11. Radiation

What is radiation?

Radiation sources Antenna: radio waves, microwaves Atoms, molecules: light, infrared light, UV, X-rays Nuclei: Gamma rays We shall consider: Point-like source at rest with time-dependent charge distribution Accelerated point charge

Point-like source d radiation zone long wavelength limit

Multipole expansion

Radiation fields Dipole fields No em monopole fields Acoustic monopole fields do exist.

Radiated power

Why is the sky blue?

Oscillating dipole Holds in general if

Radiation from a moving point charge

Holds also for velocities small as compared to c.

Arbitrary velocity Power radiated by the charge Lienard’s formula

Bremsstrahlung v and a are parallel

Wilhelm Conrad Roentgen discovered X-rays 1895 Nobelprice 1901 First commercial X-ray tube 1896

1960

Synchrotron radiation

BESSY synchrotron X-ray source in Berlin