31.

a) A convex lens of focal length 5 cm is used as a simple microscope. Where should an object be placed so that the image formed by it lies at the least distance of distinct vision (D = 25 cm)?

b) Draw a labelled diagram showing the formation of an image by a refracting telescope when the final image lies t infinity.

a) Monochromatic light of wavelength 198 nm is incident on the surface of a metallic cathode whose work function is 2.5 eV. How much potential difference must be applied between the cathode and the anode of a photocell to just stop the photo current from flowing?

b) i) What is de-broglie hypothesis?
ii) What conclusion can be drawn from Davisson and Germer's experiment?

a) i) How are various lines of lyman series formed? Explain on the basis of Bohr's theory.
ii) Calculate the shortest wavelength of electromagnetic radiation present in Balmer series of hydrogen spectrum.

b) State the effect of the following changes on the X-rays emitted by Coolidge X-ray tube:

i) High voltage between cathode and anode is increased.
ii) Filament temperature is increased.

a) Half life of a certain radioactive material is 8 hours.
i) Find disintegration constant of this material.
ii) If one starts with 60 g of this substance, how much of it will disintegrate in one day?

b) Sketch a graph showing the variation of binding energy per nucleon of a nucleus with its mass number.

a) Draw a circuit diagram for the common emitter transistor amplifier. What is meant by phase reversal?

b) Write the truth table of the following circuit. Name the gate represented by this circuit.