CBSE Class 12

Pre Boards

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Sample Papers

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 Multiple Choice QuestionsLong Answer Type



Define magnifying power of a telescope. Write its expression.

A small telescope has an objective lens of focal length 150 cm and an eye piece of focal length 5 cm. If this telescope is used to view a 100 m high tower 3 km away, find the height of the final image when it is formed 25 cm away from the eye piece.

a) Magnifying power of telescope is the ratio of the angle subtended at the eye by the image to the angle subtended at the unaided eye by the object.

Mathematically, we can write

where, fo is the focal length of the objective, fe is the focal length of the eye-piece and D is the least distance of distinct vision.

b) Using, the lens equation for objective lens,

Hence, magnification due to the objective lens is given by,

Now, using lens formula for eye-piece, we get

Therefore, magnification due to eyepiece me = 
Hence, total magnification, 

So, size of final image = 



Draw a simple circuit of a CE transistor amplifier. Explain its working. Show that the voltage gain AV, of the amplifier is given by A subscript v space equals space minus fraction numerator beta subscript a c end subscript R subscript L over denominator r subscript i end fraction, where bold italic beta subscript bold italic a bold italic c end subscript is the current gain, RL is the load resistance and ri is the input resistance of the transistor. What is the significance of the negative sign in the expression for the voltage gain?



Draw the circuit diagram of a full wave rectifier using p-n junction diode. Explain its working and show the output, input waveforms.

(b) Show the output waveforms (Y) for the following inputs A and B of

(i) OR gate  

ii) NAND gate