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Physics - 1998 ( I.S.C)
You are on Questions 6 to 13

SECTION -  B

Q 6.(a) What is meant by a 'wave front'? What is the shape of the wave front of a beam of parallel waves?
State Huygens' principle and use it to describe the process of reflection of a parallel beam of light incident on a plane mirror obliquely, with the help of a labelled diagram. Prove that the angle of incidence is equal to the angle of reflection.

(b)In a Fresnel biprism experiment, the fringe width is found to be 0.5 mm when monochromatic source of wavelength 4000 A is used. Keeping the distance between the source and the biprism constant but reducing the distance between the source and the screen to half the original value, the same biprism is used with a source of different wavelength l. What is the value of l for this new setting to produce fringes with fringe width 0-4 mm?

Q 7(a) Which plane is defined as the plane of polarisation in a plane polarised electromagnetic wave?

(b) The critical angle between a given transparent medium and air is denoted by ic. A ray of light in air medium enters this transparent medium at an angle of incidence equal to the polarising angle(ip).Deduce a relation for the angle of refraction(r) in terms of ic.

(c)A beam of paralled rays makes normal incidence on a plane transmission diffraction grating and coloured fringe lines are observed. The red line of the second order spectrum overlaps exactly on the blue lineof the third order at an angle of diffraction of 30°. If the wavelength of the red light is 7500 Å, What is the wavelength of the overlapping blue colour? What is the number of lines present per cm on the grating surface?

Q 8(a) A liquid of refractive index 1.5 is poured into a cylindrical jar of radius 20cm upto a height of 20 cm.  A small bulb is lighted at the centre of the bottom of the jar.  Find the area of the liquid surface through which the light of the bulb passes into air.

(b) Fig.7 shows a solid glass sphere of radius 5 cm that has a amll air bubble (B) trapped at a distance of 2 cm from the cnetre C. The refractive index of the mateiral of glass is 1.5. Find the apparent position of the bubble where it will appear, when seen through the surface of the sphere from an outside point E

 

Q 9(a) What is meant by the term 'pure spectrum'?
Briefly describe a method to adjust the telescope and collimeterof a spectrometer to set it for the production of a pure spectrum.

(b)The focal lengths of the eyepiece and objective of a compound microscope are 5 cm and 1 cm respectively and the length of the tabe is 20 cm. Calculate the magnifying power of this microsope when the final image is formed at infinity.  The value of least distance of distinct vision is 25 cm.

SECTION C

Q10 (a) With Reference to the photoelectirc effect define the terms 'work function' and 'threshold wavelength' for a given metal. On what factors will be following depends during photoelectric emission from a metal surface :
(i) the magnitude of photelectric current, and
(ii) the velocity of ejected dlectrons?

(b)In a certain region of space a uniform electric field 7200 V/m and a uniform magnetic field
1.8x10-3 T are simultaneouslypresents in mutually perpendicular directions. An electron beam enters this region in a direction perpendicular to both electric and magnetic fields. Calcuate the velocity of elctrons in the beam if they pass through this region without any deviation in their straight line path. if the electric field is now switched off, what kind of path will be taken by the beam? Find the radius of curvature of this path.(Mass of electron = 9x10-31kg and Charge on electron = 1.6x10-19 C).

Q 11(a) 18 KV accelerating voltage is applied across an X-ray tube. Calculate:
(i)The velocity of the fastest electron striking the target,
(ii)The minimum wavelength in the continuous spectrum of X-rays produced.(Mass of electron = 9x10-31kg, Charge on electron = 1.6x10-19 C, h = 6.6x10-34 Js, c = 3x108 m/s).

(b) What are Fraunhofer lines? Give a brief explanation for their occurence.

(c) If a light wave of wavelength 4950 Åis viewed as a continuous flow of photons what is the energy in eV (values given for h,c,e etc. in Q.no 11(a) may be used).

Q 12(a) With the help of labelled diagram explain the working of a Bainbridge Mass Spectometer. Deduce an expression for the radius of curvature of the path of an ion in terms of its mass number and charge.  How is a linear mass scale achieved for different isotopes?

(b) What is the purpose of (i) moderator and (ii) controller in a nuclear reactor?

(c) Find out the binding energy per nucleon of an a-particle in MeV. It is given that the mases of a-particle, proton and neutron are respectively 4.00150 amu, 1.00728 amu and 1.00867 amu

Q 13.(a) Distinguish between insulators, conductors and semiconductors in terms of their energy band diagrams.

(b) Draw the current versus voltage characteristic curve of Zener diode. Indicate the position of Zener voltage. Draw a labelled circuit diagram of D.c voltage regulator using Zener diode.

(c) What is the relation between emitter current (IE), base current (IB)and collector current (IC)at any instant in a properly baised common emitter transistor circuit? Which one of the three has the smallest magnitude?

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