I am attempting to do this Modern Physics Homework and i cannot wrap my mind around these three questions:

34. Three observers located at E, F, and G are watching two bombs located close to Observers E and F. Both bombs explode simultaneously. Explain how the observers see the pattern of explosions.

47. A farmer is working with Iron to make a horseshoe. If the iron had to be at least 3000K before it could be bent, how would the farmer know the temperature of the iron simply by looking at it?

49. What is meant by the threshold frequency of the photoelectric effect?


***If you can respond to any of these questions, it will be greatly appreciated.

49. What is meant by the threshold frequency of the photoelectric effect?

This is the minmum frequency of incident light required for electrons to be emitted.
According to quantum mechanics, the energy of a photon is related to its frequency by the relation

E = h f where h is the plancks constant. Thus this corresponds to the minimum energy of the photon required to knock out the electron.

This energy corresponds to the work function of the material

47. A farmer is working with Iron to make a horseshoe. If the iron had to be at least 3000K before it could be bent, how would the farmer know the temperature of the iron simply by looking at it?

When the piece of iron is white hot, its temp should be over 5000 K i think. So he should wait till it is white hot. When heated initially only its temperature increases, i.e. there is no visual emission. Later red light is emitted and gradually all other colours also are giving the impressionof it being "white hot".

Look up blackbody spectrum.

34. Three observers located at E, F, and G are watching two bombs located close to Observers E and F. Both bombs explode simultaneously. Explain how the observers see the pattern of explosions.


Obviously E will see the explosion closer to him first and G will see the explosion closer to him first as light has to travel less distance.

As for G, that would depend on where G is located. If he is at the midpoint, or at equal distances from E and F,he would find them to be simultaneous.

But in practice , the speed of light is so high, that they will appear simultaneous to all three as it will be impossible to distinguish the time difference with our eyes.

The speed of light is so high it can circle the earth many times a sec!