NDA Modern physics and everyday devices previous year questions with solutions

Answer: (a) 1 nm

Method. Wavelength range of X-rays

X-rays span about 0.01 nm to 10 nm. 1 nm lies well inside this range.
10 nm is the very edge of the range, where X-rays merge into the extreme ultraviolet; 100 nm and 300 nm are ultraviolet. 1 nm is the option that clearly corresponds to an X-ray. Option (a). Source: standard Class 12 Physics, electromagnetic spectrum.

Answer: (c) Micro Wave

Method. Order of the electromagnetic spectrum by wavelength

In order of increasing wavelength: X-rays (about 0.01 to 10 nm), ultraviolet (about 10 to 400 nm), infra-red (about 700 nm to 1 mm), microwaves (about 1 mm to 30 cm).
Microwaves have the longest wavelength of the four. Option (c). Source: standard Class 12 Physics, electromagnetic spectrum.

Answer: (c) Due to their wavelengths being shorter, these can be used for radar systems.

Method. X-ray properties and uses

(a) True: X-rays have wavelengths of about 0.1 to 100 Å, so about 1 Å is typical.
(b) True: X-rays are produced when high-energy electrons strike a metal target.
(c) False: radar uses microwaves and radio waves, which are reflected from aircraft; X-rays are not used in radar.
(d) True: X-rays are used in radiotherapy for certain cancers.
The untrue statement is (c). Source: standard Class 12 Physics, electromagnetic spectrum.

Answer: (d) 1 nm

Method. Wavelength range of X-rays

X-rays have wavelengths of about 0.01 nm to 10 nm (10^-11 to 10^-8 m). Of the options, only 1 nm lies in this range.
100 nm is ultraviolet, 500 nm is visible light (green), and 5000 nm is infra-red. Option (d). Source: standard Class 12 Physics, electromagnetic spectrum.

Answer: (d) X-rays

Method. Order of the electromagnetic spectrum by wavelength

In order of decreasing wavelength: radio, microwaves, infra-red, visible, ultraviolet, X-rays, gamma rays.
Among microwaves, infra-red, visible light and X-rays, X-rays have the smallest wavelength. Option (d). Source: standard Class 12 Physics, electromagnetic waves.

Answer: (b) 1 and 2 only

Method. Photon energy E = hc/⁠λ and the order of the spectrum by wavelength

Wavelength order: X-rays < UV < visible, so statement 1 (visible wavelength more than X-rays) is true.
Energy per photon E = hc/⁠λ is largest for the shortest wavelength: X-ray > UV > visible. Statement 2 is true.
Statement 3 says UV photons have less energy than visible photons; they have more. False.
Statements 1 and 2 only. Option (b). Source: standard Class 12 Physics, electromagnetic spectrum.

Answer: (a) 1 Å

Method. Wavelength range of X-rays

X-rays span roughly 0.01 nm to 10 nm, i.e. about 0.1 Å to 100 Å; the typical X-ray used in diffraction and imaging has λ of about 1 Å (10^-10 m), the spacing between atoms in a crystal.
1 μm, 1 mm and 1 cm are infra-red to microwave. Option (a) 1 Å. Source: standard Class 12 Physics, electromagnetic spectrum.

Answer: (a) X-rays

Method. Photon energy E = hc/⁠λ; shortest wavelength has the most energy

Energy per photon E = hf = hc/⁠λ, so the radiation of highest frequency (shortest wavelength) has the most energy per photon.
Order of frequency: radio < microwave < light < X-rays. X-rays are the highest of the four. Option (a). Source: standard Class 12 Physics, electromagnetic spectrum.

Answer: (c) Will be halved

Method. Duane–⁠Hunt law: cutoff wavelength λ_min = hc/⁠(eV)

In an X-ray tube the most energetic photon carries the whole kinetic energy eV of one electron, so eV = hc/⁠λ_min and λ_min = hc/⁠(eV) ≈ 12400/⁠V Å (V in volts).
λ_min is inversely proportional to V and does not depend on the filament–⁠target separation.
V doubled gives λ_min halved. Option (c). Source: standard Class 12 Physics, X-ray production.

Answer: (b) Cathode ray particles start from anode and move towards cathode.

Method. Properties of cathode rays

Cathode rays are streams of electrons that leave the cathode and move towards the anode; they travel in straight lines if no field acts, and a television picture tube is a cathode ray tube.
Statement (b) reverses the direction (anode to cathode), so (b) is not correct. Option (b). Source: standard Class 12 Physics, Dual Nature / J. J. Thomson's experiment.

Answer: (b) Albert Einstein

Method. Einstein's photon explanation of the photoelectric effect

Einstein (1905) explained the photoelectric effect by treating light as photons of energy hf, with K_max = hf − φ; this earned him the 1921 Nobel Prize.
Planck introduced the quantum of energy, Bohr the atomic model, Rutherford the nuclear atom. Option (b) Albert Einstein. (The printed 'Neils' is the spelling of Niels Bohr's name in option (c), which is wrong anyway.) Source: standard Class 12 Physics, Dual Nature.

Answer: (b) Photoelectric emission

Method. Photoelectric emission = electron emission by light

Emission of electrons from a metal surface when light falls on it is the photoelectric effect (photoelectric emission).
Thermionic emission is by heating; high-field (autoelectronic, cold-cathode) emission is by a strong electric field. Option (b). Source: standard Class 12 Physics, Dual Nature of Radiation and Matter.

Answer: (b) 13.6 eV

Method. Ionisation energy of hydrogen from the Bohr model

Bohr energy of the ground state of hydrogen: E1 = −⁠13.6 eV. Ionisation energy is the energy needed to take the electron to infinity (E = 0), so it is +13.6 eV.
The printed '13.6' is the British-style decimal point, the same number as 13.6. MeV is a million times too large and joule is not the atomic unit (13.6 eV = 2.18 × 10^-18 J). Option (b). Source: standard Class 12 Physics, Atoms.

Answer: (c) A mechanism to reduce CO₂ emission

Method. Parts of a nuclear reactor (standard schematic)

The standard schematic of a nuclear reactor (NCERT Class 12 Physics, Nuclei) shows the reaction chamber (core, with fuel rods, moderator and control rods), the coolant that carries the heat of fission to a heat exchanger (steam generator), and the steam that drives a turbine coupled to a generator; (a), (b) and (d) are all parts of it. Fission releases no CO₂, so a mechanism to reduce CO₂ emission has no place in a nuclear reactor; it belongs to a coal- or gas-fired plant. Option (c). Source: standard Class 12 Physics, Nuclei (nuclear reactor), and Class 10 Science, Sources of Energy.

Answer: (a) nuclear fission and its expression was proposed by Einstein.

Method. Nuclear energy and the mass–⁠energy relation

Nuclear power stations obtain energy from nuclear fission of heavy nuclei (uranium-235).
The mass–⁠energy relation E = mc², which expresses the energy released, was proposed by Einstein (1905).
Options (b), (c) and (d) pair a wrong process or a wrong scientist with the statement (Rutherford, Bohr and Heisenberg did not propose E = mc²). Option (a). Source: standard Class 12 Physics, Nuclei.

Answer: (d) 3 only

Method. Matching scientists with their discoveries

1. The neutron was discovered by James Chadwick (1932), not the photoelectric effect (discovered by Hertz, explained by Einstein). As printed, pair 1 is wrong.
2. Einstein explained the photoelectric effect; he did not discover the neutron. Pair 2 is wrong (the discoverers in pairs 1 and 2 are swapped).
3. Radium was discovered by Marie Curie (with Pierre Curie), 1898. Pair 3 is correct.
3 only. Option (d). Source: standard Class 12 Physics, history of modern physics.

Answer: (d) Pitchblende

Method. Pitchblende (uraninite) is the ore of uranium

Pitchblende, an oxide of uranium (mainly UO2/⁠U3O8), is the chief ore of uranium, from which the nuclear fuel is obtained (natural or enriched uranium).
Bauxite is the ore of aluminium; quartz is silica; feldspar is an aluminosilicate. Option (d). Source: standard reference, minerals and nuclear fuels.

Answer: (d) Controlled nuclear fission

Method. Principle of a nuclear reactor

A nuclear reactor works on the chain reaction of nuclear fission of uranium-235 (or plutonium-239), kept steady by control rods and a moderator, i.e. controlled nuclear fission.
Fusion is not used in power reactors; uncontrolled fission is the atom bomb. Option (d). Source: standard Class 12 Physics, Nuclei.

Answer: (a) GM Counter

Method. Geiger–⁠Müller counter detects and counts radiation

A GM counter detects ionising radiation (alpha, beta, gamma) and measures its count rate, i.e. radioactivity.
A polarimeter measures optical rotation, a calorimeter heat, a colorimeter the intensity of colour of a solution. Option (a). Source: standard Class 12 Physics, Nuclei.

Answer: (a) [graph (a)]

Method. Ohm's law: a uniform semiconductor wafer is a resistor

A rectangular piece of semiconductor without a junction is a plain resistor, so I = V/⁠R with constant R: a straight line through the origin.
The four sketches (first quadrant on cross axes I against V): (a) straight line from the origin rising to the upper right; (b) straight line from a positive point on the I-axis falling to a positive V; (c) curve from the origin, flat at first then rising ever more steeply, the exponential rise of a forward-biased p–⁠n diode; (d) curve falling steeply from high on the I-axis and flattening towards the V-axis, I varying like 1/⁠V.
(c) is a p–⁠n junction diode, which a bare wafer is not; (b) and (d) show current falling as voltage rises, which Ohm's law does not give. Option (a). Source: standard Class 12 Physics, Current Electricity and Semiconductor Electronics.

Answer: (c) Solar cell

Method. Photovoltaic effect: solar cell converts light to electricity

A solar cell (photovoltaic cell) is a p–⁠n junction that turns light energy into electrical energy.
An LED and a laser diode do the reverse (electrical to light), and a transistor is an amplifier or switch. Option (c). Source: standard Class 12 Physics, Semiconductor Electronics.

Answer: (c) Light Emitting Diode.

Method. Abbreviation LED

LED stands for Light Emitting Diode, a semiconductor p–⁠n junction that emits light when forward biased. Option (c). Source: standard Class 12 Physics, Semiconductor Electronics.

Answer: (a) Light Emitting Diode

Method. Abbreviation LED

LED stands for Light Emitting Diode: a p–⁠n junction diode that gives out light when forward biased. Option (a). Source: standard Class 12 Physics, Semiconductor Electronics.

Answer: (d) holes

Method. Majority carriers in p-type semiconductor

A p-type semiconductor is made by doping the pure crystal with a trivalent impurity (e.g. boron, indium); each impurity atom leaves a vacancy (hole).
Holes are the majority carriers, electrons the minority carriers. Option (d). Source: standard Class 12 Physics, Semiconductor Electronics.

Answer: (c) Tidal energy

Method. Conventional versus non-conventional sources of energy

Conventional sources are the fossil fuels (coal, petroleum, natural gas) and are exhaustible. Tidal energy is renewable and is a non-conventional source, along with solar, wind, biomass and geothermal energy. Option (c). Source: standard reference, energy sources.

Answer: (b) sound waves to electrical signals

Method. Microphone is a transducer from sound to electrical signal

A microphone converts sound waves (pressure variations) into a varying electrical signal; a loudspeaker does the reverse.
Option (b). Source: standard Class 12 Physics / general science, transducers.