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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