Photons of light scatter off molecules, and the distance you can see through a gas is proportional to the mean free path of photons through the gas. Photons are not gas molecules, so the mean free path of a photon is not given by Equation 20.3, but its dependence on the number density of the gas and on the molecular radius is the same. Suppose you are in a smoggy city and can barely see buildings 500 m away. b. How far would you be able to see if the temperature suddenly rose from 20°C to a blazing hot 1500°C with the pressure unchanged?

Key Concepts

Mean Free Path

The mean free path is the average distance a particle travels between collisions with other particles. In the context of photons scattering off gas molecules, it describes how far a photon can travel before interacting with a molecule. This distance is influenced by the number density of the gas and the size of the molecules, which affects the likelihood of scattering events.

Temperature and Gas Density

Temperature affects the kinetic energy of gas molecules, which in turn influences their density. As temperature increases, gas molecules move faster and can spread out, potentially reducing the number density of the gas. In the scenario of rising temperature from 20°C to 1500°C, the change in density will impact the mean free path of photons, altering visibility through the gas.

Scattering of Photons

Scattering occurs when photons interact with particles in a medium, such as gas molecules. The extent of scattering determines how far light can travel through a medium before being absorbed or redirected. In a smoggy environment, increased scattering due to higher particle density reduces visibility, while changes in temperature can modify the scattering behavior and thus the distance light can travel.