In which of the following phenomena do heat waves travel along a straight line with the speed of light?
- A.Thermal conduction
- B.Thermal convection
- C.Thermal radiation
- D.Both, thermal conduction and radiation
Correct answer
Explanation
The correct answer is C, Thermal radiation. Radiant heat is infrared electromagnetic radiation, so it travels in straight lines at the speed of light.
Heat moves in three ways. In radiation, energy is carried by electromagnetic waves, mainly infrared, which need no medium and move at about 3 × 10⁸ m/s in vacuum. That is how the Sun's heat crosses empty space to reach the Earth, and why a screen placed in the straight path blocks the warmth of a fire. Radiation is the fastest mode of heat transfer.
A is wrong: in conduction heat passes slowly from particle to particle through a material, as along a metal spoon, with no bulk movement and nowhere near the speed of light.
B is wrong: in convection the heated fluid itself moves in currents, as in boiling water or sea breezes; it is slow and not straight-line.
D is wrong: conduction does not travel at the speed of light, so the combined option fails.
Exam tip: conduction needs contact (solids), convection needs a fluid, radiation needs no medium and is the fastest.
Practice Questions
View allThe speed of the wind is measured by
- A.wind vane
- B.wind rose
- C.barometer
- D.anemometer
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An anemometer measures how fast the wind is blowing; its name comes from the Greek word anemos, meaning wind.
Read the full explanation →Brilliance of diamond is due to
- A.interference of light
- B.diffraction of light
- C.polarization of light
- D.total internal reflection of light
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Light that enters a cut diamond is reflected again and again inside it before it escapes, which makes the stone sparkle.
Read the full explanation →Which one of the following is NOT an electromagnetic wave ?
- A.Light wave
- B.Radio wave
- C.Sound wave
- D.Micro wave
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Sound is a mechanical wave that needs a medium to travel, while the other three are electromagnetic waves. Sound travels as a longitudinal wave of compressions and rarefactions in air, water or solids, which is why it cannot pass through a vacuum.
Read the full explanation →Which one of the following is NOT true for an electromagnetic wave ?
- A.Electromagnetic wave transports energy and momentum
- B.Electromagnetic wave can be polarized and reflected
- C.Electromagnetic wave is longitudinal
- D.Electromagnetic wave propagation is described by the varying electric and magnetic fields
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Electromagnetic waves are transverse, not longitudinal, so this statement is false. In an electromagnetic wave the electric field and the magnetic field oscillate at right angles to each other and both at right angles to the direction in which the wave travels.
Read the full explanation →Which one of the following frequency ranges is sensitive to human ears ?
- A.0-200 Hz
- B.20-20,000 Hz
- C.200-20,000 Hz only
- D.2,000-20,000 Hz only
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The normal human ear hears sound from about 20 hertz to 20,000 hertz, the audible range. Frequency, measured in hertz, is the number of vibrations per second, and it decides the pitch of a sound: the higher the frequency, the shriller the note.
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