WBCS Prelims Physics Questions and Answers (Previous Year Paper)

Answer: (A) 50 h

Explanation: The energy of a photon is given by Planck’s relation, E = hν, where h is Planck’s constant and ν is the frequency. For ν = 50 Hz, E = 50h joules. Photon energy is directly proportional to frequency.

Electromagnetic Spectrum:

Answer: (B) 15-20%

Explanation: Commercially available solar cells, mainly crystalline silicon panels, convert about 15-20% of incident sunlight into electricity. Laboratory cells and multi-junction cells for space use reach higher, but ordinary commercial panels stay in this range. The theoretical limit for a single-junction silicon cell is about 29-30%.

Solar Cell Types:

Answer: (A) zero

Explanation: Work done by a force is W = F·d·cosθ. For a planet in circular orbit, the gravitational force acts towards the sun (along the radius), while the displacement at every point is along the tangent. The angle between them is 90°, so cos 90° = 0 and the work done is zero. Hence the planet’s kinetic energy and speed remain constant in a circular orbit.

Work, Energy and Circular Motion:

Answer: (A) Proton + Neutron

Explanation: The mass of an atom is concentrated in its nucleus, which contains protons and neutrons (together called nucleons). Each has a mass of about 1 atomic mass unit, while an electron is roughly 1/1836 of a proton’s mass and contributes negligibly. Hence the mass number A = number of protons + number of neutrons.

Subatomic Particles:

Answer: (D) Silicon

Explanation: Silicon is a metalloid whose electrical conductivity lies between that of a conductor and an insulator, which makes it a semiconductor. Its conductivity rises with temperature and with doping by impurities like boron or phosphorus. Iron, zinc and mercury are metals and good conductors.

Semiconductor Materials:

Answer: (D) Thorium

Explanation: India has one of the world’s largest reserves of thorium, found mainly in the monazite sands of the Kerala, Tamil Nadu and Odisha coasts. Thorium-232 is fertile and is converted to fissile uranium-233 in a reactor, which forms the basis of the third stage of India’s three-stage nuclear power programme. Uranium reserves in India, by contrast, are limited.

India’s Three-Stage Nuclear Programme:

Answer: (D) 100 Ωcm

Explanation: Resistivity is given by ρ = RA/L. Here R = 10 Ω, A = 1 cm², and L = 1 m = 100 cm. So ρ = (10 × 1)/100 = 0.1 Ωcm… but this needs the length in the same unit as the area. Recomputing with consistent units: ρ = RA/L = (10 Ω × 1 cm²)/(100 cm) = 0.1 Ωcm.

Wait, that gives 0.1 Ωcm, which is option (A) 1/10 Ωcm, not (D). The correct answer is therefore (A) 1/10 Ωcm.

Resistance and Resistivity: Details

Answer: (B) Above the centre of gravity

Explanation: A fully submerged or floating body is in stable equilibrium when its centre of buoyancy (B) lies above its centre of gravity (G). A tilt then produces a restoring couple that brings the body back upright. For a partly submerged floating body, B can shift sideways on tilting, so the precise condition is that the metacentre (M) lies above G.

Stability of Floating Bodies:

Answer: (D) volume.

Thermal Expansion:

Answer: (D) Volume

Explanation: When a drop of water splits into two drops, no water is added or lost, so the total volume (and mass) stays the same. Radius, surface area and surface energy do not add up to the original values: the two smaller drops together have a larger total surface area, and therefore greater total surface energy, than the single drop.

Drop Splitting and Surface Properties:

Answer: (B) Its volume increases.

Explanation: When a metallic ball with a cavity is heated, every part of the ball expands as if the cavity were filled with the same metal. The cavity therefore expands exactly like a solid piece of that metal would, and its volume increases by ΔV = V₀γΔT. This holds for any material, so it does not depend on the nature of the metal.

Expansion of Cavities and Hollow Bodies:

Answer: (A) 12.5 Ω.

Explanation: Resistance is maximum when the resistors are connected in series, since the total is then the sum of the individual resistances. Here, R = 5 + 4.5 + 3 = 12.5 Ω for all three in series.

Combination of Resistors:

Answer: (C) W1 = W2

Explanation: The cage, bird and air inside form a single closed system hanging from the spring balance. When the bird flies, it stays up by pushing air downward, and that downward force is transmitted through the air to the floor of the cage. On average, this force equals the bird’s weight, so the total downward pull on the balance is unchanged. Hence the reading stays the same (W1 = W2), assuming the cage is closed and the bird flies at steady height.

Weight, Reaction and Apparent Weight:

Answer: (C) 3 × 10⁻⁴ °C⁻¹

Explanation: Density varies inversely with volume, so ρ₂ = ρ₁/(1 + γΔT). Putting in the values, 998/992 = 1 + γ(20), which gives 1.00605 − 1 = 20γ. So γ ≈ 3.0 × 10⁻⁴ °C⁻¹.

Expansion Coefficients:

Answer: (C) 40%

Explanation: The efficiency of a Carnot engine is η = 1 − T₂/T₁, where temperatures are in kelvin. Here T₁ = 227 + 273 = 500 K and T₂ = 27 + 273 = 300 K. So η = 1 − 300/500 = 0.4, i.e. 40%.

Carnot Engine and Efficiency:

Answer: (B) Gamma rays

Explanation: The energy of a photon is E = hν, so the radiation with the highest frequency (shortest wavelength) carries the most energy. Among the options, gamma rays have the highest frequency, followed by X-rays, UV rays and IR rays. Hence gamma rays carry the maximum energy.

Electromagnetic Spectrum:

Answer: (B) Current

Explanation: A step-down transformer has fewer turns in the secondary coil than in the primary, so it lowers the voltage. For an ideal transformer, power stays nearly constant (V₁I₁ ≈ V₂I₂), so when voltage decreases, current increases. Frequency does not change in a transformer.

Transformer:

Answer: (D) v²/r

Explanation: In uniform circular motion the speed is constant, but the direction of velocity changes continuously, so the particle has an acceleration directed towards the centre. This centripetal acceleration is a = v²/r = ω²r. It is never zero as long as the particle moves in a circle.

Circular Motion:

Answer: (D) It will remain same.

Explanation: The boat floats, so it displaces water equal to the total weight of the boat plus the passengers. When a passenger drinks water from the tank, that weight is removed from the tank but stays on the boat, because it is still inside the passenger. The total weight of the boat, passengers and water in the tank is unchanged, so the boat displaces the same volume of water and the level stays the same.

Floating Bodies and Water Level:

Answer: (C) insulator

Explanation: At 0 K there is no thermal energy, so all electrons in pure silicon remain in the valence band and none can cross the band gap (about 1.1 eV) into the conduction band. With no free charge carriers, pure silicon behaves as a perfect insulator. As temperature rises, electrons gain energy to jump the gap and conductivity increases.

Behaviour of Materials with Temperature:

Answer: (C) √3 : √2.

Explanation: For molecules of an ideal gas, the most probable speed is v_mp = √(2kT/m) and the rms speed is v_rms = √(3kT/m). The ratio of rms speed to most probable speed is therefore √3 : √2.

Molecular Speeds in an Ideal Gas:

Answer: (B) Pascal’s law

Explanation: A hydraulic press works on Pascal’s law, which states that pressure applied to an enclosed incompressible fluid is transmitted equally in all directions throughout the fluid. A small force on a small piston creates a pressure that acts on a larger piston, producing a proportionally larger force: F₂ = F₁ × (A₂/A₁).

Fluid Mechanics Laws and Applications:

Answer: (D) convection currents are set up

Explanation: The freezing chest (freezer) is kept near the top so that the air cooled by it, being denser, sinks and sets up convection currents. Warmer air from the lower part of the refrigerator rises to the top, gets cooled, and sinks again, so the whole interior is cooled uniformly. If it were placed at the bottom, the cold air would stay there and the upper space would not cool well.

Heat Transfer:

Answer: (C) Greenhouse

Explanation: A solar cooker works on the greenhouse effect. Sunlight passes through the glass lid and is absorbed by the blackened inner surface, which re-emits it as longer-wavelength infrared radiation. Glass does not allow this infrared to escape, so heat is trapped and the temperature inside rises. A reflecting mirror is often added to concentrate more sunlight into the box.

Solar Cooker and Related Devices:

Answer: (B) high k and low δ

Explanation: Diamond is an excellent thermal conductor, with thermal conductivity several times that of copper, because lattice vibrations (phonons) carry heat very efficiently through its rigid covalent network. It is, however, an electrical insulator, since all four valence electrons of each carbon atom are tied up in covalent bonds and there are no free electrons. Hence diamond has high thermal conductivity (k) and low electrical conductivity (δ).

Diamond and Carbon Allotropes:

Answer: (C) 6V.

Explanation: When cells are connected in series, their emfs add if they are connected correctly. A wrongly connected cell has its polarity reversed and contributes −1 V instead of +1 V. With 10 cells, 8 are correct and 2 are reversed, so the emf is 8 × 1 − 2 × 1 = 6 V.

Cells in Series and Parallel:

Answer: (C) 100.101

Explanation: Convert the integer and fractional parts separately. For the integer part, 4 = 100 in binary (4 = 1×2²). For the fractional part, multiply by 2 repeatedly and note the integer digits: 0.625 × 2 = 1.25 (1), 0.25 × 2 = 0.5 (0), 0.5 × 2 = 1.0 (1). This gives 0.101. So 4.625 = 100.101 in binary. Check: 4 + 0.5 + 0.125 = 4.625.

Number Systems:

Answer: (C) decreases.

Explanation: A person on a swing behaves as a pendulum whose effective length is the distance from the pivot to the centre of mass of the swing plus person. When the person stands up, the centre of mass rises, so the effective length decreases. Since T = 2π√(l/g), a shorter length gives a smaller time period, so the period decreases.

Pendulum and Swing:

Answer: (B) 50 cm, convex

Explanation: The focal length of a lens is the reciprocal of its power: f = 1/P. For P = +2.0 D, f = 1/2 = 0.5 m = 50 cm. A positive power means a converging lens, which is a convex lens. So the lens is convex with a focal length of 50 cm.

Lens Power and Optics:

Answer: (A) heavy body has greater momentum.

Explanation: Kinetic energy and momentum are related by KE = p²/2m, so p = √(2m·KE). For equal kinetic energy, momentum is directly proportional to √m. The heavy body therefore has the greater momentum, and the light body has the smaller one.

Kinetic Energy and Momentum:

Answer: (C) Diffraction

Explanation: A CD has a spiral track of tiny pits and lands, spaced about 1.6 µm apart, which acts like a reflection diffraction grating. When white light falls on it, each wavelength is diffracted at a different angle, so the disc shows a spectrum of colours. The effect depends on the spacing of the tracks being comparable to the wavelength of light.

Diffraction and Related Optical Phenomena:

Answer: (D) √6 times slower.

Explanation: The time period of a pendulum is T = 2π√(l/g). On the moon, g is about one-sixth of its value on the earth, so T becomes √6 times larger. A longer time period means each oscillation takes more time, so the clock runs √6 times slower.

Pendulum Clock:

Answer: (C) 2k

Explanation: The spring constant of a spring is inversely proportional to its length, since a longer spring stretches more under the same force. Cutting a spring into two equal halves gives each half half the original length, so each half has twice the spring constant, k’ = 2k. Equivalently, the two halves act as springs in series, and two springs of constant 2k in series give back the original constant k.

Springs and Spring Constant:

Answer: (C) longitudinal and transverse

Explanation: Waves on the surface of water are neither purely transverse nor purely longitudinal. Water particles move in circular or elliptical paths, which combines a back-and-forth (longitudinal) motion with an up-and-down (transverse) motion. The waves made by a motor boat are surface waves of this kind, so they are both longitudinal and transverse.

Types of Waves:

Answer: (C) dimensional change in material

Explanation: A dilatometer is an instrument used to measure the dimensional changes (expansion or contraction) of a material, such as a solid or liquid, as its temperature changes. From these measurements the coefficient of thermal expansion can be calculated.

Measuring Instruments:

Answer: (A) Scattering

Explanation: Sir C. V. Raman received the 1930 Nobel Prize in Physics for his work on the scattering of light and for the discovery of the Raman effect (1928). When light passes through a transparent medium, a small fraction is scattered with a change in wavelength, because photons exchange energy with the vibrating molecules. This inelastic scattering gives the characteristic Raman lines used to identify molecular structure.

C. V. Raman and Light Scattering:

Answer: (B) Angular momentum

Explanation: A vector quantity has both magnitude and direction. Angular momentum (L = r × p) is the cross product of position and linear momentum, so it has a direction given by the right-hand rule, along the axis of rotation. Heat, time and work are scalar quantities, having magnitude only.

Scalar and Vector Quantities:

Answer: (A) Fermi

Explanation: The size of a nucleus is of the order of 10⁻¹⁵ m, which is far too small for metres or angstroms to be convenient. The unit used is the fermi (fm), named after Enrico Fermi, where 1 fermi = 10⁻¹⁵ m. Nuclear radius follows R = R₀A^(1/3), with R₀ ≈ 1.2 fm.

Units of Small and Large Lengths:

Answer: (C) offer no resistance to the flow of current.

Explanation: A superconductor is a material whose electrical resistance drops to exactly zero when cooled below a characteristic critical temperature (Tc). Once a current is set up in a superconducting loop, it can flow indefinitely without loss. Superconductors also expel magnetic fields from their interior, a property called the Meissner effect.

Superconductivity:

Answer: (D) solids

Explanation: Sound is a mechanical wave, and its speed depends on the elasticity and density of the medium: v = √(E/ρ). Solids have a very high elastic modulus, so sound travels fastest in them (about 5000 m/s in steel), then in liquids (about 1500 m/s in water), and slowest in gases (about 340 m/s in air). Sound cannot travel in a vacuum.

Speed of Sound in Different Media:

Answer: (D) fossils

Explanation: Radiocarbon dating uses the radioactive isotope carbon-14, which living organisms absorb from the atmosphere throughout their lives. After death the intake stops and ¹⁴C decays with a half-life of about 5730 years. Measuring the remaining ¹⁴C fraction gives the age of the dead organic material. It is therefore used for fossils and other once-living remains, up to roughly 50,000 years old, and not for rocks, which are dated by uranium-lead or potassium-argon methods.

Radioactive Dating Methods:

Answer: (B) capillary action

Explanation: Blotting paper is made of a mass of fine cellulose fibres with tiny pores between them, which behave like very narrow capillary tubes. Ink rises into these pores because of surface tension and adhesion between the liquid and the fibres, which is capillary action. This is how the paper soaks up ink so readily.

Capillarity and Surface Tension:

Answer: (A) Infrared frequency

Explanation: A TV remote control uses an infrared LED that sends coded pulses of infrared light to a sensor on the TV. Infrared is invisible to the eye, is not affected by ordinary room light, and has low energy so it is safe. It needs a clear line of sight, which is why the remote must be pointed at the TV and does not work through walls.

Uses of Electromagnetic Radiations:

Answer: (C) the bomb drops vertically downward.

Explanation: At the instant of release, the bomb has the same horizontal velocity as the airplane. After release, no horizontal force acts on it (air resistance neglected), so its horizontal velocity stays constant, while gravity accelerates it downward. The bomb therefore remains directly below the airplane throughout its fall, and the pilot sees it fall vertically. An observer on the ground sees a parabolic path, since the bomb has both horizontal and vertical motion.

Projectile Motion:

Answer: (C) good emitters

Explanation: According to Kirchhoff’s law of thermal radiation, a body that is a good absorber of heat radiation at a given wavelength is also a good emitter at that wavelength. A black, rough surface absorbs most of the radiation falling on it and also emits heat rapidly when hot. A polished, shiny surface reflects most radiation, so it is both a poor absorber and a poor emitter.

Absorption, Emission and Reflection of Heat:

Answer: (C) to maintain convectional currents to keep the air fresh in the room.

Explanation: Air in a room gets warmed by people, lamps and appliances, and it also picks up carbon dioxide and moisture from breathing. This warm, impure air is lighter, so it rises to the top of the room. Ventilators placed near the ceiling let it escape, and cooler fresh air enters through the doors and windows at a lower level. This continuous circulation is a convection current, and it keeps the air in the room fresh.

Convection in Everyday Life:

Answer: (B) quality

Explanation: The quality (timbre) of a sound is the characteristic that lets us distinguish two sounds of the same pitch and loudness. It depends on the number and relative strength of the overtones (harmonics) present along with the fundamental note. Every person’s voice has its own pattern of overtones, so we can recognise a friend’s voice even without seeing them.

Characteristics of Sound:

Answer: (C) the distance travelled by light in free space in one year.

Explanation: A light year is a unit of distance, not of time. It is the distance light covers in vacuum in one year, travelling at about 3 × 10⁸ m/s. This works out to about 9.46 × 10¹² km (roughly 9.46 trillion km), and it is used to express the enormous distances to stars and galaxies.

Astronomical Distance Units:

Answer: (D) large distance of stars and fluctuations in the density of air

Explanation: Stars are so far away that they act as point sources of light. As starlight passes through the atmosphere, the air layers have varying density and temperature, so the refractive index keeps changing and the light is refracted by different amounts from moment to moment. The apparent position and brightness of the star therefore fluctuate, which we see as twinkling. Planets do not twinkle because they are nearer and appear as extended sources, so the variations average out.

Atmospheric Refraction and Optical Effects:

Answer: (C) of decrease in air pressure in the space between them.

Explanation: According to Bernoulli’s principle, where the speed of a fluid is higher, its pressure is lower. When two cars pass each other at high speed, the air in the narrow gap between them is forced to move faster than the air on their outer sides. The pressure in the gap therefore drops below the pressure on the outer sides, and the higher outside pressure pushes the cars towards each other, creating a danger of side-swiping.

Bernoulli’s Principle:

Answer: (A) 70-75 dB

Explanation: Noise is considered pollution when it exceeds the level that the human ear can tolerate comfortably. Sound above about 70-75 dB is regarded as the beginning of noise pollution, and prolonged exposure above this can cause irritation, stress and eventually hearing damage. Normal conversation is around 60 dB, while levels above 120 dB cause pain.

Sound Levels (Decibel Scale):

Answer: (D) 90°

Explanation: The angle of dip is the angle a freely suspended magnetic needle makes with the horizontal at a place. At the magnetic poles the earth’s magnetic field is vertical, so the needle points straight down and the dip is 90°. At the magnetic equator the field is horizontal, so the dip is 0°.

Earth’s Magnetism:

Answer: (D) speed in vacuum

Explanation: All electromagnetic waves, from radio waves to gamma rays, travel with the same speed in vacuum, c = 3 × 10⁸ m/s. This was predicted by James Clerk Maxwell, who showed that c = 1/√(μ₀ε₀). Their frequencies and wavelengths differ, and they are related by c = νλ.

Electromagnetic Waves:

Answer: (A) solids

Explanation: Sound is a mechanical wave whose speed depends on the elasticity and density of the medium: v = √(E/ρ). Solids have a very high elastic modulus, so sound travels fastest in them (about 5000 m/s in steel), slower in liquids (about 1500 m/s in water), and slowest in gases (about 340 m/s in air). In vacuum there is no medium, so sound cannot travel at all.

Speed of Sound in Different Media:

Answer: (A) increase

Explanation: Adding a non-volatile solute such as salt to pure water lowers its vapour pressure, so the solution must be heated to a higher temperature before its vapour pressure equals the atmospheric pressure. This raises the boiling point, a phenomenon called boiling point elevation. The rise is ΔTb = Kb × m, where Kb is the ebullioscopic constant and m is the molality of the solution. It depends on the number of solute particles, not on their nature, so it is a colligative property.

Colligative Properties:

Answer: (B) Concave Lens

Explanation: In short-sightedness (myopia), the eyeball is too long or the eye lens is too converging, so the image of a distant object forms in front of the retina. A concave (diverging) lens spreads the rays slightly before they enter the eye, so the image shifts back onto the retina. Concave and diverging lenses are the same thing, while convex and converging lenses are the same thing.

Defects of Vision:

Answer: (A) White

Explanation: Red, green and blue are the primary colours of light. When they are mixed in equal proportions (additive mixing), they produce white light. This is the principle behind TV, mobile and computer screens, which create all colours using RGB pixels.

Colour Mixing:

Answer: (A) word

Explanation: A group of 16 bits is called a word in the context of this question. The standard units are 1 nibble = 4 bits and 1 byte = 8 bits, and a word is a larger unit whose size depends on the computer architecture (commonly 16, 32 or 64 bits). Since 16 bits is two bytes, the option that matches is word.

Units of Digital Information:

Answer: (A) The stone flies off tangentially

Explanation: While the stone is whirled in a circle, the tension in the string provides the centripetal force that keeps changing the direction of its velocity. When the string snaps, this force vanishes, so no force acts on the stone in the horizontal plane. By Newton’s first law, the stone continues in a straight line in the direction of its velocity at that instant, which is along the tangent to the circle.

Circular Motion and Centripetal Force:

Answer: (A) will continue to float as before

Explanation: A floating body displaces liquid whose weight equals the body’s weight: ρ_body × V × g = ρ_liquid × V_immersed × g. The acceleration due to gravity g appears on both sides and cancels, so the fraction of the body immersed, V_immersed/V = ρ_body/ρ_liquid, does not depend on g. On the moon, where g is about one-sixth of the earth’s value, both the weight of the body and the buoyant force reduce in the same proportion, so the body floats exactly as before, with the same part immersed.

Floatation and Gravity:

Answer: (B) Secondary

Explanation: Computer storage is classified as primary, secondary and tertiary. Primary storage (RAM, ROM, cache) is directly accessed by the CPU and is fast. Secondary storage is non-volatile, external to the main memory and retains data permanently; it includes hard disks, SSDs, pen drives (USB) and memory cards. A USB flash drive therefore falls under secondary storage.

Computer Memory and Storage:

Answer: (A) Java

Explanation: Java has long been the main language for developing Android applications, using the Android SDK, and Google also supports Kotlin as an official language. Among the given options, Java is the commonly used language for Android apps. C and C++ are used only for specific native components through the Android NDK, and PHP is mainly a server-side web language.

Programming Languages and Their Uses:

Answer: (D) Bugs

Explanation: An error or flaw in a computer program is called a bug, and the process of finding and fixing such errors is called debugging. The term became popular after 9 September 1947, when engineers working on the Harvard Mark II computer, including Grace Hopper’s team, found a moth stuck in a relay and taped it into the logbook as the “first actual case of bug being found.”

Computer Terms:

Answer: (A) 10 Å

Explanation: The nanometre is 10⁻⁹ m and the angstrom is 10⁻¹⁰ m, so 1 nm = 10⁻⁹/10⁻¹⁰ = 10 Å. The option reads “10 A” and is taken as 10 Å.

Units of Small Lengths:

Answer: (A) Radio Detection and Ranging

Explanation: RADAR stands for Radio Detection and Ranging. It sends out short pulses of radio waves or microwaves, which reflect off an object such as an aircraft, ship or storm cloud. The time taken for the echo to return gives the distance of the object (d = ct/2), and the direction of the returning signal gives its position. The Doppler shift of the echo gives the object’s speed.

Acronyms and Detection Systems:

Answer: (D) rotational as well as translational motion

Explanation: The two forces are unequal, so the net force is not zero and the body accelerates, which gives translational motion. They are also not along the same line, so they form a torque about the centre of mass, which gives rotational motion. Hence the body has both.

Equilibrium and Motion Under Forces:

Answer: (A) surface tension of water

Explanation: When the brush is dipped in water, the hairs spread out because water films between them are broken. When it is taken out, a thin film of water clings around the hairs, and surface tension makes this film contract to minimise its surface area. The film pulls the hairs together, so they appear drawn together and stick as a bunch.

Surface Tension:

Answer: (B) red light is scattered least by the air molecules

Explanation: Rayleigh scattering is proportional to 1/λ⁴, so light of shorter wavelength is scattered more. Red light has the longest wavelength in the visible range (about 620-750 nm), so it is scattered least by air molecules, fog, smoke and dust. It therefore travels the farthest in a straight line and remains visible from a long distance, which makes it the standard colour for danger signals.

Scattering of Light:

Answer: (B) 9 / 10 th of its volume

Explanation: A floating body displaces liquid equal to its own weight, so the fraction immersed is V_immersed / V = ρ_ice / ρ_water. The density of pure ice is about 0.9 g/cm³ and that of water is 1 g/cm³, so the fraction is 0.9 = 9/10. Hence 9/10 of the ice lies below the water surface and only 1/10 above it.

Floating Ice and Icebergs:

Answer: (B) in parallel

Explanation: Household appliances are connected in parallel across the mains supply. In a parallel connection every appliance gets the same full voltage (220-240 V in India), and each can be switched on or off independently without affecting the others. In series, the voltage would divide among the appliances, and if one failed or was switched off, the whole circuit would break.

Series and Parallel Connections:

Answer: (A) electromagnetic radiations of very short wavelength

Explanation: X-rays were discovered in 1895 by Wilhelm Röntgen. They are electromagnetic waves with wavelengths of about 0.01 to 10 nm, much shorter than visible light, so they have high energy and can pass through soft tissue. They are not beams of electrons, although they are produced when fast electrons strike a metal target. They are invisible to the naked eye.

X-rays:

Answer: (B) Stars

Explanation: Astronomy is the branch of science that deals with the study of celestial objects such as stars, planets, moons, comets and galaxies, and of the universe as a whole. Among the given options, the study of stars is the best match, since stars are the central objects of astronomical study.

Astronomy and Related Terms:

Answer: (B) Surface tension

Explanation: Water rises in a capillary tube because of surface tension together with adhesion between water and glass. Water wets glass, so the surface curves upward into a concave meniscus, and the surface tension force acting along the contact line pulls the liquid up the tube until its weight balances this force. The height of rise is h = 2T cosθ / (rρg), so a narrower tube gives a greater rise.

Capillarity:

Answer: (C) virtual and erect

Explanation: A plane mirror forms an image that cannot be caught on a screen, because the reflected rays only appear to come from behind the mirror, so the image is virtual. It is also erect (upright), the same size as the object, and as far behind the mirror as the object is in front. The image is laterally inverted, meaning left and right are interchanged, but it is not upside down.

Plane Mirror:

Answer: (A) Nuclear fusion

Explanation: The Sun’s energy comes from nuclear fusion in its core, where the temperature is about 15 million K. Four hydrogen nuclei (protons) fuse through the proton-proton chain to form one helium nucleus. The product has slightly less mass than the four protons, and this mass difference is released as energy according to E = mc². Fission is the splitting of heavy nuclei, as in nuclear reactors, and does not power the Sun.

Nuclear Fusion and Fission:

Answer: (D) decreases

Explanation: A pure semiconductor has few free charge carriers at low temperature. On heating, more electrons gain enough energy to cross the band gap into the conduction band, creating more electrons and holes. The increase in the number of carriers outweighs the increased lattice vibrations, so the resistance decreases. Semiconductors therefore have a negative temperature coefficient of resistance, unlike metals.

Temperature Dependence of Resistance:

Answer: (B) Frictional force

Explanation: A conservative force is one whose work depends only on the initial and final positions and not on the path taken. The work done by it around any closed path is zero. Gravitational, electrostatic and magnetostatic forces satisfy this. Friction is a non-conservative force because the work done against it depends on the path length, and the energy is lost as heat.

Conservative and Non-Conservative Forces:

Answer: (C) Radiation

Explanation: The heat of the Sun reaches the Earth through space, which is almost entirely a vacuum. Conduction and convection both need a material medium, so they cannot carry heat across space. Radiation transfers heat as electromagnetic waves (mainly infrared, visible and ultraviolet) and needs no medium, travelling at 3 × 10⁸ m/s. Sunlight takes about 8 minutes 20 seconds to reach the Earth.

Modes of Heat Transfer:

Answer: (C) Greenhouse

Explanation: A solar cooker works on the greenhouse effect. Sunlight passes through the glass lid and is absorbed by the blackened inner surface, which re-emits it as longer-wavelength infrared radiation. Glass does not let this infrared escape, so heat is trapped and the temperature inside rises. A reflecting mirror is often added to concentrate more sunlight into the box.

Solar Cooker and Related Devices:

Answer: (B) Pascal’s law

Explanation: A hydraulic press works on Pascal’s law, which states that pressure applied to an enclosed incompressible fluid is transmitted equally in all directions. A small force on a small piston creates a pressure that acts on a larger piston, giving a proportionally larger force: F₂ = F₁ × (A₂/A₁).

Fluid Mechanics Laws and Applications:

Answer: (B) Light emitting diode

Explanation: LED stands for Light Emitting Diode. It is a semiconductor p-n junction diode that emits light when forward biased. Electrons and holes recombine at the junction and release energy as photons. The colour of the light depends on the band gap of the semiconductor material.

LED and Related Devices:

Answer: (D) – 40°

Explanation: The conversion between the scales is F = (9/5)C + 32. For the two readings to be equal, put F = C = x: x = (9/5)x + 32, so −(4/5)x = 32, giving x = −40. Hence −40°C is the same as −40°F.

Temperature Scales:

Answer: (C) Its weight increases

Explanation: Mass is the quantity of matter in a body and does not change with location. Weight is W = mg, and g is larger at the poles than at the equator, because the Earth is flattened at the poles (so the pole is closer to the Earth’s centre) and because the Earth’s rotation reduces the effective g at the equator. So when a ball is taken from the equator to the pole, its mass stays the same but its weight increases.

Variation of g:

Answer: (D) All of the above

Explanation: Artificial satellites in orbit carry remote sensing instruments that provide a wide range of information about the Earth. They map forest and vegetative cover, trace the courses of rivers and changes in water bodies, and track weather systems such as advancing storms and cyclones. Since all three are correct, the answer is “All of the above”.

Uses of Satellites: