Optics is about:

  • Optics of lightbeams
  • Biophysics: The eye

In geometrical optics lightbeams always travel in a straight line, unless they are mirrored or refracted. Refraction is caused by the fact that the speed of light changes when it travels from one medium to the other. The ratio between those different speeds give the refractive index. Different colors will change to different speeds (different refractive indices). That's why a white light beam decomposes into different colors.