Light Definition and 1000 Threads

Hi, I've been struggling for a long time to understand the propagation of light. Here are my key problems: - since a star can be seen from any direction, it means that photons are emitted in all these directions, which are infinite, how is this possible? infinite photons? - in case there are...

In Cosmos: A Spacetime Odyssey episode 5 Hiding in the Light, Neil Degrasse Tyson describes one of William Herschel's experiments with light in the year 1800. The episode shows Herschel's experiment as a cartoon. Herschel put two thermometers under a prism that was in the sunlight. Herschel put...

If the speed of light in the vacuum of empty space is the same for all observers, how can there be a Doppler effect on light? Doesn't the shift of color/frequency indicate a change in speed? Sorry if I posted with the wrong prefix, I'm just a curious senior (as in old guy) :oldconfused:

A light source in uniform motion emits a flash of light. A spherically symmetric wavefront propagates from a central point, the source, or the “origin” of emission. The wave front remains at c relative to the origin as measured by all observers. How fast do these origins move? Am I missing...

The constancy of the speed of light is a fundamental principle in modern physics, and it is supported by a wide range of current experimental evidence. There is no evidence to suggest that the speed of light was different in the past, and the idea that it could have been different is at odds...

1. How much is acceleration of light? 2. Why is speed of light constant if time is relative, to keep speed constant distance must be changed as well? 3.What is time by Einstien? If we state it is not absolute, it is illusion, dont exsit? 4. Can we say ratio between distance and time is...

Elevator (2m height) is accelerate upward, light is at the top pointed toward flor of elevator. Is time t=2m/c to light pass from top to bottom? Bottom moving toward light,so time would be smaller then t=2m/c?

I brought time over to the other side and multiplied to get 686 m/s but that wasn't correct

In the illustration below, a laser beam is emitted, and the time recorded, from the rear of a frame which is 150,000 km long and is moving at velocity 0.5 c to the right. When the beam reaches a half silvered mirror at the front of the frame, the frame being shown as a rectangle and using...

In Feynman's book, QED, The Strange Theory of Light and Matter, page 27 he says, "As long as the photon moves, the stopwatch hand turns (about 36,000 times per inch for red light); when the photon end up at the photomultiplier, we stop the watch." I've been puzzling about the significance of...

Is my understanding correct that if we have a moving vehicle moving to the right at speed v, as above, with a light source in center going in both directions, that (upon emitting the light at time T), a detector at D1 & D2 would both detect light reaching it at T2? (even though in the time it...

Last night I was awoken by what seemed like an extremely bright flash of light. I sat up and saw another bright flash of light just outside my bedroom that seemed to originate just down the hall. I was worried there was an electrical short so I ran out and nothing. I thought maybe I imagined it...

I once read (though I don’t remember where) that in the same way that you talk about a dimensionless ratio between Y and X in ordinary space, you can conceive of c as a dimensionless ratio between T and X in spacetime. Do you know where I can find a reliable treatment of that idea? As...

This is much like the "grain of sand" at near speed of light hiting Earth, but now targeting Jupiter with a needle! What if... Any way, which is the correct way to analyze this kind of scenarios? For example, I think than being the speed of the needle near c, is nonsense to think about the...

I perfectly understand that we cannot, as of our current methods, determine the one-way speed of light - I was, however, wondering if we could determine a relative speed using electrical transmission. the basis of which would be sending a laser light across a field into a sensor, starting a...

Hi. I am studying the wavefront evolution of light from a star. In the papers I have read, the star is often treated as a point source and the light is approximated as a line (geodesics), but this approximation is not very useful when I study the wavefront evolution, so I want to extend the...

Because when I have two carbon atoms, they create a single bond, so their two electrons are in one boding orbital, but I have also one anti-bonding orbital here. And when the electron excitates from ##\sigma## to ##\sigma^*##, how can the bond between the carbon atoms exist? It has to disappear...

Hey everyone, I am trying to couple light from an optical fiber connected to an LED (365nm) into a small glass capillary which is pulled on one end and should be used as a small light source. The idea is to use the glass as a light guide. Has anyone tips for efficient incoupling of the light? I...

Hello, people say that light has properties of particles and waves. Ok, but how exactly? We have the principle of rectilinear propagation of light as a law on Physics. Light would be a particle that propagates in a direct line pointed to all directions with origin on the source of light. The...

In relativity, momentum of a body is given by ##p=mv/\sqrt{1-v^2/c^2}##, but if mass is exactly zero and velocity is exactly ##c##, how is the photon momentum even defined? I don't think this problem can be resolved by simply stating the other formula relating energy to momentum, since it was...

Quote from NASA: My understanding of dark energy is based on NASA's report: https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy; were NASA state as follows: "It turns out that roughly 68% of the universe is dark energy. Dark matter makes up about 27%. The rest - everything on...

Hi, there. I am currently reading the paper, Gravitational Faraday rotation induced by a Kerr black hole (https://doi.org/10.1103/PhysRevD.38.472). After Eq. (2.4), it reads that The paper does not provide the derivation of the equations and no related reference is listed. Also, ##k^i## is not...

I am trying to design a simple homemade PV solar simulator. I have picked metal-halide lamps as my light source. If the PV panel has an area of 1m^2 and I wanted to achieve spatial uniformity across the irradiated surface with an irradiance of 1000 W/m^2, how do I go about selecting what...

Is red shifting of light from distant stars actually caused from them moving away from us, or is this a simplification. Doesn't this break the rule of light always going the same speed. Could the light rays be possibly lengthened because when the light left many yrs ago space was smaller, now it...

a. We know metals emit EM radiation upon heating or electric current. I'd like to understand more fundamentally how this phenomenon takes place, on the basis of the basis of band structure, and which electrons are involved ? b. Classically, charges emit radiation when accelarating or...

What kind of imaging system could be used to see without photons? I ask because I was watching a video and the furthest we can see back is Redshift Z~1090 which is the CMB. We can’t see the universe before the first stars formed or the Big Bang itself. My first guess would be some kind of dark...

In this thread, I set out an analogy illustrating what quantum entanglement is; further in my post there is a description of the experiment with polarizers and waveplates, corresponding to this model (CHSH inequalities). To understand it, you need to understand what polarization is. I have read...

https://quantum.phys.lsu.edu/old-website/seminars/abstracts/Kaushik10.pdf I have discovered an experiment in the link above where you get NOON entangled states by mixing classical laser light with quantum light from SPDC. But I cannot understand the mathematics behind it. Can you explain it to...

Dear Anyone. Please be nice and let me down gently, I know more about tapdancing than physics and I can't tapdance! It's just... Kept reading nothing could go faster than light, in a vacuum (space.) So I Googled 'why' and discovered it's because things get heavier and heavier as they get...

hello Einstein assumed the invariance of the speed of light as an hyphotesis, while I was told that : "The speed of light need not have been postulated as an invariant." in other words the invariance of the speed of light could have been proven even regardless of the special relativity is it...

Is there sufficient mass within the observable universe’s volume to form a black hole event horizon around the observable universe and, if yes would light fired tangentially at the edge of our observable universe ever loop back around in a circle or spiral inwards?

What is the difference between the GTR vacuum and the vacuum of quantum theory? What is the speed of light in the vacuums, specifically what can be predicted and what must be measured?

hello everyone According to Einstein's second principle of relativity: "The principle of the constancy of the speed of light: The speed of light in free space has the same value c in all inertial reference frames" Does free space mean vacuum? Is this principle not valid in the air and the...

I'm trying to understand the Schram plot, which is the graph of wiki here: https://en.wikipedia.org/wiki/Cosmological_lithium_problem I do not understand why the yellow box (the observed light element abundance) for H3/H (red stripe) is absent? Is there any reason for this?

I first thought that the angle would have to be maximum when it is closest to the critical angle for total internal reflection. From my lectures the equation for the critical angle is ##\theta _1>\ sin ^{-1} \left( \frac {n_2} {n_1} \right),## so as ##n_2 = 1##, we have ##\theta _1=\sin...

<unacceptable reference deleted> As our universe is expanding, that is, our vacuum is becoming more and more sparse, I believe it is possible that some key characterics of our vacuum is also variable.

hello everyone I read this text in physics book: "Another example is the so-called Cerenkov radiation, which consists of light waves emitted by charged particles that move through a medium with a speed greater than the phase speed of light in that medium. The blue glow of the water that often...

The equivalence principle tels us that we cannot distinguish between gravity and an accelleration. using that fact one can reconstruct the bending of light along the sun, but only half of it results from the equivalence principle together with the gravitational field around the sun. Is it...

Does the speed of light imply that in its property as a wave it encompasses the entire known electromagnetic spectrum?

Hello, I have a problem with the postulate of the invariance of the speed of light. When we move away from a light source it is redshift, it is the sign that the relative velocity between us and the light source has changed. If a stationary observer observes the phenomenon, he will measure that...

Assume two observers very far from each other, so far, that the accelerating expansion of the universe matters. (edit: But not outside of each others event horizons.) They will send light beams each other, and measure the energy of it. Also tie them together with a very long rope to fix their...

sir , i always taught what would happen to light traveling away from the mega sized star at the time of transformation of this star into a black hole? will it trace back its path after it taking a u - turn?

Hello and please know I am very greatful for your help! I am wanting to learn how to measure light. I have chosen a specific light for this to help me better understand. Lm- 7800 CD-620.7 So, I got that far, lol. I don't really know how to input the numbers for the Steradians equation, I have an...

I was a denizen of this forum some 15 years ago during undergrad. However I since joined the dark side working in software on ML, AI, and distributed data processing. Every now and then I pick up a physics textbook to get into the weeds of a topic I would have missed due to skipping out on grad...

Hi Everyone! I have a photonics problem that I was wondering if anyone may be able to shed some light on (no pun intended). I work for a company that is designing a fiber optic emitter/detector as a part of one of our products and we've noticed something that would appear to defy physics...

Imagine a plane 1 light minute across. Now imagine 1 person on either and of that plane. Between them, is a thin indestructible bar that is 1 light minute in length. In the center of this plane, a simple device has a wire that leads to a motion sensor on the left side of the bar. If the left...

https://en.wikipedia.org/wiki/Fluorescence You normally heard of Fluorescence coming from UV which make the objects glow. But visible light can also cause fluorescence. When you hit a colorful sample with a 532nm laser, there is fluorescence in the visible spectrum (as seen in Raman...

Are the Big Bang and inflation really needed to account for the large-scale structure of the universe, nucleosynthesis, baryon acoustic oscillations, and the anisotropies of the CMB? Can the universe not begin in an already expanded state and still have all those things happen just the same...

Hello ! I've got a problem understanding what are Lumens and how do I look at lux as well. For example here : It makes 800 Lumens in 1 m^2. Does it mean that in every direction in 1m^2 there are 800 Lumens ? Or when I have a light bult with 800 lumens : Those 800 Lumens in light bult...

While not having a professional physics background I was still interested in knowing more about special and general relativity. Therefore I was trying to find out where the space time interval was coming from in relation to the speed of light. Of course this is the first point to start I...