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Producing umbra and penumbra with light sources

  • Thread starter queen2020
  • Start date
when two souces are used, how many sources must be blocked by the object to produce the unbra? and the penumbra?

I looked it up in my text book and on the internet and i still can't find the answer.
 
help please

Can someone help me please
 

HallsofIvy

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Both have to be blocked for the "umbra", only one for the "penumbra".
 

Hurkyl

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Do you know the definitions of umbra and penumbra? The answers should have been obvious. Even doing something as crude as punching

"define umbra"
"define penumbra"

into google and looking at the web definitions gives us phrases like:

For penumbra:
"Literally partial shadow; the partially lighted area around any completely darkened area (umbra) of full shadow."
"The shadow that results when only part of the bright object is occulted"
"region of partial shadow that is outside the umbra; the light source is partially blocked."

For umbra:
"The area of total darkness in the shadow caused by an eclipse."
"The umbra is the area of total shadow (compare to penumbra)."
"The portion of a shadow that receives no direct light from the light source."
 
thank you so much
 
I need help with shadows

When three sources are used, how many sources must be blocked by the object to produce the umbra? the dark penumbra? and the light penumbra?

Could anybody help me please.
 

Hurkyl

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Have you tried at all to figure it out yourself?
 
yes i have tried to figure it out but i can't figure it out at all
 

Hurkyl

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In your own words, what are the definitions of "umbra", "dark penumbra", and "light penumbra"?

Can you at least identify the conditions on being in the umbra or penumbra?
 
Last edited:
The umbra is part of the shadow that is entirely dark. i don't know the definitions of dark penumbra or light penumbra.
 

Hurkyl

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Ok, that's good. Can you describe the penumbra? Do you have a guess as to what "light penumbra" and "dark penumbra" mean? I think that "light" and "dark" weren't being used as technical terms here, just descriptive.
 
so i would put a part where some light falls.
 

Hurkyl

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I don't understand what you've suggested.
 
When three sources are used, how many sources must be blocked by the object to produce the umbra? the dark penumbra? and the light penumbra?
 

Hurkyl

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Didn't you already figure one of those questions out?
 
i don't know
 

OlderDan

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queen2020 said:
i don't know
Draw yourself some diagrams. If you have one light source blocked by an object, there will be a shadow that has an umbra and a penumbra. If the shadow falls on a screen, you can draw the umbra and penumbra areas that hit the screen. I will assume you know what those are. If you have two light sources, each source will have its own umbra region and penumbra region behind the object. On a screen, there may or may not be some umbra overlap, some region where the umbra of one shadow overlays the penumbra of a second shadow, and some regions where only penumbra overlap. There could be no overlap at all if the screen is too far away from the object, but you can move the screen closer to get all the different kinds of overlap that are possible. With three sources you can have more combinations of overlap contitions including: three umbra overlap, two umbra and one penumbra, one umbra and two penumbra, three penumbra. Try to sketch these various possibilities as they might appear on a screen and then answer the question.
 

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