Until the end of the 19th century, people believed that atmospheric dust and water droplets caused light to be scattered. But dust and water droplets are larger than the wavelength of light. Light is scattered when it hits an object that is smaller than its wavelength. John William Strutt Lord Rayleigh (1842-1919), an English physicist, concluded that light is scattered when it hits molecules of hydrogen and oxygen in the atmosphere. There are various types of scattering, but the term for the effect described above is called Rayleigh scattering.