Core Practical: Investigating Snell's law (Edexcel IGCSE Physics: Double Science)

Aims of the experiment

• To investigate the refractive index of glass, using a glass block

Variables

• Independent variable = angle of incidence, i
• 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

Equipment

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
Perspex rectangle	To refract the light beam

• Resolution of measuring equipment:
  • Protractor = 1°
  • Ruler = 1 mm

Method

Diagram of equipment set up

Apparatus set-up to investigate Snell's Law

1. Place the glass block on a sheet of paper, and carefully draw around the block using a pencil
2. Draw a dashed line normal (at right angles) to the outline of the block
3. Use a protractor to measure the angles of incidence to be studied and mark these lines on the paper
4. Switch on the ray box and direct a beam of light at the side face of the block at the first angle to be investigated
5. 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

6. Remove the block and join the points marked with three straight lines
7. Replace the block within its outline and repeat the above process for a rays striking the block at the next angle

An example results table

Angle of incidence, i / °	Angle of refraction, r / °
0
10
20
30
40
50
60
70
80

Analysis of results

• If the angles have been measured correctly, the paper should end up looking like this:

A diagram showing how to measure the angles of incidence and refraction

• Snell's Law relates the angles of incidence and refraction
  
  • This is covered in the Snell's law revision note
• Plot a graph of sin i on the y-axis against sin r on the x-axis
  • The refractive index is equal to the gradient of the graph

A graph of the results of snell's law experiment

Evaluating the experiment

Systematic Errors:

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

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