Light as Information

Section 3 of 9

Light-Matter Interactions

Part 3: How Light Interacts with Matter

Light carries information because it doesn’t just travel — it interacts. The fundamental interactions are absorption, emission, and scattering; at a boundary we also describe transmission (it gets through), reflection (it bounces), and refraction (it bends). Every spectrum you will ever interpret is some combination of these.

Absorption: a photon is destroyed and its energy goes into the material (heat, or exciting an electron). Absorption is selective — that selectivity turns a smooth spectrum into one with informative “missing” wavelengths.
Emission: matter loses energy by creating a photon. What sets the photon’s wavelength is the energy the material loses — which is why atoms produce lines at specific wavelengths.
Transmission: light passes through without being absorbed. Glass transmits visible but absorbs UV; Earth’s atmosphere transmits visible and radio but absorbs most X-rays.

Electromagnetic spectrum diagram showing wavelength (10^2 m to 10^-12 m) and frequency (10^4 Hz to 10^22 Hz) scales against a landscape backdrop. A gray transmission curve shows atmosphere is opaque at most wavelengths but transparent in the 'Radio Window' and 'Optical Window'. Familiar objects illustrate each band: AM/FM radio towers, cell phones, microwave ovens, human body (infrared), visible light through a prism, sunburn (UV), medical X-rays, and nuclear power (gamma rays). — Figure 5Earth's atmosphere has two main 'windows' - radio and optical - where it's transparent. Most UV, X-rays, and gamma rays are blocked, which is why we need space telescopes for those wavelengths.NASA

Reflection: light bouncing off a surface. The fraction reflected (the albedo) encodes surface composition and texture.
Refraction: bending as light crosses between media. Lenses focus by refraction; atmospheric refraction makes stars twinkle. It is wavelength-dependent — blue bends more than red, which is why prisms spread white light.

Albedo

The fraction of incident light a surface reflects. A bright icy moon has high albedo; dark rock has low albedo. It encodes information about surface composition and texture.

Five panels showing light behaviors: Absorption (rays entering and stopping), Emission (rays leaving), Transmission (rays passing through), Reflection (rays bouncing off surface), Refraction (rays bending at interface). — Figure 6Light can be absorbed, emitted, transmitted, reflected, or refracted. Each interaction encodes information about the material. Blackbodies absorb all wavelengths (no reflection).JWST/STScI

Scattering: Blue Skies and Red Sunsets

Scattering redirects light into new directions. When the scattering particles are much smaller than the wavelength (like air molecules), we get Rayleigh scattering.

Rayleigh scattering

Scattering of light by particles much smaller than its wavelength, with a steep $\sigma \propto \lambda^{-4}$ dependence. It makes the daytime sky blue, sunsets red, and reddens starlight passing through interstellar dust.

The $\lambda^{-4}$ dependence is steep: blue light ( $\lambda \approx 450$ nm) scatters about $(650/450)^4 \approx 4$ times more than red ( $\lambda \approx 650$ nm), so $\sigma_{450}/\sigma_{650} = (650/450)^4$ .

Why the sky is blue: sunlight enters the atmosphere and meets countless N $_2$ and O $_2$ molecules. Short wavelengths scatter strongly in all directions, so when you look away from the Sun, much of what you see is scattered blue light arriving from all over the sky. Why sunsets are red: at sunset, light travels through much more atmosphere; along that long path the blue is preferentially scattered out of the direct beam, so the light that continues forward is redder. Same physics, different geometry.

Diagram showing sunlight entering atmosphere. Blue light (short wavelength) scatters in all directions while red light (long wavelength) passes through more directly. Viewer looking up sees scattered blue; viewer at sunset sees transmitted red. — Figure 7Rayleigh scattering explains both the blue sky (scattered short wavelengths) and red sunsets (transmitted long wavelengths). Same physics, different viewing geometry.NotebookLM

Opacity and How Far Light Gets

Often we care less about “what is this made of?” than “how far can photons travel before being absorbed or scattered?” That idea is captured by opacity — how strongly matter blocks light at a given wavelength, usually highly wavelength-dependent. In Module 2 this gives a key idea: we only see to the depth where the material becomes optically thick. For a star, that boundary is the photosphere — where photons last scattered before escaping.

Opacity

A measure of how strongly matter absorbs or scatters light at a given wavelength. High opacity means light is blocked over a short distance. It is strongly wavelength-dependent — a cloud can be transparent in the optical but opaque in the UV.