John15 said:
I am wondering if light is the right term to use. Would "we see radiated energy within certain wavelengths" be more accurate?
"Light" is just a term for electromagnetic radiation whose wavelengths fall within the visible range, i.e. the range of wavelengths to which our eyes are sensitive. In fact, in some contexts, people use the term "light" to mean electromagnetic radiation in general. In other words, they are not restricting the term to just visible light: they speak of ultraviolet light, x-ray light, infrared light, etc. So I really see no distinction between using the term "light" and using the term "radiated energy at certain wavelengths." The latter is just a description of what light IS.
John15 said:
Also what form does that energy take? I ask this because we see a red hot iron bar in a darkened room so we are not seeing reflected light in this case but radiation in the form of heat, this could also explain why we see the source - because its hot - and not the energy/photons inbetween (see 1st post) perhaps because they act as closed systems and so cannot radiate energy so cannot be detected whilst moving. So are we seeing a heat signature, light in the form of photons or a mixture? remembering that infrared shows heat so why should visible wavelengths be any different.
What is special about black regarding energy and wavelength?
Sigh. You seem very confused. Let me try to help. ANYTHING that has a temperature above absolute zero will "glow." What I mean by "glow" is that it will emit electromagnetic radiation i.e. light. We call this type of emission
thermal radiation, and in physics, there is an idealized type of radiator called a blackbody. An ideal blackbody absorbs all EM radiation that is incident on it, and re-emits that radiation with a
characteristic spectrum that depends on the equilibrium temperature it has reached. In fact, an ideal blackbody has the property that its thermal emission spectrum depends ONLY on the temperature of the object. It does not depend on anything else e.g. the surface material/composition of the object. Real objects obey this ideal case to a greater or lesser degree, depending on the object. The properties of a blackbody emission spectrum are 1. The hotter the object is, the more TOTAL radiation it will emit (when summed up over all wavelengths). 2. The wavelength at which the amount of radiation PEAKS is inversely proportional to the temperature. In other words, the emission from hotter objects peaks at a shorter wavelength in the EM spectrum than the emission from cooler objects. Properties 1 and 2 are generally obeyed by most things, even though maybe not in the strict blackbody sense. Here's practical example of that: you and I are currently at room temperature, or maybe just slightly above that. At that temperature, our thermal emission spectrum peaks in the *infrared* portion of the spectrum (meaning that the majority of the light that we emit is at infrared wavelengths). The shape of the spectrum at this temperature is such that there is negligible emission at visible wavelengths or shorter. In other words, you and I do not emit much visible light, certainly not enough to be detectable. However, if you take an object at room temperature (like an iron rod) and heat it up to higher and higher temperatures, then its thermal emission will obey both properties 1 and 2. Property 1 says that it will emit MORE overall radiation across the entire spectrum as its temperature increases, and property 2 says that it will emit more of its radiation at shorter wavelengths i.e. the peak of the spectrum will shift to shorter wavelengths. So if you heat the object enough, eventually it will get hot enough that most of its emission shifts from infrared to visible wavelengths, and it starts to "glow" visibly." At lower temperatures, this peak emission is still at the longer visible wavelengths, which is why it appears to glow red. As the temperature increases, the peak of the spectrum shifts to shorter and shorter wavelengths, and hence the colour shifts to orange, yellow, and eventually white. The reason why the object appears white is that, at this temperature, the shape of the blackbody emisson spectrum is such that the amount of emission happens to be mostly constant across the visible range (so you're seeing equal amounts of all the visible wavelengths). If you could continue to heat the object, eventually you'd reach the point where its emission spectrum would actually peak in the UV, and shorter still. There are stars whose surface temperatures are hot enough for that to be the case. I hope that this example has helped to illustrate that the emission from hot objects is electromagnetic radiation, or light. It is not some sort of "special" form of energy or radiation that is only associated with heat. Any object with a temperature will emit EM radiation, and the wavelengths at which most of that radiation is emitted will depend on the temperature of the object.
John15 said:
How do we define a photon?
Looking at the above answer a photon is defined by time rather than wavelength or energy ie taking 1 second as a start a wave with a length of 1 second will have a given amount of energy (call this 1 photon) and a wave of 10 seconds length will have energy of 1/10th per second or contain 10 photons that add up in total to the shorter wave, this would appear to give the shorter wavelength higher momentum. Or perhaps a photon or quanta could be described as the shortest wavelength highest energy wave that can exist. Or am I still barking up the wrong tree.
Still interested in an answer to amplitude.
I really have no idea what you are saying here in regards to the "time" of a photon. Classical electrodynamics says that light (meaning EM radiation in general) consists of oscillating electric and magnetic fields that propagate through space as waves. I admit to being not very knowledgeable about *quantum* electrodynamics (QED), but from what I understand, in QED, these electromagnetic fields are "quantized" in the same sense that other physical observables from classical physics are quantized in quantum physics. To be quantized means to be restricted to only being able to occupy a discrete set of energy levels (at least as far as I understand it). Somebody who is more knowledgeable than I am should comment further on what a photon actually is, all I can say is that is is the quantized version of the electromagnetic field. The last thing I can say about photons is that, just as classical electromagnetic waves have a frequency f, so too do photons. It is much more difficult to interpret (or intuit) what this "frequency" means photons, but if we just accept that "frequency" is a property of photons, we can note further that the energy E of a photon obeys the property that E = hf, where h is a constant. This is the discretization of energy that I mentioned before.
