One ray in, three ways out

Reflected · ρ0%0 photons

Absorbed · α0%0 photons

Transmitted · τ0%0 photons

Glows · ε = α0 °0 infrared photons

Pick a material

…or set the fractions yourself (they always add up to 100%)

Reflect ρ Absorb α Transmit τ

Finish Light angle40°

Fig. 1 — Every photon that meets matter has three possible fates: it is reflected (ρ), absorbed and turned into heat (α), or transmitted through (τ). Which one is down to chance, but the odds are set by the material, so the counts settle on the fractions — and because every photon goes somewhere, α + ρ + τ = 1. The extremes are the ideal mirror (ρ = 1: every ray bounces, angle in = angle out, and it never warms up), the black body (α = 1: every ray is swallowed) and a perfectly clear window (τ = 1). The finish decides how light leaves: polished surfaces bounce it in one direction (specular), rough ones scatter it everywhere (diffuse) — the same fractions, a very different look. Watch the slab, too: whatever it absorbs heats it up, and a warm surface glows in the infrared (red) in every direction. A good absorber is an equally good emitter (ε = α, Kirchhoff's law), which is why the black body glows brightest and the mirror not at all.