What is light

[NavajoEverclear]

perhaps about cliche question, but the nature what exactly i want to know is somewhat more creative. Light is electromagnetic radiation right? So whether the frequency is visible to us or not, its all light. So radio is light, and gamma is light---- right? So then how do you determine the speed of light?

Any other information on light would be nice, but those are my main concerns i guess

[Kettle]

Try this site - it comprehensively covers the "What is light?" question.

http://www.intl-light.com/handbook/registered.html

 

It points out that optical radiation sits on a different part of the electromagnetic spectrum to radio waves - so they are different.

 

You can measure the speed of light the same way you measure anything else moving at speed - time it's start at the point of origin then time it's arrival somewhere else, divide distance travelled by time taken... et voila.

 

Galileo did an early version of this experiment with lanterns on the top of hills but, of course, light travels too quickly for him to get any useful data.

[uscphysics]

Be careful, because not all gamma rays are considered to be real light. there are real photons and virtual photons, when dealing with some of the interactions in photon decay, the virtual photon must be used.

[NavajoEverclear]

So then, is our biological range the way it is because visual light is the most lightish light (by having more photons or whatever you said)--- but not all creatures see in the same range, like insects see infrared.

Ok so you measure the speed, but i know that different frequencies of electromagnetism have different speed (slower with lower frequency) so which wave do you choose to measure as the speed of light and why?

And if gamma often isn't really light, then what is it? Maybe i should look at that site link, i'll do that now . . . . . . .

 

ohh so visible light is different because it is where the radiation behaves about as much like a particle as a wave also. Well i guess that answers my question.

[Kettle]

The reason we see the "optical" spectrum is simply because of evolution - it is the most suitable spectrum for us to go about our daily business. Likewise, other species have evolved different visual ranges to suit them eg. various reptiles (e.g. snakes) use infrared to pick up on the body heat of their prey, and I believe that insects use ultraviolet to recognise favourable plant/flower species.

 

When people refer to the Speed of Light they mean the speed at which photons (the particles that make up light) travel in a vacuum. This means that it is unrefracted by anything which would give the appearance of red, green or blue light.

[NavajoEverclear]

great now we are communicating--- it is my thought that evolution being the creator of our capabilites (or our creator, who for wise reasons did not give us perfect capabilites(which belief of such possibility i know most of you think is a stupid one, so lets just forget about it and say evolution)) is the reason i asked the question. Under which case, light we see is physically as much light as any other part of the spectrum, whether visible to us or not. So when you are measuring light in a vacum, do you measure visible, infrared, microwave, gamma, radio--- whatever you call doesn't matter. Different frequencies have different speeds, and if they are all light, then which speed to you call 'the speed of light'? (i thought i had found this answer in that link, but it appears from the nonconfirmation of it in the last post, something is not fully comprehended)

 

<This means that it is unrefracted by anything which would give the appearance of red, green or blue light.>

 

That brings me to a secondary question, i want not to be answered until those former to it are addressed : is there color in different parts of the spectrum? What defines it as color since we cant optically detect it?

[uscphysics]

E=pc is the equation you would use in a vacuum, because light is massless. So the different frequencies correspond simply to the energy, ie a phpton only carries kinetic energy. This kinetic energy determines the actual frequency of the light, while the velocity remains constant(c=3e8m/s). be careful not to use p=mv in this situation, because that would really mess you up in reasoning this.

[Kettle]

when you are measuring light in a vacum, do you measure visible, infrared, microwave, gamma, radio ... Different frequencies have different speeds, and if they are all light, then which speed to you call 'the speed of light'?

When we are talking about the speed of light in a vacuum we are only talking about the speed of visible light (to us), not infrared, gamma, etc. We are also only talking about white light ie. light that has not been refracted, reflected or absorbed.

 

is there color in different parts of the spectrum? What defines it as color since we cant optically detect it?

As I understand it, colour is a property of visible light only. We can't percieve colour outside of this (if there is any) because our eyes simply are not equipped to process it.

[NavajoEverclear]

so then when they say nothing can exceed the speed of light, thats not exactly true, because the light they are talking about isn't even the fastest light there is. Is there even really a speed limit at all?

[Kettle]

Nothing in our Universe (quantum theory notwithstanding) can exceed the speed of light (186,282 miles per second apparently). It is a universal constant, a speed limit if you will.

 

I think you are maybe confusing speed with wavelength or frequency?

 

When light moves between things such as air or water, it is scattered according to the properties of the medium it encounters and its wavelength (that's why the sky appears to be blue...?).

 

So, light travelling from the Sun to here will travel at the speed of light, it then hits our atmosphere when it is scattered (refracted) - not slowed down - so that only the "blue" wavelengths are visible (all the other wavelengths pass straight through and are, therefore, invisible to us). But (and someone please correct me if I'm wrong) it is still travelling at that constant speed.

 

Edit - uscphysics put it much more succinctly than I managed in that ramble above "kinetic energy determines the actual frequency of the light, while the velocity remains constant" - boom

[uscphysics]

In reallity the speed of light does change once it is introduced to a new medium, but in a vacuum it is at its' maximum of c. If a piece of glass were placed in a vacuum and light were produced in the vacuum, when it hits the glass, its' velocity would be less than c, but the minute it exits the glas , it should propegate at c again.

That is why it is important to not confuse the speed of light in a vacumm versus in matarial. Fiber Optics is a a good example of this.

[NavajoEverclear]

BUT visible light is slower than gamma, and radio is slower than everything----- its all the same radiation, just diff speed with dif frequency. Well i thought i had figured it out, but as i said, by the nonconfirmation of my conclusion it seems not. SO when you are saying the speed of light, which frequency is that? Visible light is NOT the fastest form of electromagnetic radiation (i'm sure you know this and may think me stupid by thinking i am implying that you are, but all i mean is to state in simplest terms possible what I mean, so you can answer it)

[superchump]

Originally posted by NavajoEverclear

BUT visible light is slower than gamma, and radio is slower than everything----- its all the same radiation, just diff speed with dif frequency.

 

Where did you hear this? In a vacuum, light of all wavelenght travels at c. It doesn't get slower because of its wavelength or frequency.

[NavajoEverclear]

in a vacuum, radio to gamma all go at the same speed? I did not know this. Thank you for enlightening me. How does it work like that though?

[Radical Edward]

Originally posted by NavajoEverclear

in a vacuum, radio to gamma all go at the same speed? I did not know this. Thank you for enlightening me. How does it work like that though?

 

the velocity of electromagnetic radiation is a property, dependent, in a vacuum, on the permittivity and permeability of free space only. the energy of a photon, unlike with massive particles, depends only on the frequency of the photon, unlike with say, a proton, in which the energy can also be kinetic.

[Radical Edward]

Originally posted by Kettle

When we are talking about the speed of light in a vacuum we are only talking about the speed of visible light (to us), not infrared, gamma, etc. We are also only talking about white light ie. light that has not been refracted, reflected or absorbed.

 

this is not true. when we talk about light in a vacuum, we are using light as an umbrella term to encompass all electromagnetic radiation. furthermore, the white light comment is not correct, since white light is nothing more than a collection of many monochromatic photons of different colour. white light does not exist as such, it is simply an equal amount of all colours.

[Radical Edward]

Originally posted by uscphysics

In reallity the speed of light does change once it is introduced to a new medium, but in a vacuum it is at its' maximum of c. If a piece of glass were placed in a vacuum and light were produced in the vacuum, when it hits the glass, its' velocity would be less than c, but the minute it exits the glas , it should propegate at c again.

 

it only appears to slow down: the reason it takes longer to get through the glass is because the individual photon is interacting with the material, which takes time to react. this gives the appearance of it going slower, while no individual photon ever travels at less than c.

[Snorlax]

What about cosmic rays? Do they have longer or shorter wavelength than gamma rays?

[YT2095]

shorter. up to 10^-15 metres.

[Snorlax]

So can physicists use it to see the parts of subatomic particles?

[YT2095]

Not in itself no, Bubble chambers are usualy used to detect such particles, there is also work being done using light emmisions in heavy water from other such particles

[Snorlax]

Does the fusion of hydrogen produces harmful radiations?

[Radical Edward]

it produces gamma radiation yes, but it is not like fission in that it produces alot of long lived waste.

[Snorlax]

So the product of fusion will be shortly radioactive?

[YT2095]

only when the reactor is operating, as soon as the reaction stops, the feild collapses and there`s no trace of radiation, a bit like a microwave oven, dangerous when on and inside, the feild collapses when you open the door though