I’m confused about wavelengths and frequency and intensity of light. Please explain the relationship and which ones we can see which one’s we cannot. I look forward to your feedback
davidmich
1.Wavelength radiation is proportional to the intensity of its light. True?
2. Which wavelengths are absorbed and reflected defines the color of everything we see. I believe this is true.
3. Visible light has a short wavelength and thus a high frequency.True?
4. Short wavelengths are visible. True?
5. Longer wavelengths are beyond the visible.True?
1.Wavelength radiation is proportional to the intensity of its light. True?
False. You can have any intensity at any wavelength. The wavelength is however inversely proportional to the energy of an individual photon in the radiation. (Which means frequency is proportional to photon energy.)
2. Which wavelengths are absorbed and reflected defines the color of everything we see. I believe this is true.
Yes. The wavelengths of the radiation that reach your retina decides what colour you see.
3. Visible light has a short wavelength and thus a high frequency.True?
False-ish. It’s short wavelengths relative to radio and infrared, it’s long wavelengths relative to gamma, x-ray and UV
4. Short wavelengths are visible. True?
False. Gamma, xray and ultra violet are not visible.
5. Longer wavelengths are beyond the visible. True?
To add to the above, your questions skirt around some of the most important questions that came up in early quantum theory, and indeed were part of its genesis.
Intensity is usually a measure of power, say power per unit area of light on a surface. This is independent of wavelength.
However.
There is an effect known as the photoelectric effect - where light shone on something can liberate an electron. This is the core of an electronic photo-detector. Usually you need a very sensitive system, where you place an electrode in a vacuum, and some distance away you place another electrode, and you place a pretty large voltage between them. With no light, no current flows. Shine light on the electrode that has the negative potential, and you may see a very small current flow, as electrons are liberated from the surface and then accelerate toward the positive plate. So far so good.
Now it gets weird. If you change the electrode material it may or may not work. Also, if you change the wavelength of light used, it also may not work. If you lower the wavelength too much it stops dead, and the precise wavelength depends upon the material the electorode is made of.
Even weirder. Whether the effect works or not is independant of intensity of the light. If the wavelength is lower than needed, no increase in intensity will make it work. But if the wavelength is short enough, you can reduce the intensity to close to nothing, and although the current flowing drops, it still works.
Thus you have very clear evidence for the existance of photons, and photons that are charaterised by their energy (aka wavelength.) If you had worked this out 100 years ago you would have got a free trip to Stockholm. You would also have made the Nobel comittee’s deliberations about how to give Albert a prize much more difficult to work out.
As noted above, long and short depends upon where you sit. We can see only a tiny tiny sliver of the range of electro magnetic energy. Wavelengths can be kilometers long and stil be useful to us and can be so insanely short that we don’t bother to quote them, but just talk about energy of the photons.
Dopers have given clear correct answers. To muddy the waters slightly ( ), let me note that a photon’s wavelength is* in the eye of the beholder*. The microwave photons of the cosmic background are the very same photons whose wavelengths were mostly yellowish visible light 13.7 billion years ago.
And if you drive at them with sufficient speed they’ll be yellowish again. Neither relativity nor the expansion of space is relevant to the physics of muddy water though.
So (and I hope davidmich doesn’t mind the added question), can somebody explain what exactly is meant by the phrase “light is both a wave and a particle”? I remember being told that in physics all the time, but never got a satisfactory answer. Is the assertion that particles and waves act differently, and light can act as one or the other depending on circumstance? Or is it arguing that these two properties (wave and particle) happen to be identical, and one way of mathematically formalizing it happens to be more intuitive than the other in various cases.
I know it is a bit mean just pointing at Wikipedea, but this one gets explained so many times, that pointing to a general reference is probably the most appropriate thing.
No, no, no. You cannot drive them any faster. You can slow them in a medium. I think the speed of light in glass is about 3/4 of the speed in a vacuum. But, unless I am mistaken, that is because they are repeatedly absorbed and then re-emitted by a material. Or better, a photon is absorbed, time passes, a photon is emitted. The question of whether it is the “same” photon is meaningless.
You want it to look yellow, you have to move towards it at nearly the speed of light. But the light itseld does speed up and, if want weird, contemplate that if you measure the speed of the light, it will still be the good old speed of light.
In the macroscopic world things that are waves and things that are objects behave differently. Waves refract and diffract, objects do not. Light was known to be a wave, and then it was discovered it also had object-like properties, such as being quantized. Electrons were known to be particles, and then it was discovered it also has the properties of a wave, such as refracting and diffracting.
They’re both waves and particles is an easier way to say that our macroscopic view of the world obviously isn’t appropriate at this scale, but that we haven’t found a better way to describe things.
The double slit experiment which shows the particle- and wave-nature at the same time, if you send one quant at a time have been run with larger and larger particles/objects, including huge things such as molecules, proving that our brains are so not set up for understanding the universe at that size.
In Newtonian physics waves and particles are different categories of physical phenomena, with not really much room to mistake the two - nevertheless, depending on context, light can exhibit properties of both Newtonian waves and particles.
The ‘old quantum theory’ of de Broglie, Einstein and others gave a relationship between the two phenomena which said both particles of matter and em waves should exhibit both particle-like and wave-like behaviour according to a straightforward relationship that linked the two behaviours.
Schrodinger, Heisenberg and others, seeking to come up with a sound theoretical basis for this ad hoc relationship revolutionized physics and created quantum mechanics. In quantum mechanics the basic objects are particles, however they behave rather differently to the particles of Newton (the Newtonian description of particles is an emergent or limiting behaviour of particles in quantum mechanics) and their state is described by a, very hard to describe ontologically, abstract concept, called a wavefunction. Whilst it is not really right to think of the wavefunction as a physical wave (how to think of it is mostly a matter of interpretation), the wavelike behaviour of quantum particles can be fairly straightforwardly derived from the wavefunction.
However one area where vanilla quantum mechanics fails is describing photons, the main barrier is that vanilla quantum mechanics only pays lip service to special relativity, rather than incorporating it fully, whereas photons are highly relativistic particles. However the most straightforward extensions of quantum mechanics to include relativity still fall short of being able to properly describe a photon, so really you need quantum field theory which describes everything in terms of (quantitized) fields rather than directly as particles (or indeed waves).
), let me note that a photon’s wavelength is* in the eye of the beholder*. The 