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Author Topic: on reflection  (Read 3322 times)

Offline time-cop

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on reflection
« on: 18/05/2006 23:41:37 »
in one of my more intelligent moments i was wondering,does a reflection weigh anything,i know that light exerts a force,so a reflection must also exert some force,but does it have any mass that can be weighed.
not my usual sort of post,but what the heck,we all have off days ;)

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Offline Bass

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Re: on reflection
« Reply #1 on: 19/05/2006 03:33:19 »
My reflection in the mirror droops alot more now than it did in my younger days- so it must accummulate mass with time!

Subduction causes orogeny.
 

Offline daveshorts

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Re: on reflection
« Reply #2 on: 19/05/2006 11:22:09 »
If light reflects off something it's momentum will change, to do this it must have had a force applied to it. In fact twice as much force as if it had just stopped. I am not sure if that answers your question.

Space scientists want to take advantage of this effect, by building huge reflective solar sails which are pushed around the solar system by sunlight. This is a tiny force, but it keeps on going for ever, and you don't need to carry your own fuel, which means that a solar sail can be much faster than a conventional rocket if you have to go a long way.
 

another_someone

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Re: on reflection
« Reply #3 on: 19/05/2006 11:43:38 »
quote:
Originally posted by daveshorts

If light reflects off something it's momentum will change, to do this it must have had a force applied to it. In fact twice as much force as if it had just stopped. I am not sure if that answers your question.



Firstly, force is not itself the same thing as mass or weight.

Secondly, what is meant by reflection.  Normally, this refers to a reflected image, which could be anything from bright white to almost total black and thus could contain a lot of light or almost no light.

Clearly, the object that is causing the reflection must have sufficient mass in order to be able to reflect the light, rather than merely absorb it, but even this mass need only be on an atomic scale, and it relates to the mass of the mirror not the mass of the image.

quote:

Space scientists want to take advantage of this effect, by building huge reflective solar sails which are pushed around the solar system by sunlight. This is a tiny force, but it keeps on going for ever, and you don't need to carry your own fuel, which means that a solar sail can be much faster than a conventional rocket if you have to go a long way.




But only so long as you stay fairly close to a star I would have thought not much use in interstellar space.



George
 

Offline time-cop

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Re: on reflection
« Reply #4 on: 19/05/2006 23:50:20 »
so what i see in the mirror isnt me,its light reflecting off me and other objects around me,so really ive just answered my own question,the light does weigh something,albiet a very finite amount.
now im going to have to go away and think about this one,and see if i can find another intelligent question,thank you all.

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another_someone

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Re: on reflection
« Reply #5 on: 20/05/2006 00:18:14 »
quote:
Originally posted by time-cop

so what i see in the mirror isnt me,its light reflecting off me and other objects around me,so really ive just answered my own question,the light does weigh something,albiet a very finite amount.
now im going to have to go away and think about this one,and see if i can find another intelligent question,thank you all.

http://www.armybarmy.org



Actually, we have a slight problem here.

Light has inertia and energy, but not mass (this paradox is a privilege bestowed upon a photon in consequence to the fact that it travels at the speed of light).  That it has no mass would, in my understanding, also mean it has no weight.



George
 

Offline time-cop

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Re: on reflection
« Reply #6 on: 20/05/2006 00:30:48 »
this is going to do my head in,i thought that for something to exert a force, it must have some mass,to build momentum to exert the said force in the first place,so if light has no mass,how can it build enough momentum to exert a force to push against something else,blimey george i havent had to think this hard in years,do you understand the above ?

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another_someone

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Re: on reflection
« Reply #7 on: 20/05/2006 01:25:14 »
OK, strictly speaking, I should have said that a photon does not have any rest mass.

As objects move faster (close to the speed of light), they increase in mass.  When an object actually gets as fast as the speed of light, it has an infinite increase on its rest mass.  Most objects, having finite rest mass, will therefore have an infinite mass when they reach the speed of light.  Photons, have zero rest mass, and infinite increase on zero can be any number you choose.  This allows photons to behave as if they do have mass, but only so long as they are travelling at the speed of light as soon as their speed decreases, their mass will disappear.

Ofcourse, this does lead to the question as to what happens to the mass of a photon when it does reduce velocity (e.g. because it is travelling through dense material)?



George
 

Offline tanian

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Re: on reflection
« Reply #8 on: 21/05/2006 16:37:50 »
The original question was wether a reflection has a weight. Now mass and weight are not the same thing- weight relies upon a gravitation field to exist, and in the universe at large these fields can be negligible at times. Surely the question should be whether or not a reflection has inertia- which is all that weight can be without gravitational effects.
 

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Re: on reflection
« Reply #8 on: 21/05/2006 16:37:50 »

 

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