51 Objective Questions on Physics Part-2 (Civil Service Examination)

C. Optics and Sounds

43. When the same note is played on a sitar and a flute, the sound produced can be distinguished from each other because of the difference in: [1995]

(a) pitch, loudness and quality

(b) pitch and loudness

ADVERTISEMENTS:

(d) loudness only

44. Given below are two statements, one labelled as Assertion (A) and other labelled as Reason (R). [1995]

Assertion (A): A diamond sparkles more than a glass imitation cut to the same shape.

ADVERTISEMENTS:

Reason (R): The refractive index of diamond is less than that of glass.

In context of the above two statements, which one of the following is correct?

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and R are true and R is not a correct explanation of A

ADVERTISEMENTS:

(d) A is false but R is true

45. Optical fibre works on the principle of: [1995]

(a) total internal reflection

ADVERTISEMENTS:

(b) refraction

(d) interference

46. Suppose a rocketship is receding from the earth at a speed of 2/10th the velocity of light. A light in the rocketship appears blue to the passengers on the ship. What colour would it appear to an observer on the earth? [1995]

ADVERTISEMENTS:

(a) Blue (b) Orange

47. An air bubble in water will act like a: [1995]

(a) convex mirror (b) convex lens

ADVERTISEMENTS:

48. When a mirror is rotated by an angle 0, the reflected ray will rotate by: [1996]

(a) 0° (b) 9/2

49. Total internal reflection can take place when light travels from: [1996]

(a) diamond to glass (b) water to glass

50. Match List I (Quantity) with List II (Units) and select the correct answer using the codes given below the lists: [1999]

List-I – List-II

A. High speed – 1. Mach

B. Wavelength – 2. Angstrom

C. Pressure – 3. Pascal

D. Energy- 4. Joule

Codes:

(a) A-2,B-1,C-3,D-4

(b) A-1,B-2,C-4,D-3

(d) A-2, B-1,C-4,D-3

51. Consider the following statements: [1999]

1. If a person looks at a coin which is in a bucket of water, the coin will appear to be closer than it really is.

2. If a person under water looks at a coin above the water surface, the coin will appear to be at a higher level than it really is.

Which of the above statements is/are correct?

(a) 1 and 2 (b) 1 alone

52. A noise level of 100 decibel would correspond to: [2000]

(a) Just audible sound

(b) Ordinary conversation

(d) Noise from a machine shop

53. Assertion (A): In a motion picture, usually 24 frames are projected every second over the whole length of the film.

Reason (R): An image formed on the retina of eye persists for about 0.1 s after the removal of stimulus. [2000]

Codes:

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and R are true but R is not a correct explanation of A

(d) A is false but R is true

54. Assertion (A): Small glass beads fixed on traffic signals glow brightly when light falls upon them. [2000]

Reason (R): Light is totally reflected when the angle of incidence exceeds a certain critical value and light travelling in a denser medium is reflected from a rarer medium.

Codes:

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and R are true but R is not a correct explanation of A

(d) A is false but R is true

55. When a CD (compact disc used in audio and video systems) is seen in sunlight, rainbow like colours are seen. This can be explained on the basis of the phenomenon of: [2000]

(a) reflection and diffraction

(b) reflection and transmission

(d) refraction, diffraction and transmission

56. Assertion (A): A stick is dipped in water in a slanting position. If observed sideways, the stick appears short and bent at the surface of water. [2001]

Reason (R): A light coming from the stick undergoes scattering from water molecules giving the stick a short and bent appearance.

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and R are true but R is not a correct explanation of A

(d) A is false but R is true

57. When light waves pass from air to glass, the variables affected are: [2001]

(a) wavelength, frequency and velocity

(b) velocity and frequency

(d) wavelength and velocity

58. Consider the following statements: [2002]

1. Light of longer wavelength is scattered much more than the light of shorter wavelength.

2. The speed of visible light in water is 0.95 times the speed in vacuum.

3. Radio waves are produced by rapidly oscillating electrical currents.

4. To detect the overspeeding vehicles, police use the Doppler effect of reflected short radio waves.

Which of these statements are correct?

(a) 1 and 2 (b) 1 and 3

59. Consider the following natural phenomena: [2002]

1. Terrestrial heating 2. Reflection of light

3. Refraction of light 4. Diffraction of light

Due to which of these phenomena is mirage formed?

(a) 1 and 2 (b) 2, 3 and 4

60. Diffusion of light in the atmosphere takes place due to: [2003]

(a) carbon dioxide (b) dust particles

61. Which one of the following statements is not correct? [2003]

(a) The velocity of sound in air increases with the increase of temperature

(b) The velocity of sound in air is independent of pressure

(d) The velocity of sound in air is not affected by the change in amplitude and frequency

62. In which one among the following is the speed of sound maximum? [2006]

(a) Air at 0°C (b) Airatl00°C

63. Consider the following statements: [2007]

1. If magenta and yellow coloured circles intersect, the intersected area will have red colour.

2. If cyan and magenta coloured circles intersect, the intersected area will have blue colour.

Which of the statements, given above Is/are correct?

(a) 1 only (b) 2 only

64. Consider the following statements: [2007]

1. A flute of smaller length produces waves of lower frequency.

2. Sound travels in rocks in the form of longitudinal elastic waves only.

Which of the statements given above is/are correct?

(a) 1 only (b) 2 only

65. Assertion (A): A jet aircraft moving at Mach Number equal to 1 travels faster at an altitude of 15 km than while moving at Mach Number equal to 1 near the sea level. [2007]

Reason (R): The velocity of sound depends on the temperature of the surrounding medium.

(a) Both A are R are true and R is the correct explanation of A

(b) Both A and R are true but R is not a correct explanation of A

(d) A is false but R is true

66. Assertion (A): Radio waves bend in a magnetic field. Reason (R): Radio waves are electromagnetic in nature. [2008]

(a) Both A are R are true and R is the correct explanation of A

(b) Both A and R are true but R is not a correct explanation of A

(d) A is false but R is true

67. Which one of the following types of waves are used in a night vision apparatus? [2009]

(a) Radio waves (b) Microwaves

D. Electricity and Magnetism

68. Given below are two statements, one labelled as Assertion (A) and the other labelled as Reason (R).

Assertion (A): Transformer is useful for stepping up or stepping down voltages.

Reason (R): Transformer is a device used in D.C. circuits.

In the context of the above two statements, which one of the following is correct? [1996]

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and R are true but R is not a correct explanation of A

(d) A is false but R is true

69. Domestic electrical wiring is basically a: [1996]

(a) series connection

(b) parallel connection

(d) series connection within each room and parallel connection elsewhere

70. Which one of the following is paramagnetic in nature? [1997]

(a) Iron (b) Hydrogen

71. A fuse is used in mains electric supply as a safety device. Which one of the following statements about the fuse is correct? [1998]

(a) It is connected in parallel with the main switch

(b) It is made mainly from silver alloys

(d) It must have a very high resistance

72. Assertion (A): The temperature of a metal wire rises when an electric current is passed through it. [1998]

Reason (R): Collision of metal atoms with each other releases heat energy.

Codes:

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and R are true but R is not a correct explanation of A

(d) A is false but R is true

73. Consider the following statements: [1999]

An ordinary light bulb has a rather short life because the:

1. filament wire is not uniform.

2. bulb cannot be evacuated completely.

3. wires supporting the filament melt at high temperatures.

Which of the above statements are correct?

(a) 1 and 3 (b) 2 and 3

74. Consider the following statements regarding a motor car battery: [1999]

1. The voltage is usually 12 V.

2. Electrolyte used is hydrochloric acid.

3. Electrodes are lead and copper.

4. Capacity is expressed in ampere-hour. Which of the above statements are correct?

(a) 1 and 2 (b) 2 and 3

75. Fluorescent tubes are fitted with a choke. The choke coil: [2000]

(a) steps up the line voltage

(b) steps-down the line voltage

(d) chokes low frequency current

76. Two wires have their lengths, diameters and resistivities all in the ratio of 1: 2. If the resistance of the thiner wire is 10 ohm, the resistance of the thicker wire is: [2001]

(a) 5 ohm (b) 10 ohm

77. Assertion (A): In our houses, the current in A.C. electricity line changes direction 60 times per second.

Reason (R): The frequency of alternating voltage supplied is 50 Hz. [2004]

(a) Both A and R are individually true and R is the correct explanation of A

(b) Both A and R are individually true but R is not the correct explanation of A

(d) A is false but R is true

78. What is the order of magnitude of electric resistance of the human body (dry)? [2005]

(a) 102 ohm (b) 104 ohm

79. Which one of the following is printed on a commonly used fluorescent tube light? [2006]

(a) 220 K (b) 3K

80. Which are the materials generally employed as solder in soldering operations in electronics? [2006]

(a) Iron and tin (b) Lead and tin

E. MODERN PHYSICS

81. The technique used to transmit audio signals in television broadcasts is: [1995]

(a) Amplitude Modulation

(b) Frequency Modulation

(d) Time Division Multiplexing

82. Consider the following statements: [1996] At the present level of technology available in India, solar energy can be conveniently used to

1. Supply hot water to residential buildings.

2. Supply water for minor irrigation projects.

3. Provide street lighting.

4. Electrify a cluster of villages and small towns.

(a) 1, 2, 3 and 4 are correct

(b) 2 and 4 are correct

(d) 1, 2 and 3 are correct

83. One astronomical unit is the average distance between: [1998]

(a) Earth and the Sun (b) Earth and the Moon (c) Jupiter and the Sun (d) Pluto and the Sun

84. For reproducing sound, a CD (Compact Disc) audio player uses a: [2000]

(a) quartz crystal (b) titanium needle

85. Which one of the following does a TV remote control unit use to operate a TV set? [2000]

(a) Light waves (b) Sound waves

86. Consider the following statements: [2001]

In a nuclear reactor, self-sustained chain reaction is possible, because

1. more neutrons are released in each of the fission reactions.

2. the neutrons immediately take part in the fission process.

3. the fast neutrons are slowed down by graphite.

4. every neutron released in the fission reaction initiates further fission.

Which of these statements are correct?

(a) 1, 2 and 3 (b) 1 and 3

87. Consider the following statements: [2005]

1. A geo-stationary satellite is at an approximate height of 10,000 km.

2. FM transmission of music is a very good quality because the atmospheric or man made noises which are generally frequency variations can do little harm.

Which of the statements give above is/are correct?

(a) 1 only (b) 2 only

88. What does the term Dolby B or Dolby C printed on tape- recorders and other sound systems refer to [2006]

(a) frequency Modulated System

(b) amplitude Modulated System

(d) both DC and AC power can be used

89. Which of the following types is used by computed tomography employed for visualisation of the internal structure of human body? [2007]

(a) X-ray

(b) South waves

(d) Radioisotopes

90. Cryogenic engines find applications in:

(a) sub-marine propulsion

(b) frost-free refrigerators

(d) researches in superconductivity

91. Consider the following statements: [1996]

A person in a spaceship located half way between the earth and the sun will notice that the

1. sky is jet black.

2. stars do not twinkle.

3. temperature outside the spaceship is much higher than that on the surface of the earth,

(a) 3 alone is correct

(b) 1 and 2 are correct

(d) 1, 2 and 3 are correct

92. The tail of a comet is directed away from the sun because: [1997]

(a) as the comet rotates around the sun, the lighter mass of the comet is pushed away due to the centrifugal force alone

(b) as the comet rotates, the lighter mass of the comet is attracted by some stars situated in the direction of its tail

(d) the tail of the comet always exists in the same orientation

93. A ‘black hole’ is a body in space which does not allow any radiation to come out. This property is due to its: [2000]

(a) very small size (b) very large size

94. Consider the following statements in respect of India advanced satellite INSAT-4A: [2006]

1. INSAT-4A was launched in December, 2005 from New Mexico.

2. The European Commercial Launch Service Provider Arianespace was associated with the launch of INSA T-4A.

3. Tata-Sky; a digital cable service provider; is associated with DTH televiison broadcasting service from INSA T 4 A.

Which of the statements given above are correct?

(a) 1, 2 and 3

(b) 1 and 2 only

(d) 1 and 3 only

C. Optics and Sounds

43. (c) Sounds may be generally characterized by pitch, loudness, and quality. Sound “quality” or “timbre” describes those characteristics of sound which allow the ear to distinguish sounds which have the same pitch and loudness. Quality is then a general term for the distinguishable characteristics of a tone.

44. (c) This is a phenomenon of total internal reflection of light, dependent on the ‘critical angle’ of the incidence of light in a material medium at its bounding surface with air. The higher the refractive index of a transparent material the smaller is the critical angle and hence the larger is the range of angles of incidence for more light to be totally reflected. A diamond has a large refractive index and very small critical angle as against glass, which has a lower refractive index and large critical angle. A skilled diamond cutter exploits the large range of angles of incidence in the diamond to cut multiple faces at suitable angles. Light entering diamond from different faces, suffers multiple total internal reflection and comes out of the diamond as intense beams from selected directions. Hence, the diamond shines brilliantly.

45. (a) An optical fibre is a thin, flexible, transparent fibre that acts as a waveguide, or “light pipe”, to transmit light between the two ends of the fibre. An optical fibre transmits light along its axis, by the process of total internal reflection. When light traveling in a dense medium hits a boundary at an angle larger than the “critical angle” for the boundary, the light will be completely reflected. This effect is used in optical fibres to confine light in the core.

46. (a) The observer on the earth will detect a wavelength given by

ν = ±( λ-λ₀/ λ ) c

where λ = original wavelength

λ ₀ = apparent wavelength

ν = velocity of source

Here, source is receding, therefore +ve sign will be taken

=> 2/10 c = ( λ-λ₀/ λ ) c

=> ( λ-λ₀/ λ ) = 2/10 => 1- λ₀/ λ = 2/10

=> λ₀/ λ = 1 – 2/10 = 8/10 => λ =10/8 λ₀ = 1.25 λ₀

The wavelength of blue light varies from 450 nm to 475 nm. Therefore, the observed light will have wavelength ranging from 562.5 nm to 593.75 nm which will be yellow orange.

47. (d) Air bubble in water would be diverging lens, because the index of refraction of air is less than that of water.

48. (d) Consider a plane-mirror and a fixed incident ray of light. Before the mirror has rotated, the angle of incidence θ as is the angle of reflection. If the mirror is rotated through an angle Ф the normal is rotated by an angle Ф and thus the angle of incidence increases to θ + Ф. Therefore, the angle of reflection must also increase by Ф to θ + Ф. The difference between the final angle of reflection and the initial angle of reflection is 2Ф. Thus for a fixed incident ray, the angle of the reflection is twice the angle through which the mirror has rotated.

49. (a) Total internal reflection is an optical phenomenon happens when a ray of light strikes a medium bound; at an angle larger than a particular critical angle respect to the normal to the surface. If the refractive index is lower on the other side of the boundary, light can pass through and all of the light is reflected. The critical angle is the angle of incidence above which the total internal reflection occurs. Total intern reflection takes place when light travels from optical! denser medium to optically rarer medium. Refractive index of diamond is very high (2.417) whereas refractive index of glass is only in between (1.5 to 1.6), whereas rest all options there is no possibility of total intern reflection because refractive index of water (1.33) an air (1.0008) is less than refractive index of glass.

50. (c) Mach number (Ma or M) is the speed of an object moving through air, or any other fluid substance. It commonly used to represent the speed of an object when it is travelling close to or above the speed of sound.

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. The SI unit for pressure is the Pascal (Pa), equal to one Newton per square meter (N/m²). In physics, the wavelength of a sinusoidal wave is the spatial period of the wave-the distance over which the wave’s shape repeats. The unit for wavelength is the angstrom.

In physics, energy is a quantity that is often understood as the ability to do work. In the International System Units (SI), energy is measured in joules, but in so fields other units such as kilowatt-hours and kilocalories are also used.

51. (a) Refraction is the change in direction of a wave due to a change in its speed. This is most commonly observed when a wave passes from one medium to another at any angle other than 90° or 0°. Refraction of light is the most commonly observed phenomenon. Thus in both cases the phenomenon of refraction is applicable because there will be change in the direction and speed as light enters from one media to other. In case 1 light for observer outside water the light ray is travelling from denser medium (water) to rarer medium (air). Thus, image of coin will appear closer due to refraction. Whereas in case 2 for observer under water refraction also takes place but in opposite way as light travels from rarer medium (air) to denser medium (water) .Thus, the image of coin will appear at a higher level than actual position of coin.

52. (d) The decibel is widely known as a measure of sound pressure level, but is also used for a wide variety of other measurements in science and engineering. The decibel is commonly used in acoustics to quantify sound levels relative to a 0 dB reference which has been defined as a sound pressure level of .0002 microbar. The noise level of 100 decibel would corresponds to noise from a machine shop.

53. (c)

54. (a) Both of the statements are correct and statement 2 is also correct explanation for statement 1. Total internal reflection is an optical phenomenon that happens when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary, no light can pass through and all of the light is reflected. Thus because of this phenomena traffic light beads glow when light falls on them.

55. (d) T lie reason CDs reflect rainbow colors is because they have a clear plastic coating on top of a mirrorized surface. Light refracts (bends) when it moves from one medium (such as air) to another with a different optical density (such as the clear plastic surface of a CD). Different wave lengths of light (every color has a different wave length) travel at different speeds, so that full spectrum appears when white light passes from the air through the plastic surface of a CD, separated light rays which are then reflected back to us by the mirrorized center surface of a CD. Here the diffraction and transmission also takes place because diffraction of light rays occur when it strikes the surface of CD and transmission is obvious when light enters from one medium to another. The thickness of the different optical media, angle of source light, and brightness of source light all affect which rainbow patterns are visible on a CD.

56. (c) Statement I is true but statement 2 is false. Yes the stick dipped in water appears to be bent and short but refraction of light by water molecules is the phenomena responsible for this observation not because of scattering of light.

57. (d) When light enters from rarer (air) to denser (glass) medium wavelength and velocity will get affected. The velocity will decrease. Moreover, as the ray bent in passing through different media which results in decrease of wavelength. As such there is no change in frequency when light enters from air to glass.

58. (d) Radio waves are a form of electromagnetic radiation, created whenever a charged object (in normal radio transmission, an electron) accelerates with a frequency that lies in the radio frequency (RF) portion of the electromagnetic spectrum. Thus statement 3 is correct. The Doppler’s Effect shows a change in a wave’s frequency received when it is either reflected from a moving object or generated on a moving object. Police radar speed detectors bounce a radio frequency wave off cars and use their internal electronics to determine the speed at which the object is moving by that frequency change. Therefore, statement 4 is also correct.

59. (c) Convection currents of hot air rising off a surface have a lower density than the air directly above it. The difference in density cause light passing through the air to refract differently, which causes the formation of mirage.

60. (b) Dust particles are responsible for the diffusion of light in atmosphere.

61. (c) The velocity of sound in air is determined by the air itself and is not dependent upon the amplitude, frequency, or wavelength of the sound. The velocity of sound c depends on the temperature of air and not on the air pressure. Thus, velocity of sound in air increases with temperature and remains unaffected with increase of pressure. Therefore, statements 1,2 and 4 are wrong. Only statement 3 is right. The only other factor that has any effect on the speed of sound in air is the amount of humidity in the air. An increase in the amount of humidity in the air increases the speed by a small amount.

62. (d) The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at 20 °C (68 °F), the speed of sound is 343.2 metres per second (1,126 ft/s). In fresh water, sound travels at about 1497 m/s at 25 °C. The value of 331.3 m/s which represents the 0 °C speed. The speed of sound at 100 °C is 386 m/s. The speed of sound in wood is 3300 – 3600 m/s.

63. (c) Combination of red and blue colour results into formation of magenta colour. Where as combination of red and green colour results in to formation of yellow colour. Thus on combination of magenta and yellow colour red colour will be highlighted. On the other side combination of blue and green results into formation of cyan colour. Combination of red and blue colour produce magenta colour. Therefore, on combination of cyan and magenta colours blue colour will be more highlighted.

64. (b) The flute is a musical instrument of the woodwind family. A flute of smaller length produces waves of higher frequency. Sound waves travels through medium even through extremely dense medium like rocks in the form of longitudinal waves only. A good example is the shock wave from an Earthquake, which can be heard hundreds of miles away from the epicenter after travelling through rocks in the ground.

65. (a) Mach number, a useful quantity in aerodynamics, is the ratio of air speed to the local speed of sound. The speed of sound varies with temperature. Since temperature and sound velocity normally decrease with increasing altitude, sound is refracted upward. Mach number is a function of temperature at altitude. With decrease in sound velocity Mach number increases.

66. (a) Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. A radio wave has a much longer wavelength than does visible light. We use radio waves extensively for communications.

67. (c) Night Vision as referenced here is that technology that provides us with the miracle of vision in total darkness and the improvement of vision in low light environments. Infra-red waves are used in night vision apparatus.

D. Electricity and Magnetism

68. (c) A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors-the transformer’s coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer’s core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a voltage in the secondary winding. By appropriate selection of the ratio of turns, a transformer thus allows an alternating current (AC) or voltage to be “stepped up” by making N_s greater than N_p or “stepped down” by making N_s less than N_p.

Here N represents number of turns in primary winding.

N_s represents the number of turns in secondary winding.

As it is clear from principle involved that transformer can be used to step up and step down A.C. voltage only. This device can not be used for D.C. voltage as its working is based on the change of magnetic flux with varying current.

69. (b) In a parallel circuit, the voltage across each of the components is the same, and the total current is the sum of the currents through each component. The wiring for most homes is parallel .In parallel circuit each branch receives equal current. If one branch in the circuit is broken, electric current will still flow in other branches.

70. (c) Oxygen is a paramagnetic substance.

71. (c) A fuse is a type of sacrificial over current protection device. Its essential component is a metal wire or strip that melts when too much current flows. Short circuit overload or device failure is often the reason for excessive current. The resistance of the element used in fuse generates heat due to the current flow. It ha; low melting point so that it melts when a short-circuit or overloading takes place.

72. (a) A metallic conductor has a large number of free electron in it. When a potential difference is applied across the ends of a metallic wire, the free electrons begin to drift from a region of low potential to a region of high potential. These electrons collide with the positive ion (the atoms which have lost their electrons). In these collisions, the energy of the electron is transferred to the positive ions and they begin to vibrate more violently. As a result, heat is produced. The greater tin number of electrons flowing per second, the great will be the rate of collisions and so greater is the produced.

73. (b) The electric light bulb uses a glowing wire heated by electrical resistance to white to generate light (a process known as thermal radiation). The ‘bulb’ is the glass enclosure which keeps the filament ii vacuum or low-pressure noble gas. Complete evacuation of bulb is not possible. A small quantity gases left behind which expands on heating by through filament. Moreover, the wires supporting filament sometimes melt due to high heat generate a result of high resistance of filament. Thus,these factors leads to short life span of ordinary bulb.

74. (d) In case of motor car battery or automotive batteries a nominal 12-volt potential difference is provided connecting six galvanic cells in series. Capacity of these batteries is expressed in ampere-hour. Electrolyte used is a solution of about 35% sulphuric acid and 65% water and electrodes used are plates of lead and separate plates of lead dioxide.

75. (c) A fluorescent lamp or fluorescent tube is a discharge lamp that uses electricity to excite mercury vapour. The excited mercury atoms produce short-was ultraviolet light that then causes a phosphor fluoresce, producing visible light .In fluorescent tubes the simplest ballast for alternating current use is I inductor (choke coil) placed in series, consisting o winding on a laminated magnetic core. The inductance of this winding limits the flow of AC current on account of its large impedance. The only waste of energy is to the hysteresis loss in the iron core, which is much less than the waste of energy in comparison resistance which can also reduce the current if placed instead of the choking coil.

90. (c) A cryogenic rocket engine is a rocket engine that uses a cryogenic fuel or oxidizer, that is, its fuel or oxidizers (or both) are gases liquefied and stored at very low temperatures. Notably, these engines were one of the main factors of the ultimate success in reaching the Moon by the Saturn V rocket. Various cryogenic fuel- oxidizer combinations have been tried, but the combination of liquid hydrogen fuel and the liquid oxygen oxidizer is one of the most widely used.

91. (d) There is a presence of atmosphere at earth’s surface which consists of thick and moving layers of air. The dust particles, particulates, smog, water vapour and smoke are also present in air. Stars twinkle when we see them from the Earth’s surface because we are viewing them through thick layers of turbulent (moving) air in the Earth’s atmosphere. As their light travels through the many layers of the Earth’s atmosphere, the light of the star is bent (refracted) many times in random directions (light is bent when it hits a change in density ‘ -like a pocket of cold air or hot air). This random refraction results in the star as its twinkling but for a person in spaceship no such refractions are possible as there is vaccum in space. Thus, star will not twinkle. Similarly, sky will appear blue to us due to Rayleigh scattering which is again due to presence of atmosphere on earth’s surface. The shorter wavelength light is absorbed by the gas molecules of atmosphere. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Some of this scattered blue light reaches you. Since, you see, the sky looks blue. Whereas no atmosphere is there in space thus, no absorbing and scattering is possible that’s why for a person in spaceship sky appears black. Temperature outside the spaceship is higher in comparison to earth’s surface because of being nearer to sun,

92. (a) A comet’s tail is a result of the solar wind blowing ice and dust from its surface. Tail of comet is directed away from the sun due to centrifugal force alone which acts on a body in circular motion. When a body moves along the circular path, due to inertia the body experiences a force along the radius but away from the centre.

93. (c) A black hole is a region of space from which nothing, not even light, can escape. It is the result of the deformation of spacetime caused by a very compact mass. The simplest black hole has mass but neither electric charge nor angular momentum. The mass of a black hole is finite, the reason why a black hole has infinite density is that it’s mass is concentrated into a space of zero-volume. Thus, it has a very high density.

94. (c) Statement ‘2’ & ‘3’ is correct, but’ 1′ is incorrect. Because INSAT-4A was launched on 22 December 2005 from French Guiana.