Electromagnetic Waves

Welders wear special goggles or face masks with glass windows to protect their eyes from electromagnetic radiations. Name the radiations and write the range of their frequency. 

The condition under which a microwave oven heats up a food item containing water molecules most efficiently is

• the frequency of the microwave must match the resonant frequency of the water molecules

• the frequency of the microwave has no relation  with natural frequency of water molecules

• microwave are heat waves, so always produce heating 

• microwave are heat waves, so always produce heating 

The electric field associated with an electromagnetic wave in vacuum is given by E=i 40 cos (kz-6x10⁸t), where E, z= and t are in volt/m, metre and second respectively. The value of wave vector k is

• 2 m^-1

• 0.5 m^-1

• 6 m^-1

• 6 m^-1

During the propagation of electromagnetic waves in a medium:

• Electric energy density is equal to the magnetic energy density.

• Both electric and magnetic energy densities are zero.

• Electric energy density is double of the magnetic energy density.

• Electric energy density is double of the magnetic energy density.

The threshold frequency for a metallic surface corresponds to an energy of 6.2 eV, and the stopping potential for a radiation incident on this surface 5 V. The incident radiation lies in

• X-ray region

• ultra-violet region

• infra-red region

• infra-red region

An electromagnetic wave in vaccum has the electric and magnetic fields  and which are always perpendicular to each other. the direction of polarisation is given by  and that of wave propagation by  then