Final answer:
Sound waves, which are mechanical and require a medium to propagate, cannot travel through empty space, unlike electromagnetic waves like light and radio which can propagate through a vacuum.
Step-by-step explanation:
The correct answer to which two words cannot travel through empty space is b) Sound waves, propagate. Sound waves are mechanical waves that require a medium, such as air, water, or solid, to propagate because the waves are pressure disturbances. Unlike sound waves, electromagnetic waves such as light and radio waves can travel through a vacuum, such as outer space. This is because the electric and magnetic fields in electromagnetic waves generate each other allowing these waves to move through even empty space.
To clarify a common misconception, light waves do not require a medium to travel. This was a historical misunderstanding, leading to the hypothesis of a luminiferous aether, which is now known to be false. Light waves, being a form of electromagnetic radiation, can travel through a vacuum. This concept is reflected by the fact that visible starlight can reach us from across vast distances in space where there is no material medium.
Wavelengths and photon energy are characteristic of electromagnetic waves. The speed of light, a constant denoted by (c) at 2.99792458 x 10⁸ m/s, is always maintained in a vacuum, and Planck's law relates the energy (E) of a photon to its frequency (ν) using E = hν, where h is Planck's constant.