## CBSE  Sample Papers for Class 12 Physics Solved  2016 Set 7

### Section A

1.Why amplitude modulated signals are preferred for standard AM radio ; broadcast?

2.What is the order of energy gap in
(i)a semiconductor? (ii) an insulator?

3.Which of the following waves can be polarised
(i)heat waves? (ii) sound waves?

4.What is the advantage of using thick metallic strips to join wires in a potentiometer?

5.Can an inverted telescope work as microscope?

### Section B

6.State four properties of nuclear forces.
Or
With the help of an example, explain, how the neutron to proton ratio changes during a-decay of a nucleus.

7.Is photoelectric emission possible at all frequencies? Give reason for your answer 9.Obtain relations between (a) Earth’s magnetic field and its horizontal and vertical components, (b) angle of dip and the horizontal and vertical components of the Earth’s magnetic field.

### Section C

11.A small compass needle of magnetic moment M and moment of inertia I is free to oscillate in a magnetic field B. It is slightly disturbed from its equilibrium position and then released. Show that it executes simple harmonic motion. Hence, write the expression for its time period.

16.Show that the shortest wavelength lines in Lyman, Balmer and Paschen series have their wavelengths in the ratio 1:4:9.
Or
Prove that the radius of the nth Bohr orbit of an atom is directly proportional to n2, where n is principal quantum number.

17.Two charged conducting spheres of radii a and b are connected to each other by a connecting wire. What is the ratio of (i) charges on the spheres and (ii) electric fields at the surfaces of two spheres?

18.A parallel plate capacitor is being charged by a time varying current. Explain briefly how Ampere’s circuital law is generalised to incorporate the effect due to the displacement current.

19.With the help of a diagram show the biasing of a photodiode. Give its one use and draw its characteristic curves. State with reason why a photodiode is usually operated at a reverse bias.

20.(i) The mass of a principle moving with velocity 5xl06m/s has de-Broglie wavelength  associated with it to be 0.135 nm. Calculate its mass.
(ii)In which region of the electromagnetic spectrum does this wavelength lie?

21.(i)Explain with the help of diagram, how plane polarised light is obtained by scattering?
(ii)Between two polaroids placed in crossed position, a third polaroid is introduced. The axis of  the third polaroid makes an angle of 30 ° with the axis of the crossed polaroid. Find intensity of transmitted light from the system assuming I0 be the intensity of polarised light obtained form the first polaroid.

### Section D

23.Rashmi’s Grandfather was old. Due to some reasons doctor suggested him to go through an endoscopic tests. Grandfather had never gone through any such test ever before. So, he was afraid. He thought that these tests will cause negative effects to his body organs. Therefore, he was not ready for endoscopy. Everyone tried to let him understand and convince him. But, he got ready only when his granddaughter Rashmi convinced him and explained that it is nothing but just a test in which light wave is involved and possessing total internal reflection, which is used to view and observe the internal parts of the body.
(i)What are the values of grandfather and the granddaughter’s relation displayed here?
(ii)What is the basic principle used in optical fibre?
(iii)Explain total internal reflection with a suitable diagram.

### Section E

(i)You have learnt that plane and convex mirrors produce virtual images of objects. Can they produce real images under such circumstances? Explain.
(ii)A virtual image, we always say, cannot be caught on a screen. Yet when we see a virtual image, we are obviously bringing it on to the ‘screen’ (i.e. the retina) of our eye. Is there a contradiction?
(iii)A diver under water, looks obliquely at a fisherman standing on the bank of a lake. Would the fisherman look taller or shorter to the diver than what he actually is?
(iv)Does the apparent depth of a tank of water change if viewed obliquely? If so, does the apparent depth increase or decrease?
(v)The refractive index of diamond is much greater than that of ordinary glass. Is this fact of some use to a diamond cutter?
Or
What do you mean by interference of light waves?In Young’s double slit experiment, explain with reason in each case, how the interference pattern changes, when
(i)width of the slits is doubled,
(ii)separation between the slits is increased and
(iii)screen is moved away from the plane of slits?  ### Section A

1.Why amplitude modulated signals are preferred for standard AM radio ; broadcast?
Sol.Because AM based equipments are cheaper and technology is simpler.

2.What is the order of energy gap in
(i)a semiconductor? (ii) an insulator?
Sol.(i) For a semiconductor, energy gap is 1 eV.(ii) For an insulator, energy gap is 5 eV.

3.Which of the following waves can be polarised
(i)heat waves? (ii) sound waves?
Sol.Heat waves can be polarised, because heat waves are transverse waves whereas sound waves cannot be polarised because sound waves are longitudinal waves.
Transverse waves can oscillate in the direction perpendicular to the direction of its propagation but longitudinal waves like sound waves oscillate only along the direction of its propagation. So, longitudinal waves cannot be polarised

4.What is the advantage of using thick metallic strips to join wires in a potentiometer?
Sol.These metal strips have low resistance and need not be counted in the potentiometer length / of the null point. One measures only their lengths along the straight segments (of lengths 1 m each). This is easily done with the help of centimeter rulings or meter ruler and leads to accurate  measurements.

5.Can an inverted telescope work as microscope?
Sol.No, for a telescope, the difference in focal length of the objective and eyepiece is large while in a microscope, the difference of focal lengths is smaller. Therefore, an inverted telescope cannot work as a microscope

### Section B

6.State four properties of nuclear forces.
Or
With the help of an example, explain, how the neutron to proton ratio changes during a-decay of a nucleus.
Sol.State four properties of nuclear forces.
(i)These forces are always attractive
(ii)These are charge independent forces
(iii)These are short range force
(iv)These are non-conservative forces
Or
Consider the a decay 7.Is photoelectric emission possible at all frequencies? Give reason for your answer
Sol.No, photoelectric emission is possible only for a certain minimum frequency value (called the threshold frequency). This is because the incident photons must have at least the minimum energy equivalent to the work function W0 of the metal. Sol.  9.Obtain relations between (a) Earth’s magnetic field and its horizontal and vertical components, (b) angle of dip and the horizontal and vertical components of the Earth’s magnetic field.
Sol.  Sol. ### Section C

11.A small compass needle of magnetic moment M and moment of inertia I is free to oscillate in a magnetic field B. It is slightly disturbed from its equilibrium position and then released. Show that it executes simple harmonic motion. Hence, write the expression for its time period.
Sol.  Sol.   Sol.  Sol.    Sol.  16.Show that the shortest wavelength lines in Lyman, Balmer and Paschen series have their wavelengths in the ratio 1:4:9.
Or
Prove that the radius of the nth Bohr orbit of an atom is directly proportional to n2, where n is principal quantum number.
Sol.  17.Two charged conducting spheres of radii a and b are connected to each other by a connecting wire. What is the ratio of (i) charges on the spheres and (ii) electric fields at the surfaces of two spheres?
Sol. 18.A parallel plate capacitor is being charged by a time varying current. Explain briefly how Ampere’s circuital law is generalised to incorporate the effect due to the displacement current.
Sol.   19.With the help of a diagram show the biasing of a photodiode. Give its one use and draw its characteristic curves. State with reason why a photodiode is usually operated at a reverse bias.
Sol.  20.(i) The mass of a principle moving with velocity 5xl06m/s has de-Broglie wavelength  associated with it to be 0.135 nm. Calculate its mass.
(ii)In which region of the electromagnetic spectrum does this wavelength lie?
Sol. 21.(i)Explain with the help of diagram, how plane polarised light is obtained by scattering?
(ii)Between two polaroids placed in crossed position, a third polaroid is introduced. The axis of  the third polaroid makes an angle of 30 ° with the axis of the crossed polaroid. Find intensity of transmitted light from the system assuming I0 be the intensity of polarised light obtained form the first polaroid.
Sol.    Sol. ### Section D

23.Rashmi’s Grandfather was old. Due to some reasons doctor suggested him to go through an endoscopic tests. Grandfather had never gone through any such test ever before. So, he was afraid. He thought that these tests will cause negative effects to his body organs. Therefore, he was not ready for endoscopy. Everyone tried to let him understand and convince him. But, he got ready only when his granddaughter Rashmi convinced him and explained that it is nothing but just a test in which light wave is involved and possessing total internal reflection, which is used to view and observe the internal parts of the body.
(i)What are the values of grandfather and the granddaughter’s relation displayed here?
(ii)What is the basic principle used in optical fibre?
(iii)Explain total internal reflection with a suitable diagram.
Sol. ### Section E

(i)You have learnt that plane and convex mirrors produce virtual images of objects. Can they produce real images under such circumstances? Explain.
(ii)A virtual image, we always say, cannot be caught on a screen. Yet when we see a virtual image, we are obviously bringing it on to the ‘screen’ (i.e. the retina) of our eye. Is there a contradiction?
(iii)A diver under water, looks obliquely at a fisherman standing on the bank of a lake. Would the fisherman look taller or shorter to the diver than what he actually is?
(iv)Does the apparent depth of a tank of water change if viewed obliquely? If so, does the apparent depth increase or decrease?
(v)The refractive index of diamond is much greater than that of ordinary glass. Is this fact of some use to a diamond cutter?
Or
What do you mean by interference of light waves?In Young’s double slit experiment, explain with reason in each case, how the interference pattern changes, when
(i)width of the slits is doubled,
(ii)separation between the slits is increased and
(iii)screen is moved away from the plane of slits?
Sol.    Sol.    Sol.      