Pink sunset on the mountains, Yosemite National Park, 2015.

As the sun sets, west-facing mountains turn golden and then pink because they reflect light from the setting sun. Air, smoke and dust cause this change in the color of sunlight that reaches us. As beams of light make their way through the thick atmosphere at dusk it loses violets, blues and greens along the way. However, red light waves make it through the atmosphere, even at dusk, relatively unhindered. So as the setting sun dims and becomes redder, the mountain reflects this red light. But how does this happen? Light travels in waves. Each color of light travels with a different wavelength, with red having the longest wavelength and violet the shortest. Light waves travel in a straight line until interfered with (by gas, dust or anything really). Some of the light will be absorbed and some radiated outward, on a different path than before. Lower-frequency light waves (reds) are absorbed less easily while higher-frequency light waves (blues) are absorbed more easily. So, when light reaches our atmosphere it is refracted by all sorts of dust, water or anything else in our atmosphere! It is this refraction that most determines the color sunset we will see and why it is usually red.