Thought experiment with two point sources of light

In summary, the conversation discusses being in a dark room with no other sources of light except for two point sources of equal power placed 10 feet apart. The question is posed about what kind of shadow will be cast when a thick rod is placed in the middle of the two light sources and 4 feet in front of them. The responses explain that there will be two shadows, with each one being less dim than the other depending on the brightness of the lights. Further discussion includes the possibility of a third shadow from the intersection of the two shadows, the role of the walls and ceiling, and the concept of shadows not being able to cancel out each other. Ultimately, the answer is left ambiguous due to the lack of specific details in the initial
  • #1
PaulMuaddid
13
0
You are in a dark room with no other sources of light
The walls are painted black, so no light is scattered from walls

You have two point sources of light of equal power + same height about ground
They are 10 feet apart

You stand a thick rod vertically in middle of two, and 4 feet in front of the point sources

What kind of shadow will be cast?

1) one shadow from one lamp?
2) two shadows from both lamps?
3) only intersection shadow from both lamps?
4) other?

Explain your answer!
:smile:
 
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  • #2
depending on how bright the lights are, you'd see two shadows. If they were reaaaally bright, there'd be no shadows, but if they weren't so bright you'd see two shadows. Each shadow would be not as dim as would be if the other light weren't on.

I'm basing this on personal experience of remembering shadows on auditorium stages with multiple lights. Usually the object would cast as many shadows as there were lights above it, but the lights that were further away (or dimmer) would cast shadows that weren't as "strong" as the shadows cast by the lights that were closer (or brighter).
 
  • #3
Almost...
:smile:

With answer comes another question, but there are more than 2 shadows.
 
  • #4
No. Whether Paul Muadid intended it or not, it's a trick question.

No shadows are cast.

"...no light is scattered from walls."

:)
 
  • #5
No light is scattered from walls, but the two point sources of light are generating light.

(in other words, the walls do not enter equation)
 
  • #6
PaulMuaddid said:
No light is scattered from walls, but the two point sources of light are generating light.

(in other words, the walls do not enter equation)
So, where do these shadows of which you speak get cast? :) But OK, let's grant the walls don't play in.

You get 4 shadows.

Each source casts a shadow of the rod and the other light source.
 
  • #7
PaulMuaddid said:
No light is scattered from walls, but the two point sources of light are generating light.

(in other words, the walls do not enter equation)

if no light is scattered from the walls, then how do you see the walls? And therefore how do you tell the difference between wall and shadow?
 
  • #8
There is a floor, the shadow (whatever shadow that would be) would be cast on the floor.
 
  • #9
PaulMuaddid said:
There is a floor, the shadow (whatever shadow that would be) would be cast on the floor.

You didn't say that there was a floor, or what color it was painted. All relevant information should be given in the problem statement, otherwise it's not a properly posed problem.

Should we ask about the ceiling? :devil:
 
  • #10
There's a ceiling, it too is black and non-reflective.
Floor is white and can receive cast shadows.
 
  • #11
Perhaps the OP simply wishes to say "the walls reflect little enough light that any secondary reflections can be ignored".
 
  • #12
A "shadow" counts as a separate shadow if it is from a different source OR if its gradient is different.
We're only interested in shadows cast onto floor.
There are three.

Shadow.jpg


I'm still not sure why shadow 2 and 3 are not completely canceled out.
Shadow #2 is closer to left light source, so the left light source should completely cancel out the #2 shadow.
The same for #3.

Does it depend on how bright the light is?
Shadow #1 (the intersection of both shadows) is darker than #2 and #3
 
  • #13
PaulMuaddid said:
A "shadow" counts as a separate shadow if it is from a different source OR if its gradient is different.
We're only interested in shadows cast onto floor.
There are three.
Where did you get this statement from? Source please.
PaulMuaddid said:
Shadow.jpg


I'm still not sure why shadow 2 and 3 are not completely canceled out.
Because shadows don't "cancel out". If you stood at a point in one of those shadows and took a light meter reading, how much light do you think you'd measure? Would it be less than you'd measure out in full light of both sources? Yes? Then you're in the shadow of at least one light regardless of how many other lights are shining on you..

One light source, one shadow. Two light sources, two shadows. The darker area is simply the intersection of the two shadows. It is not a third shadow.BTW, the lights themselves will cast shadows of each other. Do you choose to ignore this?
 
  • #14
The words Umbra and Penumbra come to mind.
The problem isn't presented in an unambiguous way so it's hard to be sure what they're after.
I don't know how any shadow can "cancel out" another. The sum of the illumination 'outside' any shadow must be greater than the illumination within a shadow.
 

FAQ: Thought experiment with two point sources of light

1. What is a thought experiment with two point sources of light?

A thought experiment with two point sources of light is a hypothetical scenario used in physics to understand the behavior of light. It involves two point sources of light, which are objects that emit light in all directions, and observing how the light from these sources interacts with each other.

2. How does a thought experiment with two point sources of light help in understanding light?

By setting up this thought experiment, scientists can analyze the behavior of light in a controlled and simplified scenario. This can help in understanding complex phenomena such as interference, diffraction, and polarization of light.

3. What are some real-world applications of a thought experiment with two point sources of light?

Thought experiments with two point sources of light have practical applications in fields such as optics, astronomy, and telecommunications. They can be used to design and optimize optical devices, study celestial objects, and improve the efficiency of communication systems.

4. Can a thought experiment with two point sources of light be used to study other types of waves?

Yes, the principles of a thought experiment with two point sources of light can be applied to other types of waves, such as sound waves and water waves. This allows scientists to gain a better understanding of the behavior of waves in general.

5. Are there any limitations to a thought experiment with two point sources of light?

While thought experiments are useful for understanding complex concepts, they are not a replacement for real-world experiments. The results of a thought experiment may not always accurately reflect the behavior of light in the physical world, so it is important to validate these findings through experimentation.

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