Describe some outstanding problems that are the focus of current research. Illustrate how astrophysical theories are developed and tested against observations. Physics can teach us about astronomy, and astronomy can teach us about physics. An electromagnetic wave is a propagating disturbance in an electromagnetic field, which affects electrical charges and magnets. All forms of electromagnetic radiation are carried by parcels of energy called photons .
Photons have properties of both waves and particles.
Gamma-ray photons have the highest energy, radio photons have the lowest energy. Only visible light and radio waves can easily get through to the earth's surface. Most other electromagnetic radiation is absorbed by the atmosphere, and astronomers can observe it only from satellites. The wavelengths of waves emitted by a moving source are shifted by the Doppler effect. The shift is small if speed is much smaller than c.