Physics
Transmittance
Quick fact
The human eye can detect light transmittance over a range of about 10 orders of magnitude, but most materials we consider 'transparent' transmit over 90% of visible light.
Why this is interesting
You see your reflection in a window, but on a sunny day the same glass lets light flood into a room. Why do some materials let light through while others block it?
Read the full explanation
Understanding Transmittance
Imagine you're looking through a pair of sunglasses. Some light passes through the lenses to reach your eyes; some is blocked. Transmittance is simply the ratio of light that gets through compared to what hits the surface. If 80% of light passes, the transmittance is 0.8 (or 80%). This value depends on the material's thickness, composition, and the wavelength of light. For example, clear glass transmits nearly all visible light but blocks ultraviolet rays. In contrast, a piece of wood transmits almost nothing—its transmittance is near zero. Transmittance is not just about ‘clear vs. opaque’—it can vary with color: a red filter transmits red light while absorbing others.
A deeper explanation
Transmittance quantifies how much electromagnetic radiation passes through a medium. It arises from the interplay of absorption, reflection, and scattering at the microscopic level. When light encounters a material, its photons can be absorbed by atoms or molecules, exciting electrons to higher energy levels; they can be scattered by inhomogeneities; or they can pass through uninterrupted. The transmittance T is defined as It / I0, where It is the transmitted intensity and I0 is the incident intensity. This is related to absorbance A by A = -log10(T). The underlying mechanism depends on the material's electronic structure and the photon energy. For example, in silicon, photons with energy greater than the bandgap are absorbed, making it opaque to visible light but transparent to infrared. Transmittance matters in designing optical filters, solar panels (maximizing light transmission), and even in medicine (determining how light penetrates tissue for phototherapy).