1° resolution and go the same way at every angle, so they support a real difference; repeat traces would show whether the scatter is small enough to be sure. (your choice)
   • The protractor only reads to 1°, so these results cannot show any difference between water and glass.
   • Water bends light more than glass, because its angles of refraction are bigger.
   • In both materials the angle of refraction is directly proportional to the angle of incidence.

   Your answer

   Light changes direction less going into water than into glass: at every angle of incidence the angle of refraction in water is bigger.; The differences (3° to 6°) are several times the protractor’s 1° resolution and go the same way at every angle, so they support a real difference; repeat traces would show whether the scatter is small enough to be sure.

   2/2

   • Light changes direction less going into water than into glass: at every angle of incidence the angle of refraction in water is bigger.
   • The differences (3° to 6°) are several times the protractor’s 1° resolution and go the same way at every angle, so they support a real difference; repeat traces would show whether the scatter is small enough to be sure.

   Full marks. Clear and precise.

   Model answer: At every angle the ray in water is further from the normal than in glass, so water changes the ray’s direction less. The differences, 3° to 6°, are several times the 1° resolution and all go the same way, so the results support a real difference. Resolution alone cannot prove it: repeat traces at each angle would show whether the scatter (from lining up the block, the ray and the protractor) is smaller than the difference. (Glass and Perspex differ by under 1°, so a protractor cannot tell those two apart.)

3. 03[2 marks]

   A student uses optical pins instead of a ray box. They push two pins, P1 and P2, into the paper along the incident line. Looking through the glass block from the other side, they push in two more pins, P3 and P4, so that all four pins look as if they are in a straight line. They then lift the block and join the pin holes.

   Which TWO changes would make the measured angle of refraction more accurate? Tick (✓) two boxes.

   • ✓ Line up the bottoms of the pins, where they go into the paper, not the tops. (your choice)
   • Trace each angle three times and take a mean, to remove the error from a normal that is not at 90°.
   • ✓ Push in P3 and P4 far apart from each other. (your choice)
   • Turn off the room lights, as with a ray box.
   • Use a thicker glass block so that the ray bends more.

   Your answer

   Push in P3 and P4 far apart from each other.; Line up the bottoms of the pins, where they go into the paper, not the tops.

   2/2

   • Push in P3 and P4 far apart from each other.
   • Line up the bottoms of the pins, where they go into the paper, not the tops.

   Full marks. Your answer meets all the points for this question.

   Model answer: Pins far apart fix the direction of the line more precisely (a small error in placing a pin turns the line less), like crosses far apart on a ray. The pins may lean, so line up the points where they enter the paper. Repeating cannot remove a systematic error such as a tilted normal; a thicker block does not change the angle; pins need light to be seen.