Sound and light waves are
- A.respectively longitudinal and transverse in air
- B.respectively transverse and longitudinal in air
- C.both longitudinal in air
- D.both transverse in air
Correct answer
Explanation
The correct answer is A, respectively longitudinal and transverse in air. Sound travels as compressions and rarefactions, while light is a transverse electromagnetic wave.
In a longitudinal wave the particles of the medium vibrate back and forth along the direction in which the wave moves. Sound in air works this way: it needs a medium and travels at about 343 m/s at 20 °C. In a transverse wave the vibration is at right angles to the direction of travel. Light is made of electric and magnetic fields vibrating perpendicular to each other and to its direction, so it is transverse; it needs no medium and can be polarised, which a longitudinal wave cannot.
B is wrong because it reverses the two wave types. C is wrong because light is never longitudinal; polarisation of light proves it is transverse. D is wrong because sound cannot be transverse in air, since gases cannot support the shearing motion a transverse wave needs.
Exam tip: Sound in air = longitudinal, needs a medium; light = transverse, can be polarised, travels in vacuum.
Practice Questions
View allThe pressure exerted by a 760 mm column of mercury at 0 °C is known as
- A.1 pascal
- B.1 atmosphere
- C.1 bar
- D.1 poise
Show answer
Standard atmospheric pressure at sea level is defined as the pressure of a 760 mm column of mercury at 0 °C. In 1643 Evangelista Torricelli showed that air pressure holds up a column of mercury about 760 mm (76 cm) tall in a glass tube, inventing the mercury barometer.
Read the full explanation →Which one of the following is the purest form of Carbon ?
- A.Charcoal
- B.Coke
- C.Fullerene
- D.Carbon black
Show answer
Fullerenes are made of carbon atoms alone, with no surface to trap other substances, so they are counted as the purest form of carbon.
Read the full explanation →A planet has a mass M₁ and radius R₁. The value of acceleration due to gravity on its surface is g₁. There is another planet 2, whose mass and radius both are two times that of the first planet. Which one of the following is the acceleration due to gravity on the surface of planet 2?
- A.g₁
- B.2g₁
- C.g₁/2
- D.g₁/4
Show answer
Surface gravity is g = GM/R², so doubling the mass doubles g while doubling the radius divides it by four, leaving g₁ × 2/4 = g₁/2.
Read the full explanation →Which one of the following is the major constituent of biogas ?
- A.Carbon dioxide
- B.Nitrous oxide
- C.Methane
- D.Oxygen
Show answer
Methane makes up the largest share of biogas, usually about 55 to 75 per cent, and it is the gas that burns.
Read the full explanation →A lemon kept in water in a glass tumbler appears to be larger than its actual size. It is because of
- A.reflection of light
- B.scattering of light
- C.refraction of light
- D.polarization of light
Show answer
The lemon looks bigger because light from it bends as it passes from water through the curved glass into air.
Read the full explanation →