Determining Refractive Index
Aim of the Experiment
- To investigate the refraction of light by a perspex block
Equipment
- Ray Box - to provide a narrow beam of light to refract through the perspex box
- Protractor - to measure the light beam angles
- Sheet of paper - to mark with lines for angle measurement
- Pencil - to make perpendicular line and angle lines on paper
- Ruler - to draw straight lines on the paper
- Perspex block - to refract the light beam
- Resolution of measuring equipment:
- Protractor = 1°
- Ruler = 1 mm
Variables
- Dependent variable = angle of refraction , r
- Control variables:
- Use of the same perspex block
- Width of the light beam
- Same frequency / wavelength of the light
Method
Apparatus to investigate refraction
- Place the glass block on a sheet of paper, and carefully draw around the block using a pencil
- Switch on the ray box and direct a beam of light at the side face of the block
- Mark on the paper:
- A point on the ray close to the ray box
- The point where the ray enters the block
- The point where the ray exits the block
- A point on the exit light ray which is a distance of about 5 cm away from the block
- Draw a dashed line normal (at right angles) to the outline of the block where the points are
- Remove the block and join the points marked with three straight lines
- Replace the block within its outline and repeat the above process for a ray striking the block at a different angle
- An example of the data collection table is shown below:
Analysis of Results
- i and r are always measured from the normal
- For light rays entering perspex block, the light ray refracts towards the central line:
i > r
- For light rays exiting the perspex block, the light ray refracts away from the central line:
i < r
- When the angle of incidence is 90° to the perspex block, the light ray does not refract, it passes straight through the block:
i = r
- If the experiment was carried out correctly, the angles should follow the pattern, as shown below:
How to measure the angle of incidence and angle of refraction
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