Question

1) When an iron rod is heated to hundreds of degrees Celsius, it begins to glow red. Why does it do this? (Hint: think about Feynman’s “jiggling” atoms, atomic structure, and the nature of light)

2) We saw in lecture that light waves can be polarized. Can sound waves be polarized? If so, give an example of polarized sound. If not, explain why not.

3) Helium-neon lasers emit light with a wavelength of 633 nm (6.33×10−7 m). (a) Is this light visible? If so, what color is it? (b) What is the frequency of this light? (c) What is the energy of one photon from this laser? (d) Suppose the power output of a helium-neon laser is 0.005 W. How many photons per second are emitted?

Answer 1

1) For metals, when they are heated, the energy of the electrons increases. As the energy increases, the photons that are emitted fall in the wavelength of the visible spectrum. Initially, the metal is emitting infrared radiation invisible to the naked eye. When the emitted photons get more energy, they reach the red region of the visible spectrum. Persistent heating will take you to more colors in the spectrum i.e. Red --> Orange --> Yellow --> White.