Multiple Choice Question
1. Which statement is true for Gauss law ?
(a) All the charges whether inside or outside the Gaussian surface contribute to the electric flux.
(b) Electric flux depends upon the geometry of the Gaussian surface.
(c) Gauss theorem can be applied to non-uniform electric field.
(d) The electric field over the Gaussian surface remains continuous and uniform at every point.
Ans. (d) The electric field over the Gaussian surface remains continuous and uniform at every point.
2. An electric dipole placed in a non-uniform electric field can experience.
(a) a force but not a torque.
(b) a torque but not a force.
(c) always a force and a torque.
(d) neither a force nor a torque.
Ans. (c) always a force and a torque.
3. Some charge is being given to a conductor. then, its potential
(a) is maximum at surface.
(b) is maximum at centre.
(c) remains the same throughout the conductor.
(d) is maximum somewhere between surface and centre.
Ans. (c) remains the same throughout the conductor.
4. A positively charged particle is released from rest in an uniform electric field. The electric potential energy of the charge
(a) remains a constant because the electric field is uniform.
(b) increases because the charge moves along the electric field.
(c) decreases because the charge moves along the electric field.
(d) decreases because the charge moves opposite to the electric field.
Ans. (c) decreases because the charge moves along the electric field.
5. The resistance of a metal wire increases will increasing temperature on account of
(a) decrease in free electron density.
(b) decrease in relaxation time.
(c) increase in mean free path.
(d) increase in the mass of electron.
Ans. (b) decrease in relaxation time.
6. A battery is connected to the conductor of non-uniform cross section area. The quantities or quantity which remains constant is
(a) electric field only
(b) drift speed and electric field
(c) electric field and current
(d) current only
Ans. (d) current only
7. A current carrying wire kept in a uniform magnetic field, will experience a maximum force when it is
(a) perpendicular to the magnetic field
(b) parallel to the magnetic field
(c) at an angle of 45° to the magnetic field
(d) at an angle of 60° to the magnetic field
Ans. (a) perpendicular to the magnetic field
8. If an ammeter is to be used in place of a voltmeter, then we must connect with the ammeter a
(a) low resistance in parallel
(b) low resistance in series
(c) high resistance in parallel
(d) high resistance in series
Ans. (d) high resistance in series
9. The material which is not suitable for making a permanent magnet is
(a) Steel
(b) Ticonal
(c) Lead
(d) Alnico
Ans. (c) Lead
10. A magnetic needle is kept in a uniform magnetic field. It experiences
(a) a force and a torque
(b) a force but not a torque
(c) a torque but not a force
(d) neither a torque nor a force
Ans. (c) a torque but not a force
11. Whenever the flux linked with a circuit changes, there is an induced emf in the circuit. This emf in the circuit lasts
(a) for a very short duration
(b) for a long duration
(c) forever
(d) as long as the magnetic flux in the circuit changes.
Ans. (d) as long as the magnetic flux in the circuit changes.
12. A copper ring is held horizontally and a magnet is dropped through the ring with its length along the axis of the ring. The acceleration of the falling magnet is
(a) equal to that due to gravity
(b) less than that due to gravity
(c) more than that due to gravity
(d) depends on the diameter of the ring and the length of the magnet
Ans. (b) less than that due to gravity
13. The core of a transformer is laminated to reduce the effect of
(a) flux leakage
(b) copper loss
(c) hysteresis loss
(d) eddy current
Ans. (d) eddy current
14. Which among the following is not a cause for power loss in a transformer ?
(a) Eddy currents are produced in the soft iron core of a transformer.
(b) Electric flux sharing is not properly done in primary and secondary coils.
(c) Humming sound produced in the transformers due to magnetostriction.
(d) Primary coil is made up of a very thick copper wire.
Ans. (d) Primary coil is made up of a very thick copper wire.
15. Displacement current exists only when
(a) electric field is changing.
(b) magnetic field is changing.
(c) electric field is not changing.
(d) magnetic field is not changing.
Ans. (a) electric field is changing.
16. During the propagation of electromagnetic waves in a medium
(a) electric energy density is double of the magnetic energy density.
(b) electric energy density is half of the magnetic energy density.
(c) electric energy density is equal to the magnetic energy density.
(d) both electric and magnetic energy densities are zero.
Ans. (c) electric energy density is equal to the magnetic energy density.
17. The focal length of the objective of a compound microscope is
(a) greater than the focal length of eyepiece
(b) lesser than the focal length of eyepiece
(c) equal to the focal length of eyepiece
(d) equal to the length of its tube
Ans. (b) lesser than the focal length of eyepiece
18. Larger aperture of objective lens in an astronomical telescope
(a) increases the resolving power of telescope.
(b) decreases the brightness of the image.
(c) increases the size of the image.
(d) decreases the length of the telescope.
Ans. (a) increases the resolving power of telescope.
19. Yellow light is used in a single slit diffraction experiment with slit width of 0.6 mm. If yellow light is replaced by X-rays, then the observed pattern will reveal
(a) that the central maximum is narrower
(b) more number of fringes
(c) less number of fringes
(d) no diffraction pattern
Ans. (a) that the central maximum is narrower
20. In a single-slit diffraction experiment, the width of the slit is halved. The width of the central maximum, in the diffraction pattern, will become
(a) half
(b) twice
(c) four times
(d) one-fourth
Ans. (b) twice
21. Electrons used in an electron microscope are accelerated by a voltage of 25 kV. If the voltage is increased to 100 kV then the de Broglie wavelength associated with the electrons would
(a) increase by 2 times
(b) decrease by 2 times
(c) decrease by 4 times
(d) increase by 4 times
Ans. (b) decrease by 2 times
22. Sodium surface is illuminated by ultraviolet and visible radiation successively and the stopping potential is determined. This stopping potential is
(a) equal in both cases
(b) more with ultraviolet light
(c) more with visible light
(d) varies randomly
Ans. (b) more with ultraviolet light
23. The simple Bohr model cannot be directly applied to calculate the energy levels of an atom with many electrons. This is because
(a) of the electrons not being subject to a central force
(b) of the electrons colliding with each other
(c) of screening effects
(d) the force between the nucleus and an electron will no longer be given by Coulomb's law
Ans. (a) of the electrons not being subject to a central force
24. Paschen series of atomic spectrum of hydrogen gas lies in
(a) Infrared region
(b) Ultraviolet region
(c) Visible region
(d) Partly in ultraviolet and partly in visible region
Ans. (a) Infrared region
25. The energy in nuclear reactor is obtained due to
(a) nuclear fission
(b) nuclear fusion
(c) photoelectric effect
(d) spontaneous radioactive decay
Ans. (a) nuclear fission
26. Heavy stable nuclei have more neutrons than protons. This is because of the fact that
(a) neutrons are heavier than protons.
(b) electrostatic force between protons are repulsive.
(c) neutrons decay into protons through beta decay.
(d) nuclear forces between neutrons are weaker than that between protons.
Ans. (b) electrostatic force between protons are repulsive.
27. In an n-type semiconductor, the donor energy level lies
(a) at the centre of the energy gap.
(b) just below the conduction band.
(c) just above the valance band.
(d) in the conduction band.
Ans. (b) just below the conduction band.
28. The manifestation of band structure in solids is due to
(a) Heisenberg's uncertainty principle
(b) Pauli's exclusion principle
(c) Bohr's correspondence principle
(d) Boltzmann's law
Ans. (b) Pauli's exclusion principle
Assertion-Reason Question
(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 the correct explanation of A.
(c) A is true but R is false.
(d) A is false and R is also false.
1. Assertion (A) : A negative charge in an electric field moves along the direction of the electric field.
Reason (R) : On a negative charge a force acts in the direction of the electric field.
Ans. (d) A is false and R is also false.
2. Assertion (A) : An applied electric field polarises a polar dielectric.
Reason (R) : The molecules of a polar dielectric possess a permanent dipole moment, but in the absence of electric field, these dipoles are randomly oriented and when electric field is applied these dipoles align along the direction of electric field.
Ans. (a) Both A and R are true and R is the correct explanation of A.
3. Assertion (A) : The connecting wires are made of copper.
Reason (R) : Copper has very high electrical conductivity.
Ans. (a) Both A and R are true and R is the correct explanation of A.
4. Assertion (A) : Galvanometer cannot as such be used as an ammeter to measure the value of the current in a given circuit.
Reason (R) : It gives a full-scale deflection for a current of the order of micro ampere.
Ans. (a) Both A and R are true and R is the correct explanation of A.
5. Assertion (A) : For making permanent magnets, steel is preferred over soft iron.
Reason (R) : As retentivity of steel is smaller.
Ans. (b) Both A and R are true but R is not the correct explanation of A.
6. Assertion (A) : The presence of large magnetic flux through a coil maintains a current in the coil, if the circuit is continuous.
Reason (R) : Only a change in magnetic flux will maintain an induced current in the coil.
Ans. (d) A is false and R is also false.
7. Assertion (A) : An alternating current does not show any magnetic effect.
Reason (R) : Alternating current does not vary with time.
Ans. (d) A is false and R is also false.
8. Assertion (A) : Microwaves are better carriers of signals than optical waves.
Reason (R) : Microwaves move faster than optical waves.
Ans. (c) A is true but R is false.
9. Assertion (A) : In compound microscope, the objective lens is taken of small focal length.
Reason (R) : This increases the magnifying power of microscope.
Ans. (b) Both A and R are true but R is not the correct explanation of A.
10. Assertion (A) : Coloured spectrum is seen when we look through a muslin cloth.
Reason (R) : Coloured spectrum is due to diffraction of white light passing through fine slits made by fine threads in the muslin cloth.
Ans. (a) Both A and R are true and R is the correct explanation of A.
11. Assertion (A) : An electron microscope is based on de-Broglie hypothesis.
Reason (R) : A beam of electrons behaves as a wave which can be converged by electric and magnetic lenses.
Ans. (a) Both A and R are true and R is the correct explanation of A.
12. Assertion (A) : The electrons have orbital angular momentum.
Reason (R) : Electrons have well-defined quantum states.
Ans. (b) Both A and R are true but R is not the correct explanation of A.
13. Assertion (A) : Energy is released in nuclear fission.
Reason (R) : Total binding energy of fission fragments is larger than the total binding energy of the parent nucleus.
Ans. (a) Both A and R are true and R is the correct explanation of A.
14. Assertion (A) : When the temperature of a semiconductor is increased, then its resistance decreases.
Reason (R) : The energy gap between valence and conduction bands is very small for
semiconductors.
Ans. (a) Both A and R are true and R is the correct explanation of A.
Case-Based Question
1. Which of the following cannot be true about properties of charge ?
(a) Charges can be created or destroyed in equal and unlike pairs only.
(b) Proper sign have to be used while adding the charges in a system.
(c) Excess of electrons over protons in a body is responsible for positive charge of the body.
(d) It is not possible to create or destroy net charge carried by an isolated system.
Ans. (c) Excess of electrons over protons in a body is responsible for positive charge of the body.
2. Which of the following material can be used to make a Faraday cage ?
(a) Plastic
(b) Glass
(c) Copper
(d) Wood
Ans. (c) Copper
3. The power dissipated in R1 is
(a) 2 W
(b) 2.5 W
(c) 3 W
(d) 4.5 W
Ans. (c) 3 W
4. Loudspeaker works on the principle of
(a) detector
(b) generator
(c) amplifier
(d) motor
Ans. (d) motor
5. Susceptibility is positive and small for
(a) paramagnetic substances
(c) non-magnetic substances
(b) ferromagnetic substances
(d) diamagnetic substances
Ans. (a) paramagnetic substances
6. The jumping ring experiment based on which of the following law ?
(a) Lenz's Law
(c) Snell's Law
(b) Faraday's law
(d) both (a) and (b)
Ans. (d) both (a) and (b)
7. Which of the following statement is true ?
(a) Energy is created when a transformer steps up the voltage.
(b) A transformer is designed to convert an AC voltage to DC voltage.
(c) Step-up transformer increases the power for transmission.
(d) Step-down transformer decreases the AC voltage.
Ans. (d) Step-down transformer decreases the AC voltage.
8. Light wave constitutes
(a) electromagnetic waves
(c) mechanical waves
(b) longitudinal waves
(d) magnetic waves
Ans. (a) electromagnetic waves
9. The basic reason for the extraordinary sparkle of suitably cut diamond is that
(a) it has low refractive index
(b) it has high transparency
(c) it has high refractive index
(d) it is very hard
Ans. (c) it has high refractive index
10. What principle is responsible for light spreading as it passes through a narrow slit ?
(a) Interference
(b) Diffraction
(c) Polarisation
(d) Refraction
Ans. (b) Diffraction
11. According to Bohr's model of hydrogen atom, an electron can revolve round a proton indefinitely, if its path is
(a) a perfect circle of any radius
(b) a circle of constantly decreasing radius
(c) a circle of an allowed radius
(d) an ellipse
Ans. (c) a circle of an allowed radius
12. In Bohr model of hydrogen atom, which of the following is quantised ?
(a) Linear velocity of electron
(b) Angular velocity of electron
(c) Linear momentum of electron
(d) Angular momentum of electron
Ans. (d) Angular momentum of electron
13. Solar energy is mainly caused due to
(a) gravitational contraction
(b) fusion of proton during synthesis of heavier elements
(c) fission of uranium present in the sun
(d) burning of hydrogen in the oxygen
Ans. (b) fusion of proton during synthesis of heavier elements
14. Silicon is doped with which of the following to obtain p-type semiconductor ?
(a) Phosphorus
(b) Gallium
(c) Germanium
(d) Bismuth
Ans. (b) Gallium
Very Short Answer Question
1. What is electrostatic shielding ? How is this property used in actual practice ? Is the potential in the cavity of a charged conductor zero ?
Ans. Whatever be the charge and field configuration outside, any cavity in a conductor remains shielded from outside electric influence. The field inside a conductor is zero. This is known as electrostatic shielding. Sensitive instruments are shielded from outside electrical influences by enclosing them in a hollow conductor. During lightning it is safest to sit inside a car, rather than near a tree. The metallic body of a car becomes an electrostatic shielding from lightening. Potential inside the cavity is not zero. Potential is constant.
2. Consider two identical point charges located at points (0, 0) and (a, 0).
1. Is there a point on the line joining them at which the electric field is zero ?
2. Is there a point on the line joining them at which the electric potential is zero ?
Justify your answers for each case.
Ans.
1. Yes, electric field will zero at mid point. Electric field being a vector quantity, its resultant is zero.
2. No, potential cannot be zero on line joining the charges. Electric potential being a scalar quantity, the net potential due to two identical charges cannot be zero.
3. Define the terms (i) drift velocity, (ii) relaxation time.
Ans.
(i) Drift Velocity : The average velocity acquired by the free electrons of a conductor in a direction opposite to the externally applied electric field is called drift velocity. The drift velocity will remain the same with lattice ions/atoms.
(ii) Relaxation Time : The average time of free travel of free electrons between two successive collisions is called the relaxation time.
4. State two reasons why a galvanometer can not be used as such to measure current in a given circuit.
Ans. A galvanometer cannot be used as such to measure current due to following two reasons.
(i) A galvanometer has a finite large resistance and is connected in series in the circuit, so it will increase the resistance of circuit and hence change the value of current in the circuit.
(ii) A galvanometer is a very sensitive device, it gives a full scale deflection for the current of the order of microampere, hence if connected as such it will not measure current of the order of ampere.
5. The susceptibility of a magnetic material is -2.6 x 10-5. Identify the type of magnetic material and state its two properties.
Ans. The magnetic material having negative susceptibility is diamagnetic in nature Properties :
(i) This material has + ve but low relative permeability.
(ii) They have the tendency to move from stronger to weaker part of the external magnetic field.
6. What is the basic difference between the atom and molecule of a diamagnetic and a paramagnetic material ? Why are elements with even atomic number more likely to be diamagnetic ?
Ans. Atoms/molecules of a diamagnetic substance contain even number of electrons and these electrons form the pairs of opposite spin; while the atoms/molecules of a paramagnetic substance have excess of electrons spinning in the same direction. The elements with even atomic number Z has even number of electrons in its atoms/molecules, so they are more likely to form electrons pairs of opposite spin and hence more likely to be diamagnetic.
7. Both alternating current and direct current are measured in amperes. But how is the ampere defined for an alternating current ?
Ans. An ac current changes direction with the source frequency and the attractive force would average to zero. Thus, the ac ampere must be defined in terms of some property that is independent of the direction of current. Joule's heating effect is such property and hence it is used to define rms value of ac.
8. Explain why the reactance provided by a capacitor to an alternating current decreases with increasing frequency.
Ans. A capacitor does not allow flow of direct current through it as the resistance across the gap is infinite. When an alternating voltage is applied across the capacitor plates, the plates are alternately charged and discharged. The current through the capacitor is a result of this changing voltage (or charge). Thus, a capacitor will pass more current through it if the voltage is changing at a faster rate, i.e., if the frequency of supply is higher. This implies that the reactance offered by a capacitor is less with increasing frequency; it is given by 1/wC.
9. Explain why the reactance offered by an inductor increases with increasing frequency of an alternating voltage.
Ans. An inductor opposes flow of current through it by developing an induced emf according to Lenz's law. The induced voltage has a polarity so as to maintain the current at its present value. If the current is decreasing, the polarity of the induced emf will be so as to increase the current and viceversa. Since the induced emf is proportional to the rate of change of current, it will provide greater reactance to the flow of current if the rate of change is faster, i.e., if the frequency is higher. The reactance of an inductor, therefore, is proportional to the frequency, being given by WL.
10. Consider an induced magnetic field due to changing electric field and an induced electric field due to changing magnetic field. Which one is more easily observed? Justify your answer.
Ans. Electric field which is easily observed because an a.c. circuit displacement current can be increased by increasing the angular frequency of current. The electric field increase due to changing magnetic field can be increased by taking more no. of turns of the coil. But magnetic field set up due to this displacement current is very small so it can easily observed.
11. Write three characteristic features in photoelectric effect which cannot be explained on the basis of wave theory of light, but can be explained only using Einstein's equation.
Ans. The three characteristic features which cannot be explained by wave theory are :
(i) Kinetic energy of emitted electrons is found to be independent of the intensity of incident light.
(ii) There is no emission of electrons if frequency of incident light is below a certain frequency (threshold frequency).
(iii) Photoelectric effect is an instantaneous process.
12. Which is easier to remove: orbital electron from an atom or a nucleon from a nucleus ?
Ans. It is easier to remove an orbital electron from an atom. The reason is the binding energy of orbital electron is a few electron-volts while that of nucleon in a nucleus is quite large (nearly 8 MeV). This means that the removal of an orbital electron requires few electron volt energy while the removal of a nucleon from a nucleus requires nearly 8 MeV energy.
13. Write shortcomings of Rutherford atomic model. Explain how these were overcome by the postulates of Bohr's atomic model.
Ans. Two important limitations of Rutherford model are :
(i) According to Rutherford model, electron orbiting around the nucleus, continuously radiates energy due to the acceleration; hence the atom will not remain stable.
(ii) As electron spirals inwards; its angular velocity and frequency change continuously, therefore it should emit a continuous spectrum. But an atom like hydrogen always emits a discrete line spectrum.
Bohr's postulates overcome these limitations by :
(i) Bohr stated that negatively charged electrons revolve around positively charged nucleus in certain orbits called stationary orbits. The electrons does not radiate energy when in stationary orbits.
(ii) The quantum of energy is released or absorbed when an electron jumps from one stationary orbit to another.
14. Define the term, mass defect. How is it related to stability of the nucleus ?
Ans. The difference between mass of nucleus and the sum of the masses of its nucleons (i.e., proton (p) and neutron (n)) is called its mass defect. This mass defect is in the form of binding energy of nucleus, which is responsible for binding the nucleons in to a small nucleons. Hence, higher mass defect, higher is the stability of the nucleus.
Short Answer Question
1. Write any two important points of similarities and differences each between Coulomb's law for the electrostatic field and Biot-Savart's law for the magnetic field.
Ans. Similarities : Both electrostatic field and magnetic field :
(i) follows the principle of superposition.
(ii) depends inversely on the square of distance from source to the point of interest.
Differences :
(i) Electrostatic field is produced by a scalar source (q) and the magnetic field is produced by a vector source (Idl).
(ii) Electrostatic field is along the displacement vector between source and point of interest; while magnetic field is perpendicular to the plane, containing the displacement vector and vector source.
(iii) Electrostatic field is angle independent, while magnetic field is angle dependent between source vector and displacement vector.
2. State the underlying principle of a transformer. How is the large scale transmission of electric energy over long distances done with the use of transformers ?
Ans. The principle of transformer is based upon the principle of mutual induction which states that due to continuous change in the current in the primary coil an emf gets induced across the secondary coil. At the power generating station, the step up transformers step up the output voltage which reduces the current through the cables and hence reduce resistive power loss. Then, at the consumer end, a step down transformer steps down the voltage. Hence, the large scale transmission of electric energy over long distances is done by stepping up the voltage at the generating station to minimise the power loss in the transmission cables.
3. (a) How are electromagnetic waves produced by oscillating charges ?
(b) State clearly how a microwave oven works to heat up a food item containing water molecules.
(c) Why are microwaves found useful for the radar systems in aircraft navigation ?
Ans. (a) If a charge particle oscillates with some frequency, produces an oscillating electric field in space, which produces an oscillating magnetic field, which inturn, is a source of electric field, and so on. Thus oscillating electric fields and magnetic fields regenerate each other, and an electromagnetic wave propagates in the space.
(b) In microwave oven, the frequency of the microwaves is selected to match the resonant frequency of water molecules so that energy from the waves get transferred efficiently to the kinetic energy of the molecules. This kinetic energy raises the temperature of any food containing water.
(c) Microwaves are short wavelength radio waves, with frequency of order of few GHz. Due to short wavelength, they have high penetrating power with respect to atmosphere and less diffraction in the atmospheric layers. So these waves are suitable for the radar systems used in aircraft navigation.
4. Explain the following, giving reasons :
(i) When monochromatic light is incident on a surface separating two media, the reflected and refracted light both have the same frequency as the incident frequency.
(ii) When light travels from a rarer to a denser medium, the speed decreases. Does this decrease in speed imply a reduction in the energy carried by the wave ?
(iii) In the wave picture of light, intensity of light is determined by the square of the amplitude of the wave. What determines the intensity in the photon picture of light ?
Ans. (i) Reflection and refraction arise through interaction of incident light with atomic constituents of matter which vibrate with the same frequency as that of the incident light. Hence frequency remains unchanged.
(ii) No; when light travels from a rarer to a denser media, its frequency remains unchanged. According to quantum theory of light, the energy of light photon depends on frequency and not on speed.
(iii) For a given frequency, intensity of light in the photon picture is determined by the number of photon incident normally on a crossing an unit area per unit time.
5. Explain briefly the reasons why wave theory of light is not able to explain the observed features of photo-electric effect.
Ans. The observed characteristics of photoelectric effect could not be explained on the basis of wave theory of light due to the following reasons.
(i) According to wave theory, the light propagates in the form of wavefronts and the energy is distributed uniformly over the wavefronts. With increase of intensity of light, the amplitude of waves and the energy stored by waves will increase. These waves will then, provide more energy to electrons of metal; consequently, the energy of electrons will increase. Thus, according to wave theory, the kinetic energy of photoelectrons must depend on the intensity of incident light; but according to experimental observations, the kinetic energy of photoelectrons does not depend on the intensity of incident light.
(ii) According to wave theory, the light of any frequency can emit electrons from metallic surface provided the intensity of light be sufficient to provide necessary energy for emission of electrons, but according to experimental observations, the light of frequency less than threshold frequency cannot emit electrons; whatever the intensity of incident light may be.
(iii) According to wave theory, the energy transferred by light waves will not go to a particular electron, but it will be distributed uniformly to all electrons present in the illuminated surface. Therefore, electrons will take some time to collect the necessary energy for their emission. The time for emission will be more for light of less intensity and vice versa. But experimental observations show that the emission of electrons take place instantaneously after the light is incident on the metal; whatever the intensity of light may be.
6. Distinguish between 'intrinsic' and 'extrinsic' semiconductors.
Ans.
Intrinsic semiconductor :
(i) It is a semiconductor in pure form.
(ii) Intrinsic charge carriers are electrons and holes with equal concentration.
(iii) Current due to charge carriers is feeble (of the order of μA).
Extrinsic semiconductor :
(i) It is a semiconductor doped with trivalent or pentavalent impurity atoms.
(ii) The two concentrations are unequal in it. There is excess of electrons in n-type and
excess of holes in p-type semiconductors.
(iii) Current due to charge carriers is significant (of the order of mA).
7. Arrange the following electromagnetic waves in the order of their increasing wavelength :
(a) y-rays
(b) Microwaves
(c) X-rays
(d) Radiowaves
How are infra-red waves produced? What role does infra-red radiation play in (i) maintaining the earth's warmth and (ii) physical therapy?
Ans. y-rays < X-rays < Microwaves < Radiowaves
Infra-red waves are produced by the vibration of atoms and molecules.
(i) Maintaining Earth's Warmth : Infrared rays are absorbed by the earth's surface and reradiated as longer wave length infrared rays. These radiations are trapped by green house gases such as CO2 and maintain earth's warmth.
(ii) Physical Therapy : Infrared rays are easily absorbed by water molecules present in body. After absorption, their thermal motion increases causing heating which is used as physical therapy.