Light comprises waves.
When looking at waves of water, if plane ripples hit an obstacle larger than their wavelength, you will observe them bending around the edge of the obstacle. Light waves show the same phenomenon, called light diffraction, when they bend around an obstacle larger than their wavelength. Meanwhile, when these waves of light collide with particles and molecules smaller than their wavelengths in the atmosphere, they cause the particles and molecules to "relay" their wave motion, radiating light in the same wavelength into the surrounding air.