If an ideal capacitor is charged by DC battery, current flows momentarily till capacitor gets fully charged after that no current flows. however, when an AC source is connected, the conduction current, \(I_c=\frac{d q}{d t}\) starts flowing in the connecting wire. As charging polarity of AC current changes, the capacitor is alternatively charged and discharged with time. this causes change in electric field between plates of the capacitor which causes change in electric flux and gives rise to displacement current in the region between plates of cpacitor, as displacement current, \(I_d=\varepsilon_0 \frac{d {\phi}_E}{d t}\) and I_d = I_e at all instants.

Electric field of an electromagnetic wave is an oscillating field which causes force on the charged particle. This electric force averaged over an integral number of cycles is zero, because its direction changes with every half cycle. So, electric field is not responsible for radiation pressure.

Intensity of light is reduced to one fourth because the light beam spreads as it approaches into a spherical region of area \(4\pi { r }^{ 2 },i.e.,I\infty 1/{ r }^{ 2 }\) But laser beam does not spread, hence its intensity remains constant. Laser beam is unidirectional, monochromatic and coherent light, whereas the light from a bulb does not posses the above properties.

Here, I_D = 1 mA = 10^-3A ; C = \(2\mu F\) = 2 x 10^-6 F, \({ I }_{ D }=I=\frac { d }{ dt } \left( CV \right) =C\frac { dV }{ dt } \)
Therefore, \(\frac { dV }{ dt } =\frac { { I }_{ D } }{ C } =\frac { { 10 }^{ -3 } }{ 2\times { 10 }^{ -6 } } =500V/s\)
So, by applying a varying potential difference of 500 V/s, we would produce a displaccment current of desired value.