Light can be regarded as a wave or as a particle -- the photon. In a vacuum, the waves travel at speed c while the photons travel at a speed which
(A) always equals c.
(B) is always less than c.
(C) is always greater than c.
(D) depends on its momentum.

Questions --- What this question is about
121.2

Light can be regarded as a wave or as a particle -- the photon. In a vacuum, the waves travel at speed c while the photons travel at a speed which
(A) always equals c. --- Yes.
All of the waves which make up a packet
travel at speed c, so the packet does too.

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Questions --- What this question is about
121.2

Light can be regarded as a wave or as a particle -- the photon. In a vacuum, the waves travel at speed c while the photons travel at a speed which
(B) is always less than c.
No. --- Then some observer would see
photons at rest.

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Questions --- What this question is about
121.2

Light can be regarded as a wave or as a particle -- the photon. In a vacuum, the waves travel at speed c while the photons travel at a speed which
(C) is always greater than c.
No. --- Then some observer would see the
photons arrive before they are sent.(!!??)

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Questions --- What this question is about
121.2

Light can be regarded as a wave or as a particle -- the photon. In a vacuum, the waves travel at speed c while the photons travel at a speed which
(D) depends on its momentum.
No. --- That turns out to be true for massive
particles, but not for photons.

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Questions --- What this question is about
121.2