Reflection, refraction, transmission and absorption

When a wave meets a boundary between two media, four things may happen:

• Reflection — wave bounces back into the original medium.
• Refraction — wave passes into the new medium and changes direction (and speed).
• Transmission — wave passes through with little energy lost.
• Absorption — wave energy is taken in by the medium and converted to heat or other forms.

The fraction of energy reflected, transmitted or absorbed depends on both materials and on the wavelength.

Reflection

The law of reflection: angle of incidence = angle of reflection. Both angles are measured from the normal (the line perpendicular to the surface).

• Specular reflection — smooth surface (mirror) reflects parallel rays in one direction → clear image.
• Diffuse reflection — rough surface scatters rays in many directions → no clear image but the surface still appears bright.

Refraction (Higher tier)

When a wave enters a different medium it changes speed. If it crosses the boundary at an angle, it also changes direction.

• Light entering glass from air slows down and bends towards the normal.
• Light leaving glass into air speeds up and bends away from the normal.

The reason: different media have different optical densities. The wave’s frequency stays the same, but wavelength changes when speed changes (v = f × λ).

Ray diagram for refraction

1. Draw the boundary as a horizontal line.
2. Draw the normal as a perpendicular dashed line at the point where the ray hits.
3. Draw the incident ray with an arrow towards the boundary.
4. Draw the refracted ray in the second medium, bending towards the normal if the medium is denser.
5. Label angle of incidence (i) and angle of refraction (r), both measured from the normal.

Transmission and absorption

• A transparent material (clean glass) transmits visible light well — most light passes through.
• A translucent material (frosted glass) transmits some, scatters some.
• An opaque material (wood) absorbs or reflects, transmitting essentially none.

The fraction absorbed depends on wavelength: glass transmits visible light but absorbs UV. Black objects absorb all visible wavelengths and re-emit as infrared.

Edexcel exam tip

A frequent slip is measuring angles from the surface itself instead of from the normal. Always draw the normal first; otherwise the marker will dock the angle marks. State that the wave slows and bends towards the normal when entering a denser medium — both the speed change and the direction change must appear.