Today, I was wondering whether an object that is emitting light in all the directions can have a shadow. The idea is that there are two emitters, one of them throws off light in all the directions while the other throws light at the other emitter. So, would it be possible for the object emitting light to have a shadow? This question can be explained using an example, a star is an emitter of light and lightens up everything around it. Assuming that rthe brightness of the star is constant throughtout (Even though it's quite unlikely), is it possible for it to have a shadow? Shadows can be expressed as absence of light realtive to the object and the plane, so if a star brighter than the other star is close-by, won't the star have a shadow? (Assume that the plane is a white wall)
4 Answers
Everything will have a shadow, but in reference to some other brighter object, Think this as an absence of photons( presence of less photons) compared to the environment.
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An example of a light source casting a shadow is found in eclipsing binary stars. As the stars rotate around each other, the shadow of star A cast by the light of star B sweeps through space (and vice versa). When that shadow (temporarily) includes Earth, we see a dimming of the light because we're only getting light from one of the stars. This can be thought of as though star A is casting a shadow on us, blocking the light we'd normally get from star B.
Graphic from Encyclopedia Britannica
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Yes , emitters can also have shadows! The thing is we need to have a large source of light as the second emitter , lets take 2 emitters one is the sun and second is a candle or campfire shadows are just absence of light right , when the sun's light not able to reach the ground due to these fire's blocking it will create a shadow relative to the ground without blocking. Just relative to the ground without light blockage seems much brighter than the ground being blocked by these light particles bouncing off and creating a "less light zone". Here the light of the emitter creates a very negligible difference as the sun outshines it much more!
Yet there are no white walls to project the stars shadow towards , but when comparing the magnitude of light of both stars if one is much brighter then we will get a similar effect
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A flame can have a clear shadow under special circumstances. It makes a great classroom demo in a Modern Physics class.
Take an atomic emission light source. Sounds tricky, but sodium vapour lamps are very common, both as lab equipment and as some street lights. So lets use sodium.
Turn on your sodium vapour lamp and let it warm up to full brightness.
Next, light a bunsen burner or propane torch. Place it between the lamp and a white screen. The flame will be nearly invisible, and will not cast a shadow.
Now, spray some salt water as a mist into the flame.
Two things will happen. The flame will turn the same yellow colour as the lamp, because the heat of the flame is exciting the sodium in the flame to emit the exact same spectrum as the sodium in the lamp.The flame will add some yellow light to the lamp's light on the screen.
But the flame will also cast a shadow on the screen! The unexcited atoms of sodium in the flame will absorb the exact matching sodium light from the lamp. The flame will block some of the light from the lamp from reaching the screen: the very definition of a shadow.
Here's an illustration:
from the article at https://www.thenakedscientists.com/get-naked/experiments/dark-flames#:~:text=Sodium%20gas%20is%20very%20very,making%20the%20flame%20look%20dark.
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