## Tuesday, 2 January 2018

### CBSE CLASS 12 PHYSICS REVISION QUESTION PAPER

Class XII Physics
REVISION
Time: 3 hours                                                                                                Maximum Marks: 70

General Instructions:
(i) All questions are compulsory. There are 26 questions in all.
(ii) This question paper has five sections: Section A, Section B, Section, Section D and
Section E.
(iii) Section A contains five questions of one mark each, section B contains five questions
of two marks each, section C contains twelve questions of three marks each, section
D contains one value based question of four marks and section E contains three
questions of five marks each.
(iv) There is no overall choice. However, an internal choice has been provided in one
question of two marks, one question of three marks and all the three questions of five
marks weightage. You have to attempt only one of the choices in such questions.

SECTION A
1. State the factors on which the refractive index of a material medium for a given wavelength depends.
2. Sketch the emergent wavefront

3. The instantaneous current and voltage of an a.c circuit are given by
i= 10 sin 314t Ampere,
v= 50 sin 314t Volt.
What is the power factor of the a.c. circuit?
4. How does power of convex lens vary when red light incident on it is replaced by violet light?
5. The vertical component of Earth’s magnetic field at a place is √3 times the horizontal component. What is the angle of dip at this place?
SECTION B

6.(a)An alternating voltage E = Eo sinωt is applied to a circuit containing a resistor R connected in series with a black box. The current in the circuit is found to be I = Io sin (ωt + π/4).

(i) State whether the element in the black box is a capacitor or inductor.
(ii) Draw the corresponding phasor diagram and find the impedance in terms of R.

7. The magnetic field in a plane electromagnetic wave is given by: By = 12 × 10–8 sin (1.20 × 107 z + 3.60 × 1015 t) T.
Calculate the
(i) Energy density associated with the Electromagnetic wave
(ii) Speed of the wave
OR
What is dispalcemeny current? Write Maxwell’s Equations
8. A spherical convex surface of radius of curvature 20 cm, made of glass (μ = 1.5) is placed in air. Find the position of the image formed, if a point object is placed at 30 cm in front of the convex surface on the principal axis.

9. State one use of each of the following e.m. waves? Arrange them in the decreasing order of
their frequency.
i. X-rays
iii. Microwaves
iv. Ultraviolet light.
10. Define the following:
i. Geographic meridian
ii. Magnetic meridian

SECTION C
11. Figure shows a metal rod PQ of length l, resting on the smooth horizontal rails AB positioned between the poles of a permanent magnet. The rails, rod and the magnetic field B are in three mutually perpendicular directions. A galvanometer G connects the rails through a key ‘k’. Assume the magnetic field to be uniform. Given the resistance of the closed loop containing the rod is R.

(i) Suppose K is open and the rod is moved with a speed v in the direction shown. Find the polarity and the magnitude of induced emf.
(ii) With K open and the rod moving uniformly, there is no net force on the electrons in the rod PQ even though they do experience magnetic force due to the motion of the rod. Explain.
(iii) What is the induced emf in the moving rod if the magnetic field is parallel to the rails instead of being perpendicular?

12.With the help of a diagram, explain the principle of a device which changes a low voltage into a high voltage but does not violate the law of conservation of energy. Give any one reason why the device may not be 100% efficient.

13. In a double slit experiment, the distance between the slits is 3 mm and the slits are 2 m away from the screen. Two interference patterns can be seen on the screen one due to light with wavelength 480 nm, and the other due to light with wavelength 600 nm. What is the separation on the screen between the fifth order bright fringes of the two interference patterns?
14. What do you understand by the statement ‘Light from the sun is unpolarised’.
Explain how does sunlight gets polarized by the process of scattering?
15. In a series LCR circuit connected to a variable frequency 230 V source L=5.0H, C=80 μF and R=40 Ω.
(i) Obtain the amplitude of current at the resonating frequency.
(ii) Show that the potential drop across the LC combination is zero at the resonating frequency.
16.Derive lens makers formula with suitable diagram for a thin convex lens.
17. Draw a ray diagram showing the image formation by a reflecting type telescope. Give two advantage of reflecting type telescope over the refracting type telescope.
OR
Write the expression for the resolving power of a microscope. How does the resolving power of compound microscope change when (i) wavelength of light used is increased (ii) Diameter of the objective lens is increased.
18. A circular coil of radius 10 cm, 500 turns and resistance 2Ω is placed with its plane perpendicular to the horizontal component of the earth’s magnetic field. It is rotated about its vertical diameter through 1800 in 0.25 s. Estimate the magnitude of the emf and current induced in the coil. Horizontal component of the earth’s magnetic field at the place is 3.0 X 10-5 T.
19.An a.c source of voltage V = Vm sin ωt is connected one –by-one to three elements X,Y and Z. It is observed that the current flowing in them
(i) Is in phase with applied voltage for X
(ii) Lags the applied voltage in phase by π/2 for element Y.
(iii) Leads the applied voltage in phase by π/2 for element Z.
When three elements X, Y and Z are connected in series with the a.c source then at a particular frequency current in the circuit become maximum. Write the condition for the maximum current in the circuit and obtain the expression for the frequency at that condition.
20. State the difference between dia, para and ferro magnetic materials in terms of their (i) behavior in a non uniform field (ii) behavior in a uniform field (iii) relative permeability (iv) susceptibility and (v)temperature dependence of their susceptibility
21. What is total internal reflection? State two important use of it. Where does in nature this phenomenon can be observed?
22.Derive the expression when a ray of monochromatic light passes through a glass prism.
SECTION D
23.Akil was playing cricket with his friends, when a ball hit his friend Bharat on his left leg .Bharat screamed with pain.Akil rushed towards him and comforted him and asked him not to move his
leg. He quickly took out his cell phone and called up Bharat’s parents and briefed them about the incident. In10 minutes Bharat was taken to the nearby hospital and was examined by the doctor who advised for an X-rays test which confirmed a hairline fracture.
A) How are X-rays produced?
b) Mention one another application of X rays.
c) Mention two qualities of Akhil which are reflected from the above situation.
SECTION E

24.(a)Draw a ray diagram of compound microscope for the final image formed at least distance of distinct vision?

(b) An angular magnification of 30X is desired using an objective of focal length 1.25 cm and an eye piece of focal length 5 cm. How will you set up the compound microscope for the final image formed at least distance of distinct vision?

OR
(a) Draw a ray diagram of an astronomical telescope for the final image formed at least distance of distinct vision?
(b) An astronomical telescope has an angular magnification of magnitude 5 for distant objects. The separation between the objective and an eye piece is 36 cm and the final image is formed at infinity. Calculate the focal length of the objective and the focal length of the eye piece?
25. Using Huygen’s construction of a wave front, explain the reflection of a plane wave front at a plane surface. Draw the shape of the refracted wave front when a plane wave front is incident on (i) convex mirror (ii) concave mirror. Illustrate with the help of diagrams the action of convex lens and concave lens on a plane wave front incident on it.
OR
(a)State the essential condition for the diffraction of light to take place. A parallel beam of monochromatic light falls normally on a narrow slit and light coming out of the slit is obtained on the screen. Derive an expression for the angular width of the central bright maxima obtained on the screen.
(b)‘Diffraction defines the limit of ray optics’. Give a brief explanation of this statement.
26.What is mutual induction? Define the coefficient of mutual induction and its SI unit. Also, derive an expression for coefficient of mutual induction of two long solenoids.
OR
a)State Lenz’s law . Derive an expression for induced emf developed in a conductor of length l moving with velocity u in a transverse magnetic field of Strength B.
b) What are Eddy Currents? State the main undesirable effect of these currents and give the method used to minimize the undesirable effect. Write any two applications of eddy currents.