29.3 Photon energies and the electromagnetic spectrum  (Page 6/16)

A microwave oven produces electromagnetic radiation in the radio portion of the spectrum. These microwave photons are absorbed by water molecules, resulting in an increase in the molecules’ rotational energies. This added energy is transferred by heat to the surrounding food, which as a result becomes hot very quickly. If the energy absorbed by a water molecule is 1.0 × 10 ^–5 eV, what is the corresponding wavelength of the microwave photons?

1. 1.22 GHz
2. 2.45 GHz
3. 4.90 GHz
4. 9.80 Hz

In the intensity versus frequency curve for x rays ( [link] ), the intensity is mostly a smooth curve associated with bremsstrahlung (“breaking radiation”). However, there are two spikes (characteristic x rays) that exhibit high-intensity output. Explain how the smooth curve can be described by classical electrodynamics, whereas the peaks require a quantum mechanical interpretation. (Recall that the acceleration or deceleration of electric charges causes the emission of electromagnetic radiation.)

Why are UV, x rays, and $\gamma$ rays called ionizing radiation?

How can treating food with ionizing radiation help keep it from spoiling? UV is not very penetrating. What else could be used?

Some television tubes are CRTs. They use an approximately 30-kV accelerating potential to send electrons to the screen, where the electrons stimulate phosphors to emit the light that forms the pictures we watch. Would you expect x rays also to be created?

Tanning salons use “safe” UV with a longer wavelength than some of the UV in sunlight. This “safe” UV has enough photon energy to trigger the tanning mechanism. Is it likely to be able to cause cell damage and induce cancer with prolonged exposure?

Your pupils dilate when visible light intensity is reduced. Does wearing sunglasses that lack UV blockers increase or decrease the UV hazard to your eyes? Explain.

One could feel heat transfer in the form of infrared radiation from a large nuclear bomb detonated in the atmosphere 75 km from you. However, none of the profusely emitted x rays or $\gamma$ rays reaches you. Explain.

Can a single microwave photon cause cell damage? Explain.

In an x-ray tube, the maximum photon energy is given by $\text{hf}=\text{qV}.$ Would it be technically more correct to say $\text{hf}=\text{qV}+\text{BE,}$ where BE is the binding energy of electrons in the target anode? Why isn’t the energy stated the latter way?

What is the energy in joules and eV of a photon in a radio wave from an AM station that has a 1530-kHz broadcast frequency?

(a) Find the energy in joules and eV of photons in radio waves from an FM station that has a 90.0-MHz broadcast frequency. (b) What does this imply about the number of photons per second that the radio station must broadcast?