A person standing in a room can hear someone speaking in the next room through an open door but cannot see the speaker. The reason is that
- A.Sound travels faster than light
- B.The wavelength of sound is comparable with the width of the door, so sound diffracts strongly
- C.Sound is a transverse wave and light is longitudinal
- D.Light is absorbed by the air while sound is not
Correct answer
Explanation
The correct answer is B, the wavelength of sound is comparable with the width of the door, so sound diffracts strongly. Diffraction, the bending of a wave round an edge, is appreciable only when the obstacle or opening is of the same order of size as the wavelength. Audible sound has wavelengths from a few centimetres to a few metres and therefore spreads round a doorway, while visible light has a wavelength of only a few hundred nanometres and bends so little that it seems to travel in straight lines. Option A is wrong because light is far faster than sound and in any case speed does not cause bending. Option C is wrong because the opposite is true: sound in air is longitudinal and light is transverse. Option D is wrong because air absorbs very little visible light over the distance of a room.
Read the full article: Interference, Diffraction and Polarisation of Light Notes
Practice Questions
View allBrewster's law is (where the symbol mu stands for the refractive index and i_p for the polarising angle)
- A.i_p = 90 degrees
- B.mu = tan i_p
- C.i_p minus r = tan mu
- D.i_p = tan mu
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Brewster found that when unpolarised light falls on a transparent surface at one particular angle of incidence, called the polarising angle, the reflected light is completely plane polarised, and at that angle the refractive index of the surface equals the tangent of the polarising angle.
Read the full explanation →The headlights of a distant car are 1.4 m apart. If the diameter of the pupil of the eye is 3 mm, what is the maximum distance at which the two headlights can be resolved? Consider the headlights as point sources of wavelength 500 nm.
- A.3448 m
- B.68.96 m
- C.34.48 m
- D.6896 m
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By the Rayleigh criterion the smallest angle the eye can resolve is 1.22 times the wavelength divided by the diameter of the pupil, that is 1.22 multiplied by 500 nanometres and divided by 3 millimetres, which comes to about 2.03 times ten to the power minus four radian.
Read the full explanation →When a thin glass plate of refractive index 1.5 is introduced in one of the arms of a Michelson interferometer using light of wavelength 5890 angstrom, there is a shift of 10 fringes. The thickness of the plate is
- A.5.89 m
- B.5.89 times ten to the power minus three metre
- C.5.89 times ten to the power minus six metre
- D.5.89 times ten to the power minus four metre
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Inserting a plate of thickness t in one arm makes the light cross it twice, so the extra path introduced is twice the thickness times the quantity refractive index minus one.
Read the full explanation →The phenomenon that establishes the transverse nature of light waves is
- A.Interference
- B.Diffraction
- C.Polarisation
- D.Dispersion
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Only a transverse wave, in which the vibrations are at right angles to the direction of travel, can have those vibrations restricted to a single plane by a Polaroid.
Read the full explanation →The brilliant colours seen in a soap bubble or in a thin film of oil spread on water are caused by
- A.Interference of light reflected from the two surfaces of the film
- B.Total internal reflection inside the film
- C.Dispersion of light as in a prism
- D.Scattering of light by the molecules of the film
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The light reflected from the upper surface of the thin film and the light reflected from the lower surface travel slightly different distances and interfere.
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