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

### Section A

1.Which physical quantity has unit JCT1 ? Is it a vector or a scalar quantity? (l)

2.A convex lens of refractive index 1.5 has a focal length of 18 cm in air. Calculate the change in  its focal length when it is immersed in water of refractive index 4/3.

3.The graph shown in the figure represents a plot of current versus voltage for a given semiconductor. Identify the region, if any over which the semiconductor has a negative resistance.

4. Plot a graph showing the variation of photoelectric current with anode potential for  two light beams of same wavelength but different intensity.

5.Two nuclei have mass number in the ratio of 2 : 5. What is the ratio of their nuclear  densities?

### Section B

6.Define mobility of a charge carrier. Write the relation expressing mobility in terms of  relaxation time. Give its SI unit.

7. Define drift velocity. Write the expression for the current in a conductor of  cross-sectional area A in terms of drift velocity.

8. An electric lamp connected in series with a capacitor and an AC source is glowing with of certain brightness.

How does the brightness of the lamp change on reducing the

• capacitance and
• frequency?

9.Define the term ‘activity’ of a radioactive substance. State its SI unit. Give a plot of  activity of radioactive species versus time.

10.A parallel plate capacitor with air between the plates has a capacitance of 8 pF. The separation between the plates is now reduced by its half and the space between them is filled with a medium of dielectric constant 5. Calculate the value of the capacitance of the capacitor in the second case. (2)

Or

Two metal plates form a parallel plate capacitor. The distance between the plates is d. A metal sheet of thickness d/2 and of the same area is introduced between the plates. What is the ratio of the capacitances in the two cases?

### Section C

11. Name the constituent radiation of electromagnetic spectrum which

(i) is used for studying crystal structure.

(ii) is absorbed from sun light by ozone layer.

(iii) produces intense heating effect.

(iv) is used in cellular phones to transmit voice communication.

(v) is used for sterilizing surgical instruments.

(vi) is used for taking photograph in foggy season.

13. How is the mutual inductance of a pair of coils affected, when

• separation between the coil is increased?
• the number of turns of each coil is increased?
•  a thin iron sheet is placed between the two coils, other factors remaining the same?  Explain your answer in each case.

14. Light of wavelength 5000 A falls on a plane reflecting surface. What are the  wavelength and frequency of reflected light? For what angle of incidence is the reflected ray normal to the incident ray?

17. A bar magnet M is dropped so that it falls vertically through the coil C. Obtain the graph for voltage produced across the coil versus

• Explain which quantity is depicted by the area of graph.
• Why is the negative peak longer than the positive peak?

18. In a single slit diffraction pattern, how is the angular width of central bright maximum changed when

• the slit-width is decreased.
• the distance between the slit and the screen is increased.
• light of smaller wavelength used.

19. Define (i) magnifying power, (ii) resolving power and (iii) numerical aperture. What  is the relation between them?

20. Write Einstein’s photoelectric equation. Mention the underlying properties of photons on the basis of which this equation is obtained. Write two important observations of photoelectric effect which can be explained by Einstein’s equation

Or

The following graph shows the variation of stopping potential Vs with the frequency (v) of the incident radiation for two photo sensitive metals X and Y.

(i) Which of the metals has larger threshold wavelength? Give reasons.

(ii) Explain giving reason which metal gives out electrons having larger kinetic energy for the same wavelength of incident radiation.

(iii) If the distance between the light source and metal X is halved, then what will be the  kinetic energy of electron emitted due to this change? Give reasons.

21. (i) Give the ratio of the number of holes and the number of conduction electrons in an intrinsic semiconductor.

(ii) How does the energy gap in an intrinsic semiconductor vary when doped with a pentavalent impurity?

(iii) Draw I versus V graph of a forward biased junction diode.

22. Using Bohr’s postulates for hydrogen atom, show that the total energy (E) of the electron in the stationary states can be expressed as the sum of kinetic energy (K) and potential energy (U), where K = -2U. Hence, deduce the expression for the total energy in the nth energy level of hydrogen atom.

### Section D

23.Two best friends John and Jolly are students of IX class. John wants to be an astronaut but he does not know anything about astronomy. So, he shared his desire with his friend Jolly. Jolly asked him to take advice from his teacher.

• What would be the suggestion of the teacher?
• Draw ray diagram of reflecting type telescope.
• An astronaut is looking down on the Earth’s surface from with the eyelens having

aperture of 5mm from a space shuttle at an altitude of 500 km and the wavelength of visible light is 500 nm, the astronauts will be able to find the linear objects of the size of about?

### Section E

24. (i) Obtain the expression for the potential due to an electric dipole of dipole moment, p at a point x on the axial line.

(ii) Two identical capacitors of plate dimensions 1 x b and plate separation d have
dielectric slabs filled in between the space of the

25. What does the term LOS communication mean? Name the type of waves that are used for this communication. Give typical example with the help of suitable figure of communication systems that use space wave mode propagation. (5)

Or

• Describe briefly the three factors which justify the need for translating a low frequency signal into high frequencies before transmission.
• Figure shows a block diagram of a detector for AM signal

Draw the waveforms for the

• input AM wave at A.
• output B at the rectifier.
• output signal at C.