## Solutions to NCERT Physics Class 12 (Ray Optics)

1. A small candle, 2.5 cm in size is placed at 27 cm in front of a concave mirror of radius of curvature 36 cm. At what distance from the mirror should a screen be placed in order to obtain a sharp image? Describe the nature and size of the image. If the candle is moved closer to the mirror, how would the screen have to be moved?
Answer:Size of the candle, h= 2.5 cmImage size = h’Object distance, u= −27 cmRadius of curvature of the concave mirror, R= −36 cmFocal length of the concave mirror, f=R/2 = -18 cm

Image distance = v

The image distance can be obtained using the mirror formula:

$\frac{1}{u}+\frac{1}{v}=\frac{1}{f}$

$\frac{1}{v}=\frac{1}{f}-\frac{1}{u}$

$=\frac{1}{-18}-\frac{1}{-27}=\frac{-3+2}{54}=-\frac{1}{54}$

Therefore, v=-54cm

Therefore, the screen should be placed 54 cm away from the mirror to obtain a sharp image.

The magnification of the image is given as: $m=\frac{h'}{h}=-\frac{v}{u}$

Therefore, $h'=-\frac{v}{u}\times&space;h&space;=&space;-\frac{-54}{-27}\times&space;2.5&space;=&space;-&space;5&space;cm$

The height of the candle’s image is 5 cm. The negative sign indicates that the image is inverted and real.

If the candle is moved closer to the mirror, then the screen will have to be moved away from the mirror in order to obtain the image.

2. A 4.5 cm needle is placed 12 cm away from a convex mirror of focal length 15 cm. Give the location of the image and the magnification. Describe what happens as the needle is moved farther from the mirror.
3. A tank is filled with water to a height of 12.5 cm. The apparent depth of a needle lying at the bottom of the tank is measured by a microscope to be 9.4 cm. What is the refractive index of water? If water is replaced by a liquid of refractive index 1.63 up to the same height, by what distance would the microscope have to be moved to focus on the needle again?
4. Figures 9.34(a) and (b) show refraction of a ray in air incident at 60° with the normal to a glass-air and water-air interface, respectively. Predict the angle of refraction in glass when the angle of incidence in water is 45º with the normal to a water-glass interface [Fig. 9.34(c)].
5. A small bulb is placed at the bottom of a tank containing water to a depth of 80 cm. What is the area of the surface of water through which light from the bulb can emerge out? Refractive index of water is 1.33. (Consider the bulb to be a point source.)
6. A prism is made of glass of unknown refractive index. A parallel beam of light is incident on a face of the prism. The angle of minimum deviation is measured to be 40°. What is the refractive index of the material of the prism? The refracting angle of the prism is 60°. If the prism is placed in water (refractive index 1.33), predict the new angle of minimum deviation of a parallel beam of light.
7. Double-convex lenses are to be manufactured from a glass of refractive index 1.55, with both faces of the same radius of curvature. What is the radius of curvature required if the focal length is to be 20 cm?
8. A beam of light converges at a point P. Now a lens is placed in the path of the convergent beam 12 cm from P. At what point does the beam converge if the lens is (a) a convex lens of focal length 20 cm, and (b) a concave lens of focal length 16 cm?
9. An object of size 3.0 cm is placed 14 cm in front of a concave lens of focal length 21 cm. Describe the image produced by the lens. What happens if the object is moved further away from the lens?
10. What is the focal length of a convex lens of focal length 30 cm in contact with a concave lens of focal length 20 cm? Is the system a converging or a diverging lens? Ignore thickness of the lenses.
11. A compound microscope consists of an objective lens of focal length 2.0 cm and an eyepiece of focal length 6.25 cm separated by a distance of 15 cm. How far from the objective should an object be placed in order to obtain the final image at (a) the least distance of distinct vision (25 cm), and (b) at infinity? What is the magnifying power of the microscope in each case?
12. A person with a normal near point (25 cm) using a compound microscope with objective of focal length 8.0 mm and an eyepiece of focal length 2.5 cm can bring an object placed at 9.0 mm from the objective in sharp focus. What is the separation between the two lenses? Calculate the magnifying power of the microscope,
13. A small telescope has an objective lens of focal length 144 cm and an eyepiece of focal length 6.0 cm. What is the magnifying power of the telescope? What is the separation between the objective and the eyepiece?
14. (a)A giant refracting telescope at an observatory has an objective lens of focal length 15 m. If an eyepiece of focal length 1.0 cm is used, what is the angular magnification of the telescope?(b) If this telescope is used to view the moon, what is the diameter of the image of the moon formed by the objective lens? The diameter of the moon is 3.48 × 106 m, and the radius of lunar orbit is 3.8 × 108 m.

## CBSE Physics Solved Board Question Papers 2008, 2009, 2010 & 2011 – All versions

Download CBSE Physics Solved Board Question Papers 2008, 2009, 2010 & 2011 – All versions (Delhi, Outside Delhi & Foreign)

The question Papers are in PDF format, all question papers with solution in a single file. So, it may take some time for the download to finish.

CBSE Physics Class 12 Board Question Papers 2008 to 2011 all sets (SETS I, II & III) and versions (DELHI, FOREIGN, OUTSIDE DELHI)

## Quick Revision for Class X Physics SA1

CHAPTER -12 ELECTRICITY

GIST OF THE LESSON

1. Positive and negative charges: The charge acquired by a glass rod when rubbed with silk is called positive charge and the charge acquired by an ebonite rod when rubbed with wool is called negative charge.
2. Coulomb: It is the S.I. unit of charge. One coulomb is defined as that amount of charge which repels an equal and similar charge with a force of 9 x 109 N when placed in vacuum at a distance of 1 meter from it. Charge on an electron = -1.6 x 10-19 coulomb.
3. Static and current electricities: Static electricity deals with the electric charges at rest while the current electricity deals with the electric charges in motion.
4. Conductor: A substance which allows passage of electric charges through it easily is called a ‘conductor’. A conductor offers very low resistance to the flow of current. For example copper, silver, aluminium etc.
5. Insulator: A substance that has infinitely high resistance does not allow electric current to flow through it. It is called an ‘insulator’. For example rubber, glass, plastic, ebonite etc.
6. Electric current: The flow of electric charges across a cross-section of a conductor constitutes an electric current. It is defined as the rate of flow of the electric charge through any section of a conductor.

Electric current = Charge/Time     or        I = Q/t

Electric current is a scalar quantity.

1. Ampere: It is the S.I. unit of current. If one coulomb of charge flows through any section of a conductor in one second, then current through it is said to be one ampere.                                                                                                                         1 ampere = 1 coulomb/1 second    or      1 A = 1C/1s = 1Cs-1                                                                                                                                                                                                              1 milliampere =    1 mA = 10-3 A                                                                                                                                                                                 1 microampere = 1µA = 10-6 A
2. Electric circuit: The closed path along which electric current flows is called an ‘electric circuit’.
3. Conventional current: Conventionally, the direction of motion of positive charges is taken as the direction of current. The direction of conventional current is opposite to that of the negatively charged electrons.
4. Electric field: It is the region around a charged body within which its influence can be experienced. Continue reading “Quick Revision for Class X Physics SA1”

## Five marks questions from Electronic Devices (Long Answer Type)

1. Explain the formation of energy Bands in solids. Distinguish between metals, insulators and semiconductors on the basis of band theory.
2. Distinguish between intrinsic and extrinsic semiconductors and the conduction in P type and N type semiconductors.
3. Explain the formation of depletion region and barrier potential in a pn junction.
4. Draw the circuit diagram used to study the Forward and reverse bias characteristics and draw the graph for forward bias and reverse bias.
5. Describe the working of a half wave rectifier  with the help of a neat labeled diagram and draw the input and output wave forms.
6. Describe the working of a full wave rectifier with the help of a neat labelled diagram and draw the input and output wave forms.
7. Draw the symbols of npn and pnp transistor. Show the biasing of a transistor and explain transistor action.
8. Describe the working of an npn transistor in CE configuration as an amplifier.
9. Explain the working of a transistor in CE configuration as oscillator.
10. Explain the action of transistor as a switch.

(Have some more idea? Post them as comments)

## Questions from Physics Viva

1. What happens to the resistivity of a wire when it is doublefolded?
2. How does the resistance of a wire depend on its dimensions?
3. Why are the resistances used in a resistance box is like 1,2,2,5,10,20,20,50,100,200,200,500,1000,2000,2000,5000 etc?
4. Why constantan or manganin wires are used for making the resistance coils in resistance box?
5. What is a standard resistance?
6. What are the characteristics of a standard resistance?
7. What are the precautions to be observed while doing electricity experiments in general?
8. What is a galvanometer?
9. What is the resistance of an ideal ammeter?
10. Why is ammeter always connected in series and voltmeter always connected in parallel?
11. How can we convert a galvanometer into an ammeter or a voltmeter?
12. What is shunt?
13. What is AVO meter?
14. What is the effect of temperature on the resistance of a conductor?
15. Why does the resistance of a conductor increases with temperature, whereas that of a semiconductor decreases with temperature.
16. What is conductance?
17. What are non ohmic devices? give an example.
18. What are superconductors?
19. Define emf
20. Why emf is said to be a misnomer?
21. What happens if the battery used in the primary circuit of a potentiometer has less emf compared to the emf of the cell used in the secondary circuit?
22. If you find that the galvanometer reading is shaky, what error can you expect?
23. What do you mean by figure of merit of a galvanometer?
24. Why a moving coil galvanometer is called so?
25. What is the principle of a galvanometer?
26. What is parallax? How is it removed?
27.  Define dispersion of light. Give its cause.
28.  Show dispersion of white light into seven colours.
29. Define lateral displacements and give the factor on which it depends.
30. Define critical angle and deduce the relation between refractive index and critical angle.
31. Explain U-V graph in case of image formation in a convex lens.
32. Distinguish between primary and secondary cell. (give two differences)
33.  How does rheostat act as a variable resistor, potential divider? (draw a diagram)
34. What a non inductive coil? Give one special feature of non inductive coil?
35. Draw neat and label diagram of leclanche cell.
36. Write chemical equation in Daniel cell.
37. Why is the emf of auxiliary battery greater than the emf to be measured?
38. Why are standard resistances made up of constantan and Manganin?
39. Define figure of merit and give it is S.I unit.

## Must Practice Questions for CBSE Physics CLASS 12: Solved

We will publish here the portions one cannot afford to miss while preparing for the CBSE class 12  Physics Examination

The Chapters will be posted below. On Clicking the name of the chapter a new page will open. On that page there will be a set of collected questions. On clicking the questions, the complete solution to the questions will open.

Please note that this system is under preparation. We hope to complete this within a month. The link will be activated only after completing the project.

## A too simple question paper from Physics (Magnetic effects of current and magnetism) for class XII

If you find these question easy to answer, well and good. If you don’t, please revise these portions thoroughly. These are based on just the minimum level of learning expected from a Class XII student.

1. State Gauss theorem in magnetism.
2. Define the terms retentivity and coercivity.
3. Which direction will a magnetic compass show when it is kept horizontally at the magnetic north pole of earth?
4. Determine the magnitude and direction of magnetic field to be applied in a region with a vertical electric field 1000 V/m (downwards) where a proton has to move towards east with a speed of 100 m/s without any deflection. (Disregard the presence of earth’s magnetic field.)
5. Derive expression for the force on a current carrying conductor in a uniform magnetic field.
6. Using Ampere’s circuital law, derive an expression for the magnetic field at a point due to a straight conductor carrying current.
7. The radius of the first electron orbit of a hydrogen atom is 0.5 A. The electron moves in this orbit with a uniform speed of 2.2 x 106 m/s. What is the magnetic field produced at the centre of the nucleus due to the motion of this elecetron?
8. Derive the relation mr = 1+cm (relation between magnetic permeability and magnetic susceptibility)
9. Mention the magnetic elements of earth and define each.
10. Derive expression for the magnetic dipole moment of a revolving electron.
11. Describe the principle, construction and working of a cyclotron. Why electrons cannot be accelerated using a cyclotron?

*****************

1. Gauss theorem in magnetism states that closed surface integral of magnetic field in a closed surface is zero. That is, the net magnetic flux through a closed surface is always zero.  Mathematically,
2. Retentivity is the magnetization retained by a specimen when the magnetizing field is zero. Coercivity is the reverse magnetizing force required to completely demagnetize and specimen.
3. The compass needle will not show any particular direction as the horizontal component is zero at the magnetic poles. It can rest in any direction in which it is placed just like a non-magnetic needle.

## Unit 01 – ElectroStatics Class XII CBSE Question Bank

ElectroStatics

(At present all the questions are posted together; soon we will be categorizing according to marks)

You can either browse these questions online or download them by clicking on the link “save as pdf”. You can also send the link to your own email or your friends email address by clicking on the link “send to friend”.

Â

1. What is Frictional electricity? (1)
2. When can we say that a body is charged?(1)
3. What is electric charge? Is it a scalar or a vector quantity? (1)
4. What is electrostatics? (1)
5. Describe an experiment to show that there are two kinds of charges. (2)
6. Describe an experiment to show that like charges repel and unlike charges attract. (2)
7. Describe the electronic theory of frictional electricity. (2) Continue reading “Unit 01 – ElectroStatics Class XII CBSE Question Bank”

## Question Bank on Electromagnetic Waves for Class 12 CBSE

Summary

Properties of Electromagnetic Waves

Electromagnetic waves are composed of oscillating electric and magnetic fields at right angles to each other and both are perpendicular to the direction of propagation of the wave. Electromagnetic waves differ in wavelength (or frequency).

InÂ  an electronegative wave the electric field E(vector) and the Magnetic field B(vector) oscillate perpendicular to each other and both are perpendicular to direction of propagation of wave.

The source that produce them and methods of their detection are different, but they have the following common properties :
1. Electromagnetic waves are propagated by oscillating electric and magnetic fields oscillating at right angles to each other.
2. Electromagnetic waves travel with a constant velocity ofÂ  3 x 108 ms-1 in vacuum.
3. Electromagnetic waves are not deflected by electric or magnetic field.
4. Electromagnetic waves can show interference or diffraction.
5. Electromagnetic waves are transverse waves.
6. Electromagnetic waves may be polarized.
7. Electromagnetic waves need no medium of propagation. The energy from the sun is received by the earth through electromagnetic waves.
8. The wavelength (Î») and the frequency (v) of electromagnetic wave is related as
c = v Î» = Ï‰/k
The S.I. unit of frequency is Hertz.
1 Hertz = 1 c / s
The S.I. unit of wavelength is metre.
We however, often express wavelength in Angstrom unit [ Ã… ]
1 Ã… = 10-10 m
Also, 1 nanometer = l nm = 10-9 m

(In the chapter electromagnetic waves, only 1 mark, 2 marks or 3 marks questions will be asked in Board Exams. There is no chance for a 5 marks question. So, we’ve included all in a single section.)

1. Discuss the inconsistency in Ampere’s Circuital Law.
2. What modification was made to Ampere’s Circuital Law by Maxwell?
3. What is displacement current?
4. Conduction current and displacement current can be separately discontinuous. But their sum is continuous. Explain.
5. Can displacement current (or a changing electric flux induce a magnetic field? Explain
6. State the important consequences of displacement current.
7. State the important properties of displacement current.
8. Write Maxwell’s equations of electromagnetism and state the law underlying each equation.
9. Explain what led Maxwell to predict the existence of electromagnetic waves.
10. How are electromagnetic waves represented mathematically?
11. What is the dimension of (a)Î¼0 Îµ0(b)âˆš(Î¼0 Îµ0)
12. Discuss how an accelerating charge becomes a source of an electromagnetic wave?
13. Describe Hertz’s experiment for producing and detecting electromagnetic waves.
14. Explain how Hertz demonstrated the various properties of electromagnetic waves?
15. List the different landmarks in the history of electromagnetic waves.
16. Prove the transverse nature of electromagnetic waves mathematically.
17. Write the properties of electromagnetic waves.
18. What is electromagnetic spectrum.
19. Name the main parts of the electromagnetic spectrum and mention their frequency range and source of production. Also write their important properties and uses.

HOTS QUESTIONS FROM ELECTROMAGNETIC WAVES

1. If you find closed loops of magnetic field in a region of space, does it necessarily mean that actual charges are flowing across the area bounded by the loops? Explain.
2. Why is it that induced electric fields due to changing magnetic flux are more readily observable than the induced magnetic fields due to changing electric fields?
3. A variable frequency AC source is connected to a capacitor. Will the displacement current increase or decrease with increase in frequency?
4. Why is the quantity Îµ0 dÏ†E/dt called displacement current?
5. What oscillates in am Electromagnetic Wave?
6. Why sound waves are not considered em waves?
7. What is the phase relationship between the oscillating electric and magnetic fields in an electromagnetic wave?
8. X- rays and Î³ rays have a region of overlapping frequencies and have similar characteristics. But why they are treated as different? (OR) How are Î³ rays different from X rays?
9. Why is the ozone layer of atmosphere crucial to the existence of life on earth?
10. Why does “RADAR” use microwaves?
11. In a microwave oven, the food kept in a closed plastic container get s cooked without melting or burning the plastic container. Explain how?
12. Can an electromagnetic wave be deflected by an electric or magnetic field? Justify your answer.
13. Name the constituent radiation of electromagnetic spectrum which
(a) is used in satellite communication
(b)Is used for studying crystal structure
(c)is emitted during decay of radio active nuclei
(d) is absorbed by ozone layer Write two more uses of each
14. What is the dimension of E/B?
15. Show that the dimensions of are that of electric current
16. he sunlight reaching the earth has a maximum electric field of 810 V/m. What is the maximum magnetic field associated with it?
17. Calculate the relative permitivity of a medium of relative permeability 1.0 if the velocity of light through the medium is 2 x 108 m/s.
18. Which are the different layers of earth’s atmosphere?

(There are no limits to the Hots Questions to be framed from ANY chapter. What is important is to understand the concepts thoroughly and apply your brain)

## Numerical Problems from ELECTROMAGNETIC WAVES for CBSE Class XII

1. A parallel plate capacitor has circular plates, each of radius 5.0 cm. It is being charged so that electric field in the gap between its plates rises steadily at the rate of 1012V m-1s-1. What is the displacement current?
2. The voltage between the plates of a parallel plate capacitor of capacitance 1.0 Î¼F is changing at the rate of 5 V s-1. What is the displacement current in the capacitor?
3. A parallel plate capacitor has two metal plates of size 30 cm X 15 cm and separated by 2.0 mm. The capacitor is being charged so that the charging current has a steady value of 100 mA. Calculate the rate of change of potential difference between the capacitor plates. What is the displacement current between the capacitor plates?
4. The frequencies of radio waves in AM broadcast band range from 0.55 x 106 to 1.6 x 106 Hz. What are the longest and shortest wavelengths in this band?
5. A plane electromagnetic wave of frequency 25 MHz travels in free space along X – direction. At a particular point in space and time, E = 6.3 j V/m. What is B at this point?

There are many other similar Numerical Problems which may be asked in CBSE Exam