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Read the following paragraph and answer the questions.
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LED is a heavily doped P-N junction which under forward bias emits spontaneous radiation. When it is forward-biased, due to recombination of holes and electrons at the junction, energy is released in the form of photons. In the case of Si and Ge diode, the energy released in recombination lies in the infrared region. LEDs that can emit red, yellow, orange, green and blue light are commercially available. The semiconductor used for fabrication of visible LEDs must at least have a band gap of 1.8 eV. The compound semiconductor Gallium Arsenide – Phosphide is used for making LEDs of different colours.
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- Why are LEDs made of compound semiconductor and not of elemental semiconductors?
- What should be the order of bandgap of an LED, if it is required to emit light in the visible range?
- A student connects the blue coloured LED as shown in the figure. The LED did not glow when switch S is closed. Explain why?
OR
iii. Draw V-I characteristic of a p-n junction diode in
(i) forward bias and (ii) reverse bias
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Why a photo-diode is operated in reverse bias whereas the current in the forward bias is much larger than that in the reverse bias? Explain. Mention its two uses.
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Briefly explain how emf is generated in a solar cell.
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Draw solar cells of I-V characteristics.
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Draw the circuit diagram of an illuminated photodiode and its I-V characteristics.
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How can a photodiode be used to measure light intensity?
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A dielectric slab of thickness 't’ is kept between the plates of a parallel plate capacitor with plate separation 'd' (t < d). Derive the expression for the capacitance of the capacitor.
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Why can’t we see clearly through fog?
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Name the phenomenon responsible for it.
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Why does unpolarised light from a source show a variation in intensity when viewed through a polaroid which is rotated?
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Show with the help of a diagram, how unpolarised light from Sun gets linearly polarised by scattering.
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Draw the intensity distribution for the fringes produced in interference ?
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Draw the intensity distribution for the diffraction bands produced due to single slit ?
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Write two points of difference between the phenomena of interference and diffraction.
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Draw a labelled schematic ray diagram of astronomical telescope in normal adjustment.
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Which two aberrations do objectives of refracting telescope suffer from? How are these overcome in reflecting telescope?
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How does the resolving power of a telescope change on increasing the aperture of the objective lens? Justify your answer.
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Why does the Sun look reddish at sunset or sunrise ?
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The V − I characteristic of a silicon diode is as shown in the figure. Calculate the resistance of the diode at (i) I = 15 mA and (ii) V= −10 V.
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In the meterbridge experimental set up, shown in the figure, the null point ‘D’ is obtained at a distance of 40 cm from end A of the meterbridge wire. If a resistance of 10Ω is connected in series with R1, null point is obtained at AD = 60 cm. Calculate the values of R1 and R2.
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