Normal - perpendicular to the surface of the glass block where the ray enters or leaves the glass block

1. Place a glass block on a piece of paper

2. Shine a ray of light into the glass block using a ray box

3. Draw a line on each side of the block where the ray is

4. Join up the lines with a pencil

5. Draw the normal perpendicular to where the ray enters the block

6. Measure the angle of incidence and angle of refraction using a protractor

7. Do the same at the point where the ray emerges from the block

8. Repeat 3 times and calculate an average to improve accuracy