NCERT Physics Class 12 Exemplar Ch 8 Electromagnetic Waves Part 3

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Q 24 You are given a parallel plate capacitor. How would you establish an instantaneous displacement current of 1mA in the space between its plates?


Hence, applying a varying potential difference of would produce a displacement current of desired value.

Q 25 Show that the radiation pressure exerted by an EM wave of intensity I on a surface kept in vacuum is .



Q 26 What happens to the intensity of light from a bulb if the distance from the bulb is doubled? As a laser beam travels across the length of a room, its intensity essentially remains constant.

What geometrical characteristic of LASER beam is responsible for the constant intensity which is missing in the case of light from the bulb?


  • Intensity is reduced to one fourth. Tis is because the light beam spreads, as it propagates into a spherical region of area, but LASER does not spread and hence its intensity remains constant.

Q 27 Even though an electric field E exerts a force on a charged particle yet the electric field of an EM wave does not contribute to the radiation pressure (but transfers energy). Explain.


  • Electric field of an EM wave is an oscillating field and so is the electric force caused by it on a charged particle.

  • This electric force averaged over an integral number of cycles is zero since its direction changes every half cycle.

  • Hence, electric field is not responsible for radiation pressure.

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