Sunlight reaching the earth is
Answer: A
Thermal emission from the sun's surface has no preferred direction, so all polarisations are present in random proportion. Scattering in the atmosphere partially polarises the light of the sky, which is why a polarising filter darkens it.
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A wave incident on a boundary where the second medium has the higher impedance is reflected with
Answer: B
The reflection coefficient is positive when the second impedance is the larger, so the reflected electric field keeps its sign. Going the other way, toward a lower impedance, reverses it, and a conductor is the extreme case of that.
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The number of decibels corresponding to one neper is
Answer: C
One neper is a field ratio of e, and twenty times the logarithm to base ten of e is 8.686 decibels. Attenuation constants are quoted in nepers per metre in field theory and in decibels per metre in communication engineering, so the conversion is often needed.
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A field falling to half its value corresponds to a loss of about
Answer: D
Twenty times the logarithm of one half is minus 6.02 decibels. Halving the power, which is halving the square of the field, gives three decibels instead, and confusing the two factors is the commonest slip with decibels.
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The wavelength of a 2.4 GHz wave in free space is about
Answer: A
Dividing three times ten to the eighth by 2.4 times ten to the ninth gives 0.125 metres. A half-wave antenna at that frequency is about six centimetres long, which is why wireless equipment at this band can be so small.
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A dielectric slab is used as a radome in front of an antenna. To avoid reflection its thickness should be
Answer: B
A half-wave slab transforms the impedance behind it into itself, so the antenna sees free space and nothing is reflected. A quarter-wave slab of the geometric mean impedance is the matching case for two different media, which is a different problem.
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A wave in a medium with a complex permittivity loses energy to
Answer: C
The imaginary part of the permittivity accounts for the lag of the polarisation behind the field, which dissipates energy quite apart from any free carriers. It is what heats food in a microwave oven, since water's relaxation falls in that band.
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The Poynting vector of a plane wave points
Answer: D
Being the cross product of the two fields, it is perpendicular to both and therefore along the axis of travel, and its magnitude is the power per unit area carried. The detailed treatment of energy flow belongs to the following topic.
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The energy in a plane wave in free space is divided between the two fields
Answer: A
Substituting the impedance relation into the two energy density expressions shows them to be equal, which is a satisfying symmetry hidden by the very different numerical values of the fields. The total density is therefore twice either one.
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A wave whose amplitude falls off as the inverse of distance is characteristic of
Answer: B
The power spreads over a sphere whose area grows as the square of the radius, so the power density falls as the inverse square and the amplitude as the inverse first power. Far from the source a small patch of that sphere is very nearly a plane wave.
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