Emission of Radiation

Incandescence: the emission of light from a substance, whose atoms are excited by high temperature, is called incandescence. The light from the heated tungsten filament of an incandescent bulb (the common electric bulb) consists of many frequencies and therefore appears white. The sun is also an incandescent source.

Fluorescence: Besides heating or bombarding with fast-moving particles, an atom can also be excited by the absorption of photons of light. Many substances undergo excitation when illuminated with photons of ultraviolet light and then emit visible light upon de-excitation. Such substances are called fluorescent substances and the phenomenon is called fluorescence. 

X- Ray Emission X- rays are produced in an evacuated X-ray tube, which has a source of electrons (a heated filament) and a tungsten target. A potential difference of nearly 50, 000 to 100,000 V is maintained between the filament and the target. Due to the high potential difference, electrons are accelerated towards the tungsten target and strike it with very high speeds. The atoms of tungsten are excited and then emit X-rays in the process way have fixed frequencies and are known as characteristic  X-rays, X-rays with a continuous range of frequencies , known as continuous x-rays , are produced when some electrons are decelerated in the vicinity of target nuclei.