Optionals IAS Mains Physics: Questions 139 - 144 of 307

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Question 139

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Appeared in Year: 2016

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Essay▾

What is the different between direct and indirect band gap semiconductors? Which one is suitable for use in solar cells?

Explanation

In semiconductor physics, the band gap of a semiconductor is of two types, a direct band gap and an indirect band gap. The minimal – energy state in the conduction band and the maximal – energy state in the valence band are each characterized by a certain crystal momentum vector in the Brillouin zone. If the vectors are the same, it is called a …

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Question 140

Fresnel Diffraction - Half-Period Zones and Zones Plates

Appeared in Year: 2007

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Essay▾

The radius of the first zone in a zone plate is . What will be the position of the first image of a point source of light of light of wavelength placed at a distance of from the zone plate.

Explanation

For , the radius of the zone, the relation can be written as

is the distance of the source from the zone plate and is the distance of screen from the plate. Since (given) , the relation becomes

Question 141

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Appeared in Year: 2007

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Essay▾

In the free electron theory of metals, a conductor s regarded as consisting of free electrons in a three – dimensional box. Using the results of (a) , obtain an expression for the density of states.

Explanation

  • Density of states, is the total number of available electronic states (or orbitals) per unit energy range at energy . To obtain the expression for , we consider the linear momentum which, in quantum mechanics, is represented by the operator

  • This equation represents a standing wave solution. it is however more convenient to work with the plane trav…

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Question 142

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Appeared in Year: 2016

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Essay▾

Obtain Laue՚s equation for x – ray diffraction by crystals. Show that these are consistent with the Bragg՚s law.

Explanation

  • Van Laue treated the phenomenon of diffraction in a more general way by considering the scattering of X – rays from individual atoms in the crystal followed by their recombination to obtain in the direction of the diffraction maxima. It will be shown below that diffraction maxima appear in some specific directions which obey certain conditions know…

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Question 143

Appeared in Year: 2007

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Essay▾

Explain the difference between and transistors. Give their device structure and biasing circuits. Draw a circuit for a single – state common – emitter amplifier.

Explanation

  • Transistor consists of a silicon or germanium (preferably silicon because of its smaller cut off current , smaller variations in due to variations in temperature and higher operating temperature) crystal in which a layer of type material is sandwiched between two layers of type material, as shown in fig (1a) . Alternatively, a transistor may con…

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Question 144

Appeared in Year: 2007

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An unstable particle has a lifetime of in its own frame of reference and is moving towards the earth at a speed of 0.8 c. what will be the lifetime of the particle to an observer on the earth?

Explanation

.

Time as measured by stationary observer

Time measured by moving object

Time measured by the observer on the earth will be whereas the lifetime of the particle in its own frame of reference was .