Q8 of 30 Page 213

Explain why elemental semiconductors cannot be used to make visible LED’s.

Elemental semiconductors cannot be used to make visible LED’s Because the energy released by combination of holes and electrons in elemental semiconductors does not lie in the visible spectrum.


Specially designed diode which give out light radiations when forward biases LRED’s are made up of GaAsp,Gap etc


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6

What is the shape of the wave front on earth for sunlight?

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An electron (mass m) with an initial velocity is in an electric field . It’s de Broglie wavelength at time t is given by


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Five charges, q each are placed at the corners of a regular pentagon of side ‘a’ as shown.


(i) What will be the electric field at O if the charge from one of the corners (say A) is removed?


(ii) What will be the electric field at O if the charge q at A is replaced by –q?


 10

Two cells of emf E1 and E2 have internal resistance r1 and r2. Deduce an expression for equivalent emf of their parallel combination.