Q15 of 26 Page 1

(a) Which one of the following electromagnetic radiations has least frequency: UV radiations, X-rays, Microwaves (b) How do you show that electromagnetic waves carry energy and momentum? (c) Write the expression for the energy density of an electromagnetic wave propagating in free space.

(a) From the electromagnetic spectrum we infer that the Microwaves have the least frequency as compared to UV

radiations and X-rays.


(b) Electromagnetic waves carry both energy and momentum. This is evident from motion of charged particles placed in an electromagnetic field. If they didn’t carry a momentum or energy, a particle placed in a field will remain stationary. However, we know that the electric and magnetic fields are able to change the speed (and thus energy) of the particles and their directions. This happens in nearly all particle accelerators.


(c)The energy density(U) of electromagnetic wave is due to the electric field and magnetic field.



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13

Answer the following:

(a)In what way is diffraction from each slit related to the interference pattern in a double slit experiment?


(b)When a tiny circular obstacle is placed in the path of light from a distant source, a bright spot is seen at the centre of the shadow of the obstacle. Explain, why.


(c)How does the resolving power of a microscope depend on (i) the wavelength of the light used and (ii) the medium used between the object and the objective lens?


 14

Define the term ‘intensity of radiation’ in photon picture of light. Ultraviolet light of wavelength 2270 Å from 100 W mercury source irradiates a photo cell made of a given metal. If the stopping potential is – 1 �3 V, estimate the work function of the metal. How would the photo cell respond to a high intensity () red light of wavelength 6300 Å produced by a laser ?

OR


Set up Einstein’s photoelectric equation using the photon picture of electromagnetic radiation. Explain briefly how this equation accounts for all the observations in the photoelectric effect.


 16

Draw the magnetic field lines distinguishing between diamagnetic and paramagnetic materials. Give a simple explanation to account for the difference in the magnetic behavior of these materials.

 17

Explain, with the help of suitable diagram, the two important processes that occur during the formation of p-n junction. Hence define the terms: depletion region and barrier potential.