Where does light "go"?

[Olgar Shiverstone]

I like this theory the best as it is backed up by the most evidence.

Let's test it! I'll hold the torch; you take a stroll down yonder dark, twisty labyrinthine corridor ...

 

[Umbran]

It spreads out. There is the same amount of it, but it has to fill a larger area.

A 2D analogy would be a ripple of water in a pond: As the ripple moves away from the source, the ring expands.

Correct.

A curious thing: The wavelength of the light doesn't change. When the light finally interacts with something, it does so with the same wavelength as when it is emitted. With particle mechanics, that means the interaction over a large possible area occurs at a single point.

Somewhat correct. If you were in a static, flat universe, the wavelength of light doesn't change. Over short distances in our actual universe, (and by "short" I mean "within the same galaxy") this is largely true. However, in an expanding universe, as light travels over truly great distances (sometimes called "cosmological distances") it gets red-shifted. This is where the "microwave background" you hear about comes from. That's the leftovers of very high-frequency light that's been travelling since the beginning of the universe has gotten shifted down, down, down into microwaves.

 

[Bullgrit]

The "shotgun effect" works for light as particles, but how does it jibe with light as waves?

And a related question: You know how a shadow can have a sharp outline at one distance but a fuzzy outline at a different distance? Why?

Bullgrit

 

[tomBitonti]

The "shotgun effect" works for light as particles, but how does it jibe with light as waves?

And a related question: You know how a shadow can have a sharp outline at one distance but a fuzzy outline at a different distance? Why?

Bullgrit

For wave mechanics, the wave gets "shorter" (the wave front loses intensity). Think of a soap bubble of an initial fixed width expanding. As the bubble gets larger the bubble must get thinner to cover the larger area. Wikipedia has a good enough page on this: http://en.wikipedia.org/wiki/Inverse-square_law

For shadows, the blurring can be two different effects.

One depends on the light source: Unless the light is a point source, the shadow has two regions, the penumbra (partial shadow) and umbra (full shadow). See http://van.physics.illinois.edu/qa/listing.php?id=2068 for a good diagram.

Two, light diffracts. Classically, this is demonstrated with a single slit (see http://www.google.com/imgres?biw=20...tbnh=202&tbnw=249&start=0&ndsp=41&tx=75&ty=70). Search for "light diffraction physics"; there is a huge amount of information out there. This works for any edge -- you can think of the edge of an object casting a shadow as one of the edges of a single slit.

The result is a blurring of the edge of the shadow.

Thx!

 

[tomBitonti]

To followup, there are two other possible causes:

An object with a fuzzy border (say, a tennis ball), will have a similarly fuzzy shadow. But, I'm pretty sure your question was about objects with sharp outlines casting fuzzy shadows, not about fuzzy objects casting fuzzy shadows. Unless it was a trick question!

There will be some observational limit to the perception of the shadow. That is, optical distortions, and fundamental limits to the ability to resolve details.

Out of all of this, there is a real question of which of these effects matters in everyday experience. All of the effects are real, but one or another may surpass the others, perhaps to a very large degree.

My hunch is that most light sources not being point sources has the largest effect, for everyday shadows under simple viewing by the human eye.

Thx!

TomB

 

[Umbran]

The "shotgun effect" works for light as particles, but how does it jibe with light as waves?

Think of it using the ripple in the pond analogy. When you start, all the energy of the wave is in a circle of a small radius. As the wave moves outwards, it doesn't gain in energy, but hte total circumference increases - so the same energy is spread over a greater area.

And a related question: You know how a shadow can have a sharp outline at one distance but a fuzzy outline at a different distance? Why?

While tomBitoni mentions diffraction, I am pretty sure that's not the dominant thing for the objects in most of your everyday experiences. The umbra/penumbra effect is more likely the one you're thinking of. I'd need to be able to draw you a diagram to make it most clear, I'm afraid.

 

[tomBitonti]

While tomBitonti mentions diffraction, I am pretty sure that's not the dominant thing for the objects in most of your everyday experiences. The umbra/penumbra effect is more likely the one you're thinking of. I'd need to be able to draw you a diagram to make it most clear, I'm afraid.

Yeah, realized that a few minutes after finishing my first post. A question of practicality in context!

Would be interesting to figure out the penumbra width for different cases, e.g., the shadow of Jupiter's moons against Jupiter, compared with the shadow of a person caused by a standard incandescent bulb, or caused by the sun, or the shadow of the moon on the earth. Or, the crispness of the transition when an asteroid occults a star.

Thx!

TomB

 

[Fast Learner]

The Wikipedia article on umbra/penumbra/antumbra has some nice diagrams that help explain it.

 

[Janx]

sorry i don't know about it

Are you just posting to boost your # of posts as a new guy or a as a future spammer account. Because you keep posting the same "sorry i don't know about it" to everything. I hate to be rude to a honest new person, but a spammer I got no qualms.

If you ain't got anything to say about a thread's topic, you don't need to post.