Investigating Reflection
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
Method
Apparatus to investigate reflection
- Set up the apparatus as shown in the diagram
- In the middle of the paper use a ruler to mark a straight line of about 10 cm long
- Use a protractor to draw a 90° line that bisects (cuts in half) the 10 cm line
- Place the mirror on the first line as shown in the diagram above
- Switch on the ray box and aim a beam of light at the point where the two drawn lines cross at an angle
- 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
- Remove the ray box and mirror
- Use a ruler to join the two marked positions to the point where the originally drawn lines crossed
- 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
- Repeat the experiment three times with the beam of light aimed at different angles
- An example of the data collection table is shown below:
Example Results Table
Angle of Incidence, i / ° | Angle of Reflection, i / ° |
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 should be the same, as shown below:
Correct Results of the Experiment
Law of reflection demonstrated correctly
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 minute
- 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