B = .08768 T This magnetic field can be easily produced with a permanent magnet. So, electromagnetic waves of frequency MHz can be used for heating and cooking food because they are strongly absorbed by water molecules. . TYPES OF SPECTRUM- EMISSION AND ABSORPTION SPECTRUM- FRAUNHOFER LINES When an object burns, it emits radiations.
That is, it emits electromagnetic radiation which depends on temperature. If the object becomes hot, it glows in red colour. If the temperature of the object is further increased, then it glows in reddish- orange colour and becomes white when it is hottest. The spectrum in Figure .
usually observing the progress of healing bones. Further, in a finished metal product, it is used to detect faults, cracks, flaws and holes. Gamma rays It is produced by transitions of radioactive nuclei and decay of certain elementary particles. They produce chemical reactions on photographic plates, fluorescence, ionisation, diffraction.
The frequency range is Hz and above. Gamma rays have higher penetrating power than X-rays and ultraviolet radiations; it has no charge but harmful to human body. Gamma rays provide information about the structure of atomic nuclei. It is used in radio therapy for the treatment of cancer and tumour, in food industry to kill pathogenic microorganism.
EXAMPLE . A magnetron in a microwave oven emits electromagnetic waves (em waves) with frequency f = MHz. What magnetic field strength is required for electrons to move in circular paths with this frequency?. Solution Frequency of the electromagnetic waves given, f = MHz The corresponding angular frequency is ω = 2π f = × .
× × = , × Hz = . × s – The required magnetic field, B e ω Mass of the electron, m e = . × – kg Charge of the electron, =− ⇒ Figure . Black body radiation spectrum – variation with temperature Intensity K K K Infrared Ultraviolet Wavelength Bodies radiating at similar temperatures Surface of the sun