Light Definition and 1000 Threads

Hi, I read various threads in PF about the concept of invariant speed and the speed of light in vacuum that in our universe happens to be the same as the 'invariant speed'. My doubt is about the speed of the light in vacuum as measured from a non-inertial frame (basically in the context of SR...

My answers are quite different and here are my attempts a) What is the spatial separation ##\ell'## between the point of emission and absorption? (In the frame going ##v = \beta c##) From what I know is that moving causes lengths to be contracted according to the formula ##\ell_{travel} =...

Which one does it mean: "phase velocity" or "group velocity" or "speed of the wave front"? In the postulate of constant speed of light .

The concept of that when a photon's trajectory intersects with the Schwarzschild Radius/event horizon, said photon will never exit the Schwarzschild Radius/event horizon. Or any other object besides a photon for that matter. So far what has been the strongest evidence for this prediction?

What is the equation/formula for the Speed Of Light? E=mc²? I thought it would be simple to find in a search.

In our galaxy, two stars, A and B, move about 20 km/s in slightly different directions to each other. The stars are about 15 light years apart. Initially, the light beams are exactly anti-parallel. Will light beams emitted from star A traveling towards light beams emitted from star B stay...

The phys.org summary had me intrigued and the resulting new paper makes for heavy reading at my level of knowledge, but it was the underlying reasoning of the methodology that I really question: One of the authors notes "It's so rare, that they should not have seen any." From that, the...

5.879e+12. So that is what comes up if you convert one light year to miles. My question is, what does the 'e' mean? I see it quite a bit.

I don't even know where to start with this problem. What kind of slit makes linearly polarized light circularly polarized? The correct answer is d = lambda/(4(n1 - n2)) = 856nm. But how do I get there? Thanks in beforehand!

Hello everyone! Let's say that you were to attempt to go as fast as possible on a spaceship with the mass of an average car in an absolute perfect vacuum. What I am wondering is, that if you were to reach a certain speed, and stop applying energy to this imagined spaceship, would the spaceship...

Hi :) I just had a (most likely totaly unoriginal) thought and thought I should try to ask someone who's thinkings on physics are somewhat more refined than mine. So after thinking twice about private messaging Mr Tyson and Mr Cox :) I decided to google 'physics forum'. So here I am with my...

I'm in Surrey, London, UK. I've noticed this stationary light in the sky near the moon when there are no stars visible. What is it?

Isn't the meaning of speed, a variable of distance divided by a variable of time? Therefor isn't the meaning of a constant speed of light, a constant distance divided by a constant time? If there is any truth in this saying and there probably isn't, then what is the meaning of light constant...

If i am moving away from an object at a certain constant speed close to the speed of light, is that object also moving away from me at the same constant speed?

Would it be correct to say, that we are moving away from stars at the edge of the universe, at the same rate that these stars are moving away from us? I am relating to stars that are moving in relation to us, at a speed that is faster than the speed of light. Is the symmetry that maintains that...

To describe the movement of the planets, Newton assumed that there was such a thing as gravity. But he didn't know what gravity was. To derive the Lorentz transformation, Einstein assumed that the speed of light was absolute (not relative), but is it also known why the speed of light is absolute?

If space is transparent how can we not view light from a black hole

I ran into something on a non-technical message board that is familiar to me - a poster, clearly without any scientific knowledge, saying that Relativity is wrong. Unfortunately, the point he used as a battering ram is one to which I don't know the answer. He suggested that the light...

I understand that the meter is defined from the speed of light (distance light travels in 1/299792458 of a second). But how did man measure this exact distance to this level of precision? With any apparatus, isn't there an unknown amount of bottleneck somewhere?

I need to enlarge the light produced by a small laser beam that I have for a science project. I can't buy a larger laser because they're too expensive and not in the right color. Any suggestions? Thanks!

Is there any reason to believe that the one way speed of light is constant? Not isotropic, but constant in the sense of being independent of position.

"When electrified rods are brought near light objects, a similar effect takes place. The rods induce opposite charges on the near surfaces of the objects and similar charges move to the farther side of the object." -from a high school physics book. NCERT Class 12th part 1 to be precise. can...

I think that what happens is that the amplitude becomes sqrt(2)/2A in the slit filtered, as opposed to A in the other slit. I suppose we will still get constructive interference from the slits, so the value (1+sqrt(2)/2)A will be reached as opposed to 2A in the usual experiment. However, the...

How to calculate the wavelength of light coming from 1 light year away. Is there any formula for it. Please help.

The parabolic mirror pictured below is such that all incident rays, neglecting diffraction, are reflected towards a focus. A question states that the light cannot be focused to a point precisely, since there exists a circular diffraction pattern in the focal plane. The following diagram is...

I have A2 as 150, A4 as 150, A5 as 300 and A3 as 0. I just need these answers checked to make sure I’m right.

Hi all! Breaking down the question bit by bit: AC is coated with a silver film which acts as a mirror - Okay, nothing as of yet right..? A light beam is an incident onto prism at Point Q such that angle PQB is 40 degrees - This means that my incident angle is 50 degrees as shown below...

I take the following example to explain my question : when a « black-hole » is attracting a planet there is a Force which is proportional to the 2 masses and inversely proportional to the distance between the 2 masses. When the planet moves towards the « black-hole » this attraction force...

In A.P. French's Special relativity the author said, The mass and length of the box are irrelevant here. He said the momentum of the radiation is ##E_{radiation}/c##. We know that the momentum of a single photon with energy ##E_{photon}## is ##p_{photon}=E_{photon}/c##. So is...

Hello, Light, laser or not, is fundamentally electromagnetic radiation with visible wavelengths. Laser light has both high spatial coherence and temporal coherence (highly monochromatic) while regular light has both very low spatial and temporal coherence. Spatial coherence is not about spectral...

Hello to all, In a short pulse laser emission setup, can a pulse be emmited with beam length shorter than one wavelenght? (can a pulse have a duration shorter than its period?) Lets say a laser emmiter shoots a quarter cycle pulse, what would happen to this short beam? (lets supose the...

Imagine that we have an electromagnetic wave or light propagating in x direction, and \mathbf{E} is oscillating in z direction and \mathbf{B} in y direction. The picture looks something like this Now, if there exists a charged particle q on the xx axis at rest, then our B field can't do...

When we say speed of light = c m/s. Who is measuring meters and second. From SR perspective, that photon is moving towards you at speed of light so there should be time dilation, correct?

I set up the equation .5((cos 12)^2)^x = 1/9 Solving for x gets me 34.037 34 + 1 = 35 I've entered answers of both 34 and 35 and both have been marked as wrong. Does anybody know what I'm doing wrong here?

A thin lens has an upper radius of curvature 𝑅1 and a lower radius of curvature 𝑅2. When the lens is completely surrounded by air, it has a focal distance 𝑓. The lens is then placed in the interface between air and water inside a vessel (see figure). Calculate the refractive index for the glass...

In the process of trying to understand holography I encountered a detail that I kind of knew before, but just now I realized I have no idea how it works, how is it even possible. I’m talking about a simple pinhole camera where supposedly no matter how tiny a hole is a beam of light would still...

Summary:: need to increase the wavelength of light. I am an intro physics student working on a project and need to increase the wavelength of photons. I only have visible light sources available but need to have an emission of a wavelength larger than 1mm. is there a way to shift wavelength...

In regards to human vision and specifically the retina and ganglion cells. I find it hard to comprehend that somehow the light can only hit one part of the retinal ganglion cells (either the centre or the surround) and not the other? Doesn't light go everywhere? Is the lens focusing light so...

How was it discovered that electromagnetic radiation is electromagnetic? -- Jeff, in Minneapolis

My problem started when I took my alternator apart to grease bearings when I put it back to together dash light came on so took it apart seen wire to one of the brushes was broken soldier back together it work for a short while the light was back on seen soldier had broke again so I changed...

I'm a high school teacher. In the curriculum, students are required to explain that polarization of light shows that light is a transverse wave. My answer: In order to explain polarization, we have to consider the electric field vibration direction. For example, on a wave that propagates...

I am reading up on the special theory of relativity, and watched a video. In the video we have a train moving at ½ c towards a station, where an observer is waiting. The train's headlamp is on. How would the lamp's light appear to the observer? I was thinking that it¨'d appear to the observer...

How to distinguish elliptical polarized light from partially-polarized light?

Incandescent light lamp flickering due to ? Voltage or Frequency ?

Summary: Can one observe visible light bremsstrahlung when an electron beam stops in a transparent medium? The theoretical form of a bremsstrahlung spectrum is flat at low photon energies. This means that even a high energy electron beam incident on matter should cause the emission of visible...

Summary: If the furthest quasars we can see are let's say 13 billion light years away from us, then does this mean that the distance between us and that quasar was 13 billion light years at 13 billion years ago? To anyone educated in physics this might be a silly question but to me this is...

I am not a physics guy I'm just a husband trying to read in bed at night without disturbing my wife. Does infrared light reading glasses exist?Scientificly, can they be made?

Homework Statement: Lens questions. Homework Equations: idk Hello, so I was wanting to use a laser beam and purchase a few lens' online for a project and wanted to get something similar to the image below, where i would end with a horizontal light ray i was wondering if anyone knows the...

Hello everyone, Most of you know the Holographic Sights used on weapons to aim. I have a 3d printer and I will try to replicate one of them. I just wonder how the red dot/sight is reflected on the glass without making the light reflected anywhere else? It doesn't have to have exact same...

I have unfortunately no attempt for 31.i. I don't know how to start the problem. 31.ii is attached. Can someone give me a tip on how to start 31.i? I also have troubles solving 29.iii. I have attached that as well.