Photons travel in vacuum following straight lines. Their eight-minute flight from the Sun to the surface of the Earth is a peaceful journey almost all the way through. Hundreds of kilometers before reaching the ground, though, they start facing several layers of obstacles. The upper atmosphere reflects the most energetic photons back into space. From the remaining photons, a portion is absorbed by some of the tens of molecules identified across the following layers of the atmosphere. The rest reach their destination on time, except for a few that may be still delayed in the next-to-last stop. Much like Ulysses in Homer's Odyssey, who spent ten years bouncing between islands and coasts to cross the few kilometers of Aegean waters from Troy to Ithaca, these last few photons undergo a random walk from molecule to molecule in the concluding layers of the atmosphere. Eventually they land, continuously and from all directions, filling with bright blue the eyes that look at the sky.

The above-described adventure of photons results in an inexhaustible source of information about the composition and behavior of atmospheric gases. The HITRAN (high resolution transmission molecular absorption) spectroscopic database, maintained and developed at the Harvard–Smithsonian Center for Astrophysics in Cambridge, Massachusetts, archives the spectral parameters (the identity card) of the molecules absorbing and scattering light in the sky. People working at HITRAN like to consider it as the genome project for molecules and their isotopic variants. HITRAN provides data to academia and industry around the world, aids in the remote sensing of other planets' atmospheres, and helps track the presence and concentration of polluting agents and allergens in the Earth's air.

Such analysis of absorption and scattering of light may be particularized for the detailed study of any diffusive medium. The specific study of the diffusion of light in human tissues is a resourceful method in medical sciences, where physiological features can be assessed non-invasively and often in real time.