Reflection and Refraction

Reflection

Reflection is when a wave bounces off a surface. Mirrors, echoes and seeing most everyday objects all depend on reflection.

Some key words:

• Incident ray: the ray coming in.
• Reflected ray: the ray going out.
• Normal: an imaginary line drawn at 90° to the surface where the ray hits. Always measure angles from the normal, not from the surface.
• Angle of incidence ($i$): between the incident ray and the normal.
• Angle of reflection ($r$): between the reflected ray and the normal.

Smooth vs rough surfaces

• A smooth surface (mirror, still water) reflects all rays the same way. You get a clear image. This is regular (specular) reflection.
• A rough surface (paper, a wall) scatters rays in all directions. No image forms. This is diffuse reflection. It is why you can see a page from any angle.

Image in a flat mirror: the same size as the object, the same distance behind the mirror as the object is in front, upright, and laterally inverted (left and right swapped). That is why AMBULANCE is printed backwards on the front of ambulances.

Refraction

Refraction is when a wave changes direction as it passes from one medium into another, because its speed changes.

Light travels fastest in a vacuum (and air). It slows down in water and slows even more in glass.

Effects of refraction

• A straw in a glass of water looks bent at the surface.
• Pools look shallower than they really are.
• Ava's fish looked higher than it was.
• Lenses in glasses, cameras and microscopes bend light to focus it.
• A prism splits white light into a rainbow, because each colour bends by a slightly different amount. This is called dispersion.

Quick summary

• Reflection: a wave bounces off a surface. Angle of incidence = angle of reflection.
• Measure angles from the normal, a line at 90° to the surface.
• Smooth surfaces give clear images. Rough surfaces scatter light (diffuse reflection).
• Refraction: a wave changes direction because its speed changes in a new medium.
• Into a denser medium: slows down, bends towards the normal. Out: speeds up, bends away.
• Frequency never changes on refraction. Speed and wavelength do.

Quiz