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14.6 Required Practical: Reflection and Refraction

GCSE Physics · AQA · Chapter 14: Light

A free GCSE Physics lesson for AQA (8463) from OBA School: required practical: investigating the reflection of light by different surfaces and the refraction of light by different substances. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe how to measure angles of incidence, reflection and refraction using a ray box, a protractor and pencil-and-paper ray tracing
  • Identify the independent, dependent and control variables and the hazards in the investigation
  • Compare how different surfaces reflect light and how different substances refract it
  • Process results: tables, means, uncertainty and graphs of r against i, and draw conclusions
  • Evaluate the method: sources of error, resolution and improvements

Key words

  • Ray box: A lamp in a box with a narrow slit that produces a thin ray of light.
  • Independent variable: The variable you change: here the angle of incidence (and the surface or substance).
  • Dependent variable: The variable you measure: the angle of reflection or refraction.
  • Control variable: A variable kept the same so the test is fair.
  • Resolution: The smallest change an instrument can show: 1° for a typical protractor.
  • Uncertainty: The range within which the true value probably lies: about half the range of repeat readings.

Aim and apparatus

Aim: to investigate (1) how light is reflected by different types of surface and (2) how light is refracted by different substances, by measuring the angles a ray makes with the normal.

  • ray box with a single narrow slit, and its low-voltage power supply; Why it is used gives a thin ray whose path can be marked accurately
  • plane mirror (plus other surfaces: white card, black card, coloured card, rough and smooth materials); Why it is used surfaces to reflect the ray from
  • rectangular blocks of glass and clear acrylic; a thin-walled clear plastic box filled with water; Why it is used substances to refract the ray
  • sheets of plain white paper, sharp pencil, ruler; Why it is used to trace the outline of the mirror or block and mark the rays
  • protractor; Why it is used to draw normals at 90° and to measure angles (resolution 1°)

Method part 1: reflection

1. Draw a straight line across a sheet of plain paper. Use the protractor to draw a normal at 90° to it.

2. Stand the mirror with its reflecting surface exactly on the line. Dim the room lights.

3. Aim the ray from the ray box at the point where the normal meets the mirror, at a measured angle of incidence (e.g. 20°).

4. Mark the incident ray and the reflected ray with two small crosses each, far apart.

5. Remove the mirror and join the crosses with a ruler. Measure the angle of reflection from the normal.

6. Repeat for a range of angles of incidence (e.g. 10° to 70° in 10° steps).

7. Replace the mirror with the other surfaces. Record how bright and how sharp the reflected ray is, or whether the light is scattered.

Comparing different surfaces

Shine the same ray at the same angle on each surface in turn and compare what you see. Keep the room equally dim each time.

  • plane mirror; Typical observation one bright, sharp reflected ray; r = i; Why smooth, shiny: specular reflection
  • white card; Typical observation no clear reflected ray; the card glows with scattered light; Why rough: diffuse reflection of all colours
  • red card (white light); Typical observation card looks red; dim, scattered red light; Why pigment absorbs other colours, reflects red
  • black card; Typical observation very little light seen; Why absorbs most of the light

Method part 2: refraction

1. Place the block in the middle of a sheet of paper and trace round it with a sharp pencil.

2. Draw a normal at 90° to the top face, where the ray will enter.

3. Aim the ray at the point where the normal meets the block, at a measured angle of incidence.

4. Mark the incident ray and the ray leaving the opposite face with two crosses each.

5. Remove the block. Join the crosses, then join the entry and exit points to show the ray inside the block.

6. Measure the angle of refraction from the normal at the entry point.

7. Repeat for different angles of incidence, then repeat everything with the other blocks (acrylic, water).

Variables and safety

  • independent; In this practical angle of incidence; the type of surface (part 1) or substance (part 2)
  • dependent; In this practical angle of reflection or angle of refraction (and how the reflected ray looks)
  • control; In this practical same ray box and slit width; same point of incidence on the normal; same room lighting; same face of the block; the same person measures the angles
  • ray box lamp gets very hot; Risk burns to fingers; Control measure do not touch the lamp housing; let it cool before moving it
  • working in a dimmed room; Risk tripping over bags or stools; Control measure keep the floor and walkways clear
  • glass block with chipped edges; Risk cuts; Control measure check the block before use; handle it carefully
  • water near the ray box’s mains power supply; Risk electric shock; Control measure keep the water box away from the power supply; wipe up spills at once

Processing the results

Record results in a table with units in the headings, e.g. “angle of incidence / °”. For reflection the results should show r = i. For refraction, plot a graph of angle of refraction (y-axis) against angle of incidence (x-axis), one line for each substance.

Conclusion from the graph: as i increases, r increases, but r is always less than i (the ray bends towards the normal on entering). The relationship is not proportional: the line curves. Glass bends the light more than water, because light slows down more in glass.

Worked example: mean and uncertainty of repeat readings

At i = 50° a group measured the angle of refraction in acrylic three times: 32°, 30°, 31°.
mean = (32 + 30 + 31) ÷ 3 = 31°
range = 32 − 30 = 2°, so uncertainty = ± half the range = ± 1°

Answer: r = 31° ± 1°

Errors and improvements

  • Wide ray: a thick ray makes it hard to judge its centre. Use a narrow slit and measure to the centre of the ray.
  • Marking the ray: crosses close together give a big error in the direction. Put them as far apart as possible.
  • Normal not at 90°: every angle would be wrong by the same amount (a systematic error). Draw normals carefully with a protractor.
  • Reading the protractor: resolution is 1°. Read it at eye level, with its centre exactly on the point of incidence. A larger protractor helps.
  • Random errors: repeat each angle (or let several people measure) and calculate a mean; identify and ignore anomalous results.
  • Room too bright: the ray is faint and hard to see. Dim the lights.

Exam tip: Measuring every angle from the surface instead of the normal is a systematic error: all your angles would be 90° minus the correct value.

Common mistakes

  • Measuring angles from the mirror or the edge of the block instead of from the normal.
  • Drawing the line along the back of the mirror glass instead of its reflecting surface, or letting the block move while marking the rays.
  • Plotting angle of incidence on the y-axis. The independent variable (i) goes on the x-axis.
  • Drawing a straight line of best fit through refraction results that clearly curve.
  • Giving only one hazard and no control measure: always pair each hazard with how you reduce the risk.

Revision questions

Try these, then check your answers below each one.

Question 1. A student investigates how the angle of refraction depends on the angle of incidence. What is the independent variable? (Choose from: angle of incidence, angle of refraction, type of ray box, thickness of the block.)

Answer: angle of incidence

Working: The student chooses the angle of incidence (independent) and measures the angle of refraction (dependent).

Question 2. Why should the crosses marking a ray be placed far apart? (Choose from: so the direction of the ray can be drawn more accurately, so the ray is brighter, so the block does not move, so the angle of incidence is bigger.)

Answer: so the direction of the ray can be drawn more accurately

Working: A small error in placing crosses that are close together causes a large error in the direction of the line. Far apart crosses reduce this.

Question 3. True or false? The angles in this practical are measured from the surface of the block.

Answer: False

Working: False. All angles are measured from the normal, the line at 90° to the surface.

Related lessons

Continue in the full course

This lesson is section 14.6 (AGCPH1406) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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