(a) 10⁹ Hz - Radiowaves
Application: For radio & television communication.
(b) 10¹¹ Hz - microwaves
Application: used in Radar for aircraft navigation, (microwave oven for cooking).

(i) It is produced by transitions of atomic nuclei and decay of certain elementary particles. They produce chemical reactions on photographic plates, fluorescence, ionization, diffraction.
(ii) Gamma rays provide information about the structure of atomic nuclei. It is used in radiotherapy for the treatment of cancer and tumor, in the food industry to kill pathogenic microorganisms.

It is produced by Sun, arc, and ionized gases. The wavelength range is 6 x 10^-10m to 4 x 10^-7m and the frequency range is 5 x 10¹⁷Hz to 7 x 10¹⁴Hz. It has less penetrating power. It can be absorbed by atmospheric ozone and harmful to human body. It is used to destroy bacteria, sterilizing the surgical instruments, burglar alarm, detect the invisible writing, finger prints and also in the study of molecular structure.

It provides electrical energy to satellites by means of solar cells. It is used to produce dehydrated fruits, in green houses to keep the plants warm, heat therapy for muscular pain or sprain, TV remote as a signal carrier, to look through haze fog or mist and used in night vision or infrared photography.

(i) If the electromagnetic wave incident on a material surface is completely absorbed, then the energy delivered is U, and momentum imparted on the surface is P = \(\frac{U}{c}\)
(ii) If the incident electromagnetic wave of energy U is totally reflected from the surface, then the momentum delivered to the surface is \(\Delta p=\frac { U }{ c } -\left( -\frac { U }{ c } \right) =2\frac { U }{ c } \)
(iii) The rate of flow of energy crossing a unit area is known as the pointing vector for electromagnetic waves.