# Homework Help: Light + other problems

1. Apr 25, 2005

### vangto

I have my final exam coming up on tuesday and I've been going over past examinations. I mainly have problems with E &M and the particle/wave nature of light. I have some questions that I wasn't able to answer myself that I was hoping people could help me out with.

1. Why does the photoelectric effect suggest that light has particle-like
properties? Orange light can generate photoelectrons from the metal potassium,
but red light cannot. Name another color of light which can produce the

2. Why do the magnetic field lines outside of an isolated bar magnet never
cross?

3. What does the double slit experiment using electrons tell us about the
wave and particle nature of electrons?

4. Can atoms ever be at rest? Answer this question from the point of view of
classical and quantum mechanics. Think about cooling a gas of atoms to near
absolute zero temperature.

5. What is the perihelion shift of the planet Mercury? How does the
measurement of this quantity support the predictions of the General Theory of Relativity?

for this last question, I know that the perihelion is the closest point of a planets orbit to the sun. I also know that the axis of perihelion shifts slowly over time. Basically I do not know why, and how it is related to general relativity.

Thanks in advance for any help that can be offered.

2. Apr 25, 2005

### OlderDan

Too late to touch on all those topics, but I'll do one paragraph on the first one. In wave theory, the energy of the light is related to the intensity. Bright light has a lot of energy; dim light has little energy. In the photelectric effect, no matter how bright the red light is it cannot induce potassium to give up any electrons. Yet even a dim orange light causes some photoelectrons to be released, and the number of phototelectrons increases with the intensity of the orange light. This suggests that light energy can only be absorbed in discrete amounts, and that it can only be absorbed if one "quantum" of energy is sufficient to release an electron. A lot of quanta with sufficient energy will release a lot of electrons.