ISC Class 12th Physics - Paper 1 Exam 2024 : Most Important Short Answer Type Questions for Last-Minute Revision

ISC Class 12 exams have started and you have very little time left for the Physics - Paper 1 exam. Therefore, we are providing Important Short Answer Type Questions in this article. You can study them well and score well in your exams.

To help you prepare effectively, we have prepared a list of Important Short Answer Type Questions with detailed explanations. This guide aims to make your exam preparation easier by providing information on these questions.

Important Short Answer Type Questions are a common format in competitive exams including ISC Class 12 Physics - Paper 1  exam. By practicing "Short Answer Type Questions", you not only reinforce your knowledge but also learn to apply it effectively.

These questions cover various topics from the Physics - Paper 1 syllabus. Remember, it is important to understand the logic behind each answer to score well.

ISC Class 12 Physics Short Answer Type Questions

Q1. Give three points of distinction between charge and mass.

Ans.

(a) The force between two masses is always attractive whereas force between two charges may be attractive or repulsive depending on their nature.

(b) Mass is measured in kilograms and charge in coulombs.

(c) Charge on a body is always constant and does not change with the velocity of the body but the mass of a body changes with velocity.

Q2. How does an electrically polarised object differ from an electrically charged object ?

Ans. Electrically polarised objects have their charges separated, one side will be positively charged and the other side will be negatively charged. This usually happens when an insulated object is brought close to (but doesn’t touch) a charged object. Electrically charged objects will lose or gain charge, so the whole thing will either be positive or negative. If you run comb through dry hair few times, it will become electrically charged. You can then use the comb to pick up some tiny pieces of paper because the bits of paper become electrically polarised.

Q3. What is an electrostatic shielding ? Give atleast one practical example.

Ans. The vanishing effect of the electric field inside a cavity of a conductor is called electrostatic shielding. During a thunderstorm accompanied by lightning, it is safe to be inside a car, rather than near a tree or an open ground, because the interior of a car acts like a cavity where the electric field would be zero, regardless of the charge imparted by lightning to the body of the car.

Q4. (i) Mention two important uses of capacitor.

(ii) On what factors does the capacitance of a parallel plate capacitor depends ?

(iii) Can we give any amount of charge to a capacitor ? Why ?

Ans. (i) (a) Capacitors are used in tuning circuits of radio and television. (b) Capacitors are used in rectifiers.

(ii) It depends on the area of the plates, separation between the plates and the dielectric constant of the dielectric between the plates.

(iii) No, because when we give charge to a capacitor continuously, the potential difference between its plates will go on increasing. Finally, a stage will come when the electric field between the plates will exceed the dielectric strength of medium, thus puncturing the dielectric i.e., breaking the dielectric into positive and negative charges, so capacitor will be discharged giving sparking.

Q5. What is the physical importance of dielectric constant ?

Ans. The physical importance of dielectric between the plates of a capacitor is as follows :

(a) It solves the mechanical problem of maintaining two larger metal sheets at an extremely small separation, without any actual contact.

(b) Any dielectric material, when subjected to a sufficiently large electric field, experiences dielectric breakdown, a partial ionization, which permits conduction through a material that is supposed to insulate. Many insulating materials can tolerate stronger electric fields without breakdown as compared to when only air is used.

(c) The capacitance of a capacitor of given dimensions is larger when there is dielectric medium between the plates than when the plates are separated only by air or vacuum.

Q6. What is electric current ? Explain the mechanism of flow of electric current through a conductor.

Ans. Electric current : The directed flow of electric charges through a conductor is called electric current.

The S. I. unit of electric current is A (ampere).

Mechanism of flow of electric current : When an electric charge is set in motion through a conductor, it constitutes an electric current. A conductor contains large number of free electrons which are in the state of random motion. So, the net charge crossing any cross-section of conductor is zero. However, when a source of e.m.f. is connected to the conductor, an electric field is developed at every point of conductor. Therefore, the electrons being negatively charged particles, travel in a direction opposite to the electric field and so the electric current flows through the conductor. The direction of flow of electric current is conventionally opposite to the direction of flow of electrons.

Q7. Why is a potentiometer preferred over a voltmeter in measuring the e.m.f. of a cell ?

Ans. When e.m.f. of a cell is measured by potentiometer then it is done in the position of null point, when no current flows in the cell circuit. Thus, the actual value of e.m.f. of cell is obtained. In case of a voltmeter, it always draws some current and there may be some errors in the reading.

Q8. Why does a moving charge produce a magnetic field ?

Ans. When there is no current in the wire, the electrons move in the wire due to their thermal motion. Each electron acts as a source of magnetic field. As the thermal motions are randomly distributed due to collisions, these fields cancel out. Hence, there is no net field as there is no net flow of charge in any direction. But when electric current flows in the wire then the electrons move in a particular direction and all the electrons contribute to a net magnetic field.

Q9. An electron is passing through a field but no force is acting on it. Under what conditions it is possible if the motion of the electron be in the : (i) magnetic field, (ii) Electric field.

Ans. (i) If the direction of motion of the electron is parallel to the magnetic field then no force will act upon it.

(ii) In an electric field, there is always a force on the electron. The force will be zero only when the electric field is zero.

Q10. Both the electric and the magnetic fields can deflect an electron. What is the difference between these deflections ?

Ans. The force exerted by a magnetic field on the moving electron is perpendicular to the motion of the electron, hence the work done by this force on the electron is zero, and so the kinetic energy of the electron does not change. In an electric field, the deflection is in the direction of the field hence the speed and kinetic energy of the electron changes.

Q11. How can paramagnetic and diamagnetic material rods be distinguished in a magnetic field ?

Ans. On being suspended in a magnetic field, the paramagnetic rod stays parallel to the field while the diamagnetic rod stays perpendicular to the field.

Q12. Permanent magnets are made of steel, while the core of transformer is made of soft iron, why ?

Ans. Domains in steel are very rigidly arranged and so their alignment is difficult. But once aligned, they can not be disturbed easily. Hence, permanent magnets are made of steel. On the other hand, soft iron can be magnetized and demagnetised easily. The transformer core requires temporary magnetism, it should be demagnetized as soon as the current stops. Hence, it is made of soft iron.

Q13. State Lenz’s law. Explain how in the phenomenon of electromagnetic induction, the law of conservation of energy is not violated.

Ans. Lenz’s law states that the induced current produced in a circuit always flows in such a direction that it opposes the change or the cause that produces it. This law is in accordance with the principle of conservation of energy. The electrical energy is produced at a cost of mechanical energy. To produce induced current, equivalent work must be done by the agent in pushing the magnet towards the solenoid. Thus, magnet must be moving against some forces as warranted by Lenz’s law and the work done in moving the magnet against this force results in the form of induced current in the circuit. The direction of induced current can be found using Fleming’s right hand rule and dynamo rule.

Q14. Two co-axial circular coils carrying equal currents, flowing in the same direction, approach each other. What do you expect to happen to the currents in them ?

Ans. As coils approach each other, the flux linked with each coil increases. A current will be induced in each coil which will try to decrease the flux. This implies that, induced current in each coil will be opposite to initial current. So, current in each coil decreases as the coils approach each other.

Q15. When a fan is switched off, a spark can be seen in the switch while this is not so when the fan was switched on. Explain.

Ans. During making or breaking of the circuit, the current changes through the coil of the fan, thus, causing an induced e.m.f. to be set up in the coil due to property of self induction. At make of the circuit, there is no gap anywhere in the circuit so there is no spark. At the breaking of the circuit, there is a small gap in the circuit. Moreover, the rate of change of magnetic flux and hence the induced e.m.f. is quite larger at break than that at make of the circuit. This high self induced e.m.f. across the gap results in a spark.

Q16. When a car is started, the intensity of light of its head light undergoes a momentary decrease. Why is it so ?

Ans. When starter is pushed, car gets started. In the beginning, the speed of armature is small. Therefore, there is a small back e.m.f. Thus, there is greater fall of potential across it. So the intensity of light decreases. When the armature has acquired sufficient speed, there is appreciable back e.m.f. which reduces the fall of potential across it, thereby restoring the brightness of head lights.

Q17. How do the energy losses due to eddy currents are reduced ?

Ans. To minimize the energy loss due to eddy currents, the cores are made of thin sheets of iron, which are insulated from each other by thin paper, varnish, etc.

Q18. How are eddy currents produced ? Mention two application of eddy currents.

Ans. When a metallic plate is placed in a varying magnetic field, the magnetic flux associated with the plate changes, and hence current is induced.

There may be number of current loops on the plate and hence, currents are induced on the surface along a variety of paths. Such currents are called eddy currents.

Applications of eddy currents :

(a) Induction motors : A rotating magnetic field is produced by means of two single phase currents. A metallic motor placed inside the rotating magnetic field starts rotating due to large eddy current produced in it. These motors are commonly used in fans.

(b) Induction brakes : By setting up a strong magnetic field across a metal drum coupled with the wheels of the train, eddy currents are produced, which in turn brings the train to a halt.

Q19. What do you mean by self-inductance of a coil ?

Ans. Self inductance of a coil is defined as the magnetic flux linked with the circuit when unit current flows through it.

Q20. Name the various methods of producing induced e.m.f.

Ans. Induced e.m.f. may be generated :

(a) by changing the magnetic field.

(b) by changing the relative orientation of coil w.r.t. magnetic fields.

(c) by changing the relative orientation between a coil and a magnetic field.

Q21. On what factors does the instantaneous e.m.f. induced in a coil rotating in a magnetic field depend ?

Ans. The induced instantaneous e.m.f. depends on the following factors :

(a) Number of turns in the coil.

(b) Face area of the coil.

(c) Strength of the magnetic field.

(d) Speed of rotation of the coil.

(e) Rate of change of magnetic flux.

Q22. What is the phase relationship between current and voltage in case of a : (i) pure resistor, (ii) pure inductor, and (iii) pure capacitor ?

Ans.

(i) Voltage and current are always in same phase in a purely resistive a.c. circuit. Angle between them is zero.

(ii) Voltage leads current by p/2 in a purely inductive a.c. circuit.

(iii) Voltage lags behind current by p/2 in a purely capacitive a.c. circuit.

Q23. Why did Maxwell introduce displacement current in Ampere’s circuital law ?

Ans. Ampere’s circuital law was found inconsistent when applied to the circuit for charging a capacitor. Therefore, Maxwell added displacement current to usual conduction current.

Q24. What does an electromagnetic wave consist of? On what factors does its velocity in vacuum depend ?

Ans. The electromagnetic waves consists of coupled electric and magnetic field vectors, which vary sinusoidally with time, and are mutually perpendicular to each other and perpendicular to the direction of propagation. They do not contain any particle. The velocity, c depends on m0 and e0.

Q25. The electromagnetic waves are the radiations of large range of wavelengths. What are their velocities : (i) in vacuum and (ii) in a medium ?

Ans. The wavelength of electromagnetic waves ranges from 6 × 10–14 m to 6 × 106 m. These waves travel with the same velocity (= 3 × 108 ms–1) in vacuum but with different velocities in a medium. In fact, the velocity of an electromagnetic wave is less in a medium than in vacuum and a medium provides different values of refractive index to the electromagnetic waves of different wavelengths.

Q26. Name any two electromagnetic waves. State any one similarity and one difference between them.

Ans. The two electromagnetic waves are X-rays and microwaves.

Similarity : Both these waves do not require any medium for propagation.

Difference : X-rays have penetrating power so they are used for medical diagnosis, whereas microwaves do not have penetrating power. Microwaves are used in long distance communication purpose.

Q27. Identify the following electromagnetic radiations as per the wavelengths given below :

(i) 10–3 nm

(ii) 10–3 m

(iii) 1 nm

Write one application of each.

Ans.

(i) 10–3 nm : Gamma radiation.

Application : Radiotherapy or to initiate nuclear reactions.

(ii) 10–3 m : Microwaves.

Application : In RADAR for aircraft navigation.

(iii) 1 nm : X-ray.

Application : In medical science for detection of fractures, stones in kidney, gall bladder etc.

Q28. Identify the following electromagnetic radiations as per the frequencies given below :

(i) 10²⁰ Hz

(ii) 10⁹ Hz

(iii) 10¹¹ Hz

Write one application of each.

Ans.

(i) 10²⁰ Hz : Gamma radiation.

Application : For treatment of cancer.

(ii) 10⁹ Hz : Radiowaves.

Application : For broadcasting radio-programmes to long distances.

(iii) 10¹¹ Hz : Microwaves.

Application : For cooking in microwave oven.

Q29. State any two properties of microwaves. 

Ans. (a) Microwaves are reflected by metal surfaces. (b) Microwaves have low frequency and high wavelength.

Q30. (i) State any one use of infrared radiations.

(ii) State any one source of ultraviolet radiations. 

Ans. (i) IR rays are used in remote control systems (TV remote is an example). Other uses are as night vision camera, infrared thermometer, drying in sunlight etc.

(ii) Sun emits UV radiations (along with other radiations) Other sources are mercury lamp, halogen lamp etc.

Q31. Bees can see objects in the ultraviolet light while human beings cannot do so. Why?

Ans. Ultraviolet light has wavelength shorter than that of visible light. Bees have some retinal cones that are sensitive to ultraviolet light, so they can see objects in ultraviolet light. Human eyes do not possess retinal cones sensitive to ultraviolet light, so human beings cannot see objects in ultraviolet light. In other words, human beings are ultraviolet blind.

Q32. Write three properties of a plane mirror.

Ans. Three properties of plane mirror are :

(a) The image formed is laterally inverted.

(b) The image formed is as far behind the mirror as the object is in front of it.

(c) The size of the image equals the size of the object.

(d) Image formed is always virtual and erect.

Q33. What are paraxial rays ?

Ans. Paraxial rays are those, that subtend a small angle with normal to the surface at the point where they strike. The rays are paraxial :

(a) If they are closer to the principal axis, or

(b) If the mirror has a small aperture.

Q34. State the three conditions for the formation of a pure spectrum.

Ans. The three conditions for the formation of pure spectrum are :

(a) The slit should be narrow.

(b) The rays falling on the prism should be parallel to each other.

(c) The prism should be placed in minimum deviation position with respect to the mean ray and the refracting edge of the prism should be parallel to the slit.

Q35. A ray of white light shows no dispersion on emerging from a glass plate (slab). Is there any dispersion inside the glass plate ?

Ans. The light disperses into different colours on entering the glass slab and therefore dispersion takes place inside the glass slab. But when it emerges from the other parallel face, the different colours combine to form white light. In glass slab, dispersion occuring at one face is neutralised by refraction at other face.

Q36. Name any two phenomena which take place in the formation of a rainbow.

Ans. The formation of rainbow involves all the three phenomenon, Refraction, Dispersion and Total internal reflection.

Q37. How can you produce dispersion without deviation ?

Ans. Dispersion without deviation : A single prism is known to produce both dispersion and deviation. However, we can produce dispersion without deviation by using two prisms one of crown glass and the other of flint glass so that the deviations produced by them for a particular wavelength of light are equal and opposite.

Q38. Why do the stars twinkle and the moon does not ?

Ans. A star is a hot self luminous body. The light emitted from it has to pass through atmosphere containing various layers of gases of varying temperatures. The continuously changing atmosphere refracts the light from the stars by different amount on every moment. When the atmosphere refracts more star light towards us, the star appears to be bright and when the atmosphere refracts less star light, then the star appears to be dim. In this way, the star-light reaching our eyes increases and decreases continuously due to atmospheric refraction. So, the stars appear twinkling.

The moon is cold and so, not self-luminous. It only reflects the sun’s light incident on it. Therefore, the twinkling is not observed in the case of moon.

Q39. Why does Sun look a little oval when it is at horizon ?

Ans. The effect of refraction due to air is to distort the shape of the Sun and proportions of object seen near the horizon. When Sun approaches horizon it appears oval in shape. Since the amount of refraction increases as the altitude diminishes, the lower portions of the Sun’s disc are raised more than the upper, the vertical diameter is thereby shortened while the two extremities of its horizontal diameter are equally raised. Therefore, sun appears relatively enlarged near horizon.

Q40. Distinguish between magnification and angular magnification produced by a lens.

Ans. The magnification produced by a lens is the ratio of the size of the image to the size of the object. It is also equal to the ratio of the distance of the image from the lens to the distance of the object from the lens. Thus, magnification, m = v/u. On the other hand, angular magnification produced by a lens is defined as the ratio of the angle subtended by image at the eye to the angle subtended by object seen directly, when both lie at the least distance of distinct vision. It is also called magnifying power of the lens. The angular magnification produced = D/u; where, D is least distance of distinct vision. The magnification becomes equal to the angular magnification, when v = D.

Q41. Although the surface of a goggle lens is curved, it does not have any power. Why ?

Ans. The outer and inner surfaces of sun glasses are parallel which possesses same radius of curvature. Since one surface is convex and other concave, they possesses equal powers but of opposite in nature, hence resultant power is zero.

Q42. State one advantage of a reflecting telescope over refracting telescope.

Ans. One advantage of a reflecting telescope over refracting telescope is that the image formed by reflecting telescope is larger and brighter than the image formed by a refracting telescope.

Q43. When viewing through a compound microscope, our eye should be positioned not on the eyepiece but a short distance away from it. Why ? How much should be the distance between the eye and eyepiece ?

Ans. The image of the objective lens in eyepiece is called the eye-ring. All the rays from the object refracted by the objective of microscope go through the eye-ring. Therefore, it is an ideal position for our eyes for viewing. If we place our eyes too close to the eye piece, then the field is reduced and the eyes do not collect much of the light. If we position our eyes on the eye-ring and the area of the pupil of our eye is greater or equal to the area of the eye-ring, our eyes will collect all the light refracted by the objective. The precise location of the eye-ring naturally depends on the separation between the objective and eyepiece.

Q44. Why is a reflecting type telescope preferred in astronomy ?

Ans. Reflecting type telescopes are preferred because images formed by reflection are usually brighter than images formed by lenses under similar conditions. There is no chromatic aberration in mirrors. A large aperture is required to increase the light gathering power, to be able to view very faint stars. Spherical aberration, in case of large concave mirrors can be eliminated by using parabolic mirrors.

Q45. Very distant stars which are not visible to the eye, are visible in telescope. Why ?

Ans. The aperture of the objective of the telescope is much larger than the aperture of eye, therefore, it collects sufficient light from the star and forms a bright image, which is visible to the eye. Thus, feeble stars are visible in the telescope because of the ‘large’ aperture of the objective.

Q46. In a telescope, the objective is larger and the eyepiece is smaller. In a microscope the objective is smaller and the eyepiece is larger. If a telescope is inverted, will it work as microscope ? Is the reverse of this possible ?

Ans. No, because the difference in the focal length of the two lens in a microscope is very small. Hence, on inverting the telescope, its magnifying power in the form of microscope will be very small. Similarly, on inverting a microscope, its magnifying power in the form of telescope will be very small.

Q47. You are given three lenses having powers P and apertures A as follows :

P₁ = 6 D, A₁ = 3 cm.

P₂ = 3 D, A₂ = 15 cm.

P₃ = 12 D, A₃ = 1·5 cm.

Which two of these will you select to construct; (i) a telescope and (ii) a microscope ? State the basis for your answer in each case.

Ans. Given : Three lenses having powers P and apertures A as follows :

Lens 1 P₁ = 6 D A₁ = 3 cm.

Lens 2 P₂ = 3 D A₂ = 15 cm.

Lens 3 P₃ = 12 D A₃ = 1·5 cm.

(i) For constructing a telescope the objective lens should have large aperture. Hence, lens 2 will be preferred as an objective. Moreover, the eyepiece should have small focal length. So lens 3 will be preferred as an eye piece. [... P ∝ 1/f]

(ii) For constructing a microscope, both the objective and the eyepiece should have short focal lengths and the focal length of the objective should be shorter than the focal length of eye piece. Therefore, lens 3 will be used as the objective and lens 1 will be used as an eyepiece.

Q48. State the main characteristics of wave front.

Ans. Characteristics of wave front are :

(a) Wave fronts travel with the speed of light in all directions in an isotropic medium.

(b) The phase difference between any two points in the same phase on the two consecutive wave fronts is 2π.

(c) It always travels in the forward direction.

Q49. State Huygen’s principle .

Ans. Huygen’s principle : Huygen’s principle is a geometrical construction which is used to determine the position of a wave front at a later time from its given position at any instant.

It is based on the following assumptions :

(a) Each point on the given or primary wave front acts as a source of secondary wavelets, sending out disturbance in all directions in a similar manner as the original source of light does.

(b) The new position of the wave front at any instant (called secondary wave front) is the envelope of the secondary wavelets at that instant.

Q50. Why the frequency of wave does not change on refraction ?

Ans. If the source is in one medium and the observer in another (both media be at rest), the time taken to travel from one to the other is fixed for all points of a wave front. Thus, two wave fronts separated by one time period are separated by a same time period both at the source and at the observer. Therefore, the frequency of a wave remains same as the wave travels from one medium to another.

Since v = n l, a change in speed implies a change in wavelength, as the wave travels from one medium to another.

Q51. Why should we have a narrow source to produce good interference fringes ?

Ans. It is because a broad source is equivalent to a multiple number of narrow sources lying close to each other. Different pairs of narrow sources will produce their own interference patterns which will overlap each other. So the fringe system is lost.

Q52. What are coherent sources of light ? Why is it not possible to observe interference with non-coherent sources ?

Ans. Coherent sources of light are the sources which emit light waves of same frequency, same wavelength and have a constant initial phase difference.

Two independent light sources such as two bulbs or candles, cannot produce interference fringes, this is because the phase difference between the light waves emitted by such sources does not remain steady but varies continuously.

This variation takes place so rapidly that due to persistence of vision, our eyes are unable to note it. The intensity of light to an eye at every point appears uniform, that’s why interference pattern is not seen.

Q53. What happens to the interference pattern if the phase difference between the two sources varies continuously ?

Ans. The positions of bright and dark fringes will change rapidly. Such rapid changes cannot be detected by our eyes. A uniform illumination is seen on the screen i.e., interference pattern disappears.

Q54. Why does a soap bubble show beautiful colours when illuminated by white light ?

Ans. Light waves reflected from the upper and lower surfaces of a thin film interfere. Since the conditions for bright and dark fringes are satisfied at different positions for different wavelengths, so coloured fringes are observed.

Q55. Why do we fail to observe the diffraction from a wide slit illuminated by monochromatic light ?

Ans. When the size of the slit is large, the central maximum is small in size and the variation in the intensity of the other maxima and minima is so small that they cannot be distinguished. That is why we fail to observe the diffraction pattern.

Q56. Radio waves diffract pronouncedly around the buildings while light waves which are also electromagnetic waves, do not. Why ?

Ans. For diffraction to take place, the wavelength should be of the order of the size of the obstacle. The radio waves (particularly short radiowaves) have wavelengths of the order of the size of the building and other obstacles coming in their way and hence, they easily gets diffracted. Since, wavelength of the light waves is very small, they are not diffracted by buildings.

Q57. What is plane diffraction grating ?

Ans. Plane diffraction grating is an arrangement consisting of a large number of parallel slits of equal width separated from one another by equal opaque spaces.

Grating contains 12,000 'slits' distributed over 1-inch. Width is 21,000 Å. Gratings are used by engraving equally spaced parallel lines on a glass, using a diamond cutting point. Once such a grating has been prepared then a thin layer of colloidal solution is poured on the surface. When this solution becomes hard then it is stripped off and fastened to a flat piece of glass. This serves the purpose of a plane transmission grating.

Q58. Why longitudinal waves cannot be polarized ?

Ans. In polarization, vibrations are perpendicular to the direction of propagation which are restricted to just one direction. This is possible in transverse waves which have such vibrations. In longitudinal waves, vibrations occur along the direction of propagation. So their polarization is not possible.

Q59. What will be the effect on the width of the central bright fringe in the diffraction pattern of a single slit if :

(a) monochromatic light of smaller wavelength is used ?

(b) slit is made narrower ?

Ans. (a) Width of the central bright fringe in the diffraction pattern decreases when light of smaller wavelength is used.

(b) Width of central bright fringe increases when slit is made narrower.

Q60. If light propagation takes place through photons then why does the light arriving at the eye not appear discontinuous ?

Ans. The effect of a photon on the retina persists for nearly (1/10) second. If during this time another photon arrives, then the combined effect will be continuous. When visible light enters our eye, nearly 10-18 photons strike the retina per second. It means that every point on the retina is receiving photons continuously. Hence, their effect on the mind is continuous.

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ISC Class 12 Study Material