Introducing the different ways we can characterize the brightness of an object and how we use brightness to study astronomy. Let us know what you think of these videos by filling out our short survey at tinyurl.com/ast.... Thank you!
Good video. I am also gratified to see that good videos posted eleven years ago are still around on RU-vid, for those of us who missed it when it first came out.
When talking about the flux associated with some transfer mechanism (here it's light energy flowing through a surface but could be flow of heat, mass, energy, charge, even a probability flow in quantum mechanics) you're exactly correct, it's the flow rate per unit area. That mirror sentence doesn't make sense to me. We can also have electric and magnetic fluxes, related to how many electric (or magnetic) field lines pass through a unit of area, and in this there isn't anything actually flowing
Great video! My only question is how the equation relating flux and luminosity changes for different light sources like LEDs. I'm assuming the 4*pi in the equation is due to stars being spherical and emit in all directions. If a light source only emits in a cone, do you divide by a different number? I assume it would be the solid angle that the light source emits.
In the wave model of light, the amplitude determines the intensity, not the rate of wave fronts (frequency), and like a circular ripple in a pond, this will decrease with distance. In the particle model, the energy of an individual photon is determined by the frequency yes, but it's the number of photons times their energy which gives you the intensity, and as they propagate, they will spread out decreasing the intensity. Also, you still need the lens to make the distant star look like a point.
Thanks for all your replies. I really had in mind the pin-hole principle - wave diffraction - that images form over the entire hemisphere behind any aperture. I see that to examine a part of a spherical image you'd still have to focus it.
I'm not exactly sure what you mean at the beginning of this part, but even with a perfect plane wave moving through space you still need a lens to get an image of a star. The waves that you're saying come from different parts of the star don't really interact strongly with each other since (unless it's a laser) the light is usually incoherent, and a lens can separate these parts of the light. I hope that clarifies some of that and I do appreciate your taking the time to ask questions.
Since you're post is rather long, I'll try to respond to the specific points you discuss as separate replies. First your general question of why a source of light isn't washed out in our detectors is that most (not all) detectors have some sort of focusing mechanism. Our eyes (or telescopes) have lenses which focus light incoming from a given direction to a specific part of our retina (or CCD sensor). This allows us to localize the direction of a given light source. (continues)
A lens will focus these slightly off angle wave fronts differently, allowing them to be resolved. If you envision what happens as these off angle wave fronts continuing on, they will spread out and you will get the described drop off in light intensity with distance
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(cont) "The reason a distant star appears dimmer than a nearer star is not that its light is 'less intense' when it reaches us but that its light is even more restricted and planar and the rate of this restriction correlates with the familiar inverse-square law." Just as with our own star there will be superposition resulting in a circular spreading wave at every point, so too with a distant star its wave fronts will arrive at any point with almost equal direction from the source and combine."
(cont) "Stars are points of light. The intensity is 'how many' wavefronts pass per second of observation. This frequency is essentially the same at whatever remove (ignoring cosmic expansion). What has changed with distance is the virtual absence of any light-angle: there is practically only one direction from which the original light source can now be detected, almost zero angular extent (so stars in a telescope still appear as 'points')." (continues)
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Thanks for the terrific video! My understanding from your explanation is that flux is a quantitative expression so it would correspond to the "flow rate" as opposed to the "flow" and so saying something like the "a mirror is retracted from the flux" wouldn't make sense. Is my understanding correct?
(last) "This 'circular' wave will spread out much more rapidly (it is virtually flat) allowing almost not opportunity to collect (or detect) energy passing through the point. Thus there is a unique or preferred direction - the line of sight - along which energy cannot dissipate into space, but must remain constant. This accounts for the intensity spike observed. " THANKS
I have an open query (a speculation if you like) about how we don't just see a 'source of light' as energy of fixed intensity (which would wash out on our detectors) but having definite spatial extent. It sort of relates to this discussion. I wonder if you will critique it? "If you picture the 'wavefronts' coming from the Sun, because the Sun is extended in space rather than a point-source, these waves will be crossing any point from a range of angles." (continues)
(cont) "This range repesents both the angular 'size' of the Sun (the cone-angle) as well as its curcular appearance. We could also picture the superposition of such oblique, circularly symmetric wave fronts as giving rise (in an apparent time-reversal of ripples on a pond) to convergent circular waves through any point." Now consider the unique wavefront due (i.e. normal) to any one direction: this would register as a 'point' in that direction, and that direction alone." (continues)
So, let me get this right: Luminosity(L)= _Energy emitted per second, totalled over surface area around source._ Radiative Flux(Φ)= _(Energy transmission through unit area) per second_ Luminosity equation: L=Φ/A
Luminosity would be flux * area, not flux / area, also requiring that area to be a surface that entirely envelops the source (I was using a sphere of radius r, centered on the source, so as we get further away, that area becomes larger and the flux (related to apparent brightness) becomes less intense)
Again, there are fewer photons which originated from the star which are actually reaching the collecting area of our detector because they have spread out. Even for the Sun the wave fronts arriving at our eyes would only be different from the wave fronts of a point source by a factor of (size of Sun/distance to Earth) ~ 0.5%. This can hardly account for the brightness difference between the Sun and the stars.
Dear Michael, Thanks for this lesson. It is a good explanation of the inverse square law, as regards Luminosity and Flux. . . However, I'm still finding it hard to grasp the concept of "brightness". Perhaps I'm confusing different models of light, and if so, I wonder whether there is a more unified model of light. On one hand, I hear that brightness is the measure of the amplitude of the electromagnetic wave. Greater amplitude equals greater brightness. However, the energy of light is not determined by amplitude, but by frequency. (Therefore, greater brightness does not equal greater energy) . . . . . On the other hand, I hear that brightness is actually the measure of the number of photons received by a detector in a given unit of time. . . . What then do we call the number of photons per second? Would that not be the frequency? . . . If so, does that mean that light possesses two entirely different "frequency" characteristics? . . . And how then does the wave amplitude relate to the photon frequency, since they both are responsible for perceived "brightness"? Please forgive my ignorance on the subject.
+chinacharlatan Great question. In considering the energy of a light wave you have to consider two things, the amount of energy per photon (which depends on frequency) and the number of photons passing by (which classically would be related to the amplitude of the EM wave). The flux that we're discussing here would be the energy flux; the amount of light energy passing through a given area per second, so it depends again on energy per photon times the number of those photons passing through that area per second. Sometimes we only care about the number of photons passing by, in which case we would be looking at the particle flux. The term flux is used in a lot of different cases where we want to describe something flowing through a surface. Hope that makes a little more sense and thanks for watching.
if source emitting 300 lumen then if I use reflector, I got 10000cd at particular point. if candela is lumen / sr. so at that point I got 10000lumen / sr. How Can I get 10000lumen? source only emitting 300lumen
The explanation is very ambiguous... you should've at least pointed out that the simplified formula that you wrote is only valid if we consider a spherical surface centered in the source also using planes to represent "detectors" is only valid if we are considering a differential surface element.
