Reflection, and the glass block practical
Angles are measured from the normal
The normal is an imaginary line at 90° to a surface, drawn where the ray hits it.
Every angle in a ray diagram is measured from the normal, never from the surface.
At a mirror, the angle of reflection equals the angle of incidence.
- A ray makes 25° with the surface of a mirror
- angle of incidence = 90° − 25° = 65°
- angle of reflection = 65°
- the incident and reflected rays are 65° + 65° = 130° apart
The classic slip: using the angle to the mirror surface. Draw the normal first, then measure.
Smooth or rough
- Specular reflection: a smooth surface (a mirror, still water) reflects parallel rays in a single direction, so you see a clear image.
- Diffuse reflection: a rough surface (paper, a matt wall) scatters parallel rays in many directions. You see the surface, but no image.
Each tiny part of a rough surface still obeys the law of reflection: the normals just point in different directions.
The glass block practical
You met refraction in the waves unit: light changes speed, and so direction, as it crosses into a new material.
This practical measures it.
- Put the block on paper and draw round it
- Draw a normal at 90° to one side with a protractor
- Aim a narrow ray from a ray box at the normal, at a measured angle
- Mark the ray in and out with crosses, then remove the block
- Join entry and exit points; measure the angle of refraction from the normal
- Repeat for several angles of incidence
Independent variable: the angle of incidence (or the material). Dependent: the angle of refraction. Control: the same block, the same narrow ray.
Going into glass the ray bends towards the normal; coming out it bends away. Along the normal it does not bend at all.
Try it in the refraction sim
Set Top to Air and Bottom to Glass. Set the angle of incidence to 20°, then 40°, then 60°, and write down the refraction angle each time (about 13°, 25° and 35°). Does doubling the angle of incidence double the angle of refraction? Now set Top to Glass and Bottom to Air at 20°: which way does the ray bend? Turn on the reflected ray: some light is always reflected at the boundary too.
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This question is in Brainlag Plus