Investigating Reflection (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

Extended tier only

Aims of the experiment

• To investigate reflection by a Plane mirror

Variables

• Independent variable = angle of incidence, i
• Dependent variable = angle of reflection, r
• Control variables:
  • Distance of ray box from mirror
  • Width of the light beam
  • Same frequency / wavelength of the light

Equipment list

Equipment	Purpose
Ray Box	To provide a narrow beam of light that can be easily refracted
Protractor	To measure the angles of incidence and refraction
Sheet of Paper	To mark the lines indicating the incident and refracted rays
Pencil	To draw the incident and refracted ray lines onto the paper
Ruler	To draw the incident and refracted ray lines onto the paper
Plane mirror	To reflect the light beam

Method

Apparatus to investigate reflection

1. Set up the apparatus as shown in the diagram
2. In the middle of the paper use a ruler to mark a straight line about 10 cm long
3. Use a protractor to draw a 90° line that bisects (cuts in half) the 10 cm line
4. Place the mirror on the first line as shown in the diagram above
5. Switch on the ray box and aim a beam of light at the point where the two drawn lines cross at an angle
6. Use the pencil to mark two positions of the light beam:
   • A point just after leaving the ray box
   • The point on the reflected beam about 10 cm away from the mirror

7. Remove the ray box and mirror
8. Use a ruler to join the two marked positions to the point where the originally drawn lines crossed
9. Use the protractor to measure the two angles from the 90° line. The angle for the ray towards the mirror is the angle of incidence, and the other is the angle of reflection
10. Repeat the experiment three times with the beam of light aimed at different angles

Results

Example results table

Angle of incidence, i (°)	Angle of reflection, r (°)
10
30
45
80

Analysis of results

• The law of reflection states:

i = r

• Where:
  • i = angle of incidence in degrees (°)
  • r = angle of reflection in degrees (°)

• If the experiment was carried out correctly, the angles of incidence and reflection should be the same

Evaluating the experiment

Systematic errors:

• An error could occur if the 90° lines are drawn incorrectly
  • Use a set square to draw perpendicular lines

• If the mirror is distorted, this could affect the reflection angle, so make sure there are little to no blemishes on it

Random errors:

• The points for the incoming and reflected beam may be inaccurately marked
  • Use a sharpened pencil and mark in the middle of the beam

• The protractor resolution may make it difficult to read the angles accurately
  • Use a protractor with a higher resolution

Safety considerations

• The ray box light could cause burns if touched
  • Run burns under cold running water for at least five minutes

• Looking directly into the light may damage the eyes
  • Avoid looking directly at the light
  • Stand behind the ray box during the experiment

• Keep all liquids away from the electrical equipment and paper
• Take care using the mirror
  • Damages on the mirror can affect the outcome of the reflection experiment