Reflectivity, Light and The Mirror

[Turtle]

you have an answer for everything dont you Turtle! by average I meant my bathroom mirror..

___I certainly create that illussion, don't I.:hihi: But then you make it easy by posing such excellent questions.:)

 

It's a bathroom mirror, whats front-surfaced silvered and back surface silvered.

 

___The front surface mirror is silvered on the surface the incoming rays of light strike first. A polished piece of metal is a front surface mirror.

___The back surface mirror has the silvering on glass & the incoming light rays pass through the glass before reaching the silvering & then they are reflected back out through the glass. The 'common' household mirrors are of this type.

___In the back silvered arrangement, many of the factors we already discussed still matter, e.g. the flatness of the glass, and the kind of material used for the silvering. Additionally the chemical composition of the glass, its color, and its thickness play a role in the character of the light reflected from it.

___The big drawback of the back surface mirror is the multiple reflections it gives & the bending of the light path. Think about looking through a pool of water as you poke a stick into it. The stick appears to bend & go at a different angle under the surface. When light traverses a boundary from one medium to another it is bent. The same happens in a bathroom mirror as the light passes the air/glass boundary and the glass/silvering boundary.

___The front surface of the glass is reflective as well & some light never reaches the silvering at all. Inside the bathroom mirror the same is true, where some light rays reflected from the silvering reflect from the inside of the glass/air boundary back toward the silvering.

___I need some coffee now!;) :D

[LJP07]

After reading that, I need something a little stronger than coffee :hihi:

[LJP07]

So from what I gather, light will go in all different directions, but what is light? It's a difficult concept for me to understand even though it probably has a simple answer, if I turn on a light bulb, power will be generated for light to appear, but is light just that, light?....Lighting up the area around it.

 

Please clarify light using the above example in fine detail?

[Jay-qu]

Light has a dual nature. Some models see light as a wave of electro-magnetic radiation. While others see it as a particle. It is useful to think of it as both at different times.

[cwes99_03]

That is to say, light acts as both a wave and a particle (photon) all the time. How we describe the way it is acting depends on our understanding of how particles and waves act. So for some applications we describe light like a massless particle that bounces around and travels at c. Otherwise we treat it like a wave that vibrates with a particular frequency.

 

When discussing lenses we pretty much treat it like a wave, but when we talk about reflection or scattering, we largely treat it like a particle.

[LJP07]

So the light I see when I turn on the lightbulb is massless particles moving in all different directions called Photons. Also, they are waves in certain respects.

 

But where do the photons come from when I turn on the light bulb, how can they instantaneously appear when I turn on the light, wheres their origin?

[Jay-qu]

They dont instantanesouly apear, the photons travel at a finite speed, which is c in vacuum and slower in more optically dense materials such as air.

 

The photons origin is from the electons that are in the filliment of wire in the lightbulb - but I dont know exactly how they emit the photons.. someone else may wish to fill that blank

[Qfwfq]

Incandescence.

 

The filament becomes very, very hot. Look up blackbody spectrum.

 

Other types of light don't have a filament but use gas instead, the first to be used was neon so it became common to call them all "neon lights", this gas emits its spectrum according to its orbital energy levels. Many street lights use mercury vapour. The most common non-incandescent kind, for domestic lighting purposes, is the flourescent light in which the gas produces ultraviolet but a thin layer of stuff on the inside of the glass turns the UV into visible light.

[Jay-qu]

is the flourescent light in which the gas produces ultraviolet but a thin layer of stuff on the inside of the glass turns the UV into visible light.

 

Phosphorus I think..

 

Ok I get how the electons heat it up and then it emits as a black body, but why? is black body physics hard to explain?

[cwes99_03]

Nope. I've already stated how. Heat is just another form of energy. The gas inside a fluorescent light doesn't really heat up, but is instead excited by acceleration.

 

All that is required for light production is to bump the energy of a valence electron enough to cause it to jump up an energy orbital and then collapse back down to the lower state. This produces finite amounts of energy which are released in the collapse of the electron in different colors of light (which depend on the difference between the excited state and the collapsed state).

 

Thus a light emitting diode has "holes" in its substrate where the electrons can exist at a lower potential. When the electrons cascading across the substrate (due to an electrical field) fall into one of these "holes" they give off the energy in the form of light.

[LJP07]

How would it be described for the Sun, or that of any other thing that emits light, there are no filaments in the sun, so where are the photons emitted from?

 

" This produces finite amounts of energy which are released in the collapse of the electron in different colours of light" = Then if it's different colours, why is white light given off and not any preference to any other colour for the bulb?

[cwes99_03]

To answer the first question, the gas that the sun is made up is what fluoresces.

 

To answer the second question, a single gas can only fluoresc (sp?) at certain frequencies because it has a finite set of orbitals which are free for electrons to move between. Metals have many more orbitals (and electrons), and that leaves a much larger set (though still finite) of frequencies from which to produce visible light. Look up the visible spectrum emitted by hydrogen gas compared to neon, and then compare that to tungsten. The number of lines in the tungsten spectrum is huge.

 

There is also something to be said for black body radiation. These obey a physical law which specifies the spectrum range and the intensity of a body radiating heat.

 

I'm not an expert, just regurgitating things I remember from Physics 201.

[LJP07]

Could you describe Fluorescent and Incandescent in laymans terms, because it seems to be about them and I'd like an understanding of them.

[Qfwfq]

Flourescence is quite loosely when a substance, struck by radiation of one kind, emits another kind. Flourine compounds were the first kind of substance in which it was observed.

 

Incandescence is when the blacksmith heats up a piece of iron in the furnace; when he takes it out it glows for quite a while and we say that it is "red hot". As it cools it will become duller. If it is heated even more it will glow orange, then yellow, then white and even bluish. By that time though, it would burn if there is air, to prevent this happening the inside of a light bulb has no oxygen.

[LJP07]

Maybe a sill question, however, I was watching a documentary about Quantum Mechanics, Light etc,... and it was talking about how time differs with increasing velocity, that another person nearer the speed of light, time would be different there than it would be here. Would it be the same for light travelling at that speed. For example, if I was in an aircraft travelling at near the speed of light or even over it, would the light be affected in any way on the aircraft, seen as time differs, I'm wondering would light?

[eric l]

Maybe a sill question, however, I was watching a documentary about Quantum Mechanics, Light etc,... and it was talking about how time differs with increasing velocity, that another person nearer the speed of light, time would be different there than it would be here. Would it be the same for light travelling at that speed. For example, if I was in an aircraft travelling at near the speed of light or even over it, would the light be affected in any way on the aircraft, seen as time differs, I'm wondering would light?

Isn't that the famous Doppler-effect ? The spectral lines from the light of any star (or object) shift more towards the red as the speed with which the object is speeding away increasen ?

[cwes99_03]

Basically correct. The frequencies observed would be affected by the speed you are traveling at, however the speed of the light would be measured the same.