Light Definition and 1000 Threads

We know that photons (light) are massless but they have momentum. Now suppose I am in the space far away from planets/stars that there is no external force exerts on me, if: 1- I turn on a flashlight (torch), would I be pushed in the opposite direction which the flashlight is facing (Newton's...

Is there a way to modify a thin overhead LED light fixture that fits over a ceiling box to make the direction of the light easily adjustable? Perhaps add some sort of adjustable shade to it? In remodeling a bedroom, I'd like to install ceiling boxes ( e.g...

C= mirror center O = vertex I don't know if the design is right, if someone can show me the correct one. I also don't know how to proceed in geometry Solution: Lf / ( L - f( 1 + d/D))

UV causes dryness of plastic and rubber and lights have UV so keeping it in the dark away from light inside boxes prevents this but notice this photo where the box where the arrows are marked the clarity q enters this region of the closed box is enough for the long term the UV of this clarity...

I wanted to filter out reflections from glass. So I bought a camera with a "circular polarizing filter." It filters out polarized light, adjustable for orientation. The result confuses some cameras, so it also has a second stage which induces circular polarization. The results were...

In optical communications, one of the modulation methods is to control the optical power (Simplest case, for example, bright = bit 1, dim = bit 0). I learned that we can achieve this by a Mach-Zehnder modulator (MZ modulator). Simply speaking, the principle of MZ modulator is to split the input...

Refractive index of a medium is defined as : n = c / v; v is speed of light in medium. I believe n is never measured directly as here is no way to directly verify c / v. So what I guess is that all refractive index values are experimentally measured using n = sin α₁ / sin α₂. But then there is...

I have written a Javascript/HTML5 app that simulates a Light Clock as a way to understand the kinematics of relativity. It shows special and general relativistic effects, and how they are related. I would appreciate any feedback on the app, it's correctness, and it's usefulness as a learning...

if you put a light on object you will see shadow in the shape of the object . it would looks like the following picture the blue part is the object. but if you point a laser or light on specific object you will be able to observe it from differences angles like in the following picture the...

Question 1; The conditions for total internal reflection are: -That light is traveling from an optically denser medium (higher refractive index) to an optically less dense medium (lower refractive index) - That the angle of incidence is greater than the critical angle. Therefore, I conclude that...

Why does neutral gas before reionization block the flow of light, and how does ionization help the flow of light?

Dear Physics Forum, I need help with this problem. In the diagrams above I try to show my difficulty. My main problem is working out how shadow will fall on a completely flat surface with the light source at more or less a 0 or 180 degree angle (depending on how you want to look at it) ...

Approaching for the first time to these "higher level" topics is mind-blowing, and indeed I cannot understand why is the speed of light a constant... why doesn't it vary relatively to the emitter state of motion? And isn't it affected by gravity(I know it is affected in the sense of a spacetime...

Hello! My kid asks if this is theoretical idea is correct and I just don't know this stuff very well: It is impossible to travel at light speed but not impossible to travel just below. So the highest theoretical speed should be: "The distance light has traveled in one second" minus "One Planck...

Radio wave travels at the speed of light 3x10^8 (m/s) Converting the distance to meter: 1.3 x 3.1x 10^16 = 4.03x10^16m The time it takes in our Earth frame of reference is: 4.03x10^16m/3x10^8 (m/s) = 4.26 years The answer is B But wouldn't the time in light's frame of reference be 0 and it's...

When traveling into an optically denser medium, the speed of light reduces and as per the principle of least time, light bends towards the normal and takes the shortest path. But why isn't this followed when light passes into a rarer medium? With its speed increased in the rarer medium, if it...

I know that there are at least a couple of threads related to COVID-19 scattered over PF, but I want to get a specific feedback on this latest research in terms of human health and safety. This paper was published in Nature, and it presented a rather fascinating result if it is true. The...

Hi everyone, been reading this site for a while, and this question has been stuck in my head for 3 days and i can't shake it, so i figured I'd step out of the shadows and ask. There are some follow up questions that came on the same train of thought too. Can light pass by a Black Hole and get...

I feel like the emerging idea of quantum communications; is that you can exchange data via the synchronous states of entangled particles, across any distance, in real-time. But, there's also this dilation in the physical world, at the speed of light, according to relativity. So- how would the...

light is electromagnetic wave ,so does it also have magnetic and electric field,like all others waves(micro,gama,xray,radio waves etc..)? i never heard that some one talk about light in sense of magnetic and electric field.. if it has ,why than compass don't response to light?

My Basic Question is- Why can we see our inverted and real image inside a concave mirror when the image is formed in front of it and not behind? If you say that our eyes tries to image the real image formed by mirror on the mirror itself then- Imagine a situation where we have a concave mirror...

Hello! I recently had a discussion with a person who's well-read on quantum physics and I was suprised by his claim that "light is in no sense regarded as a wave" in quantum mechanics. His support for this claim was that there are no wave crest or wave trough, there is nothing moving. What...

In this article it discusses the generation of something called super chiral light and claims with metamaterials they can make it have very high angular momentum like l=100. What does that really mean? How does that relate in magnitude to the normally computed linear momentum of a photon p=h/λ...

I was curious if the relative speed of an object can exceed the speed of light. Specifically, I am curious about the following thought experiment. I am not a physicists (and if I were asking the following would make me a poor one) and it has been 20 years since college physics. If a vessel is...

Hello, everyone, I have a LED, 620nm, with a viewing angle of 20°. I would like to bring this light in a distance of about 20cm on a lens with as many photons as possible. My idea is to position a converging lens between the LED and the lens. To be exact, I would position the lens at the...

My initial guess... Third dark fringe is 2nd order dark fringe and second bright fringe is 1st order bright fringe. Is this the correct guess?

In the above diagram, I have illustrated what is written in the summery. So, if the thickness and refractive index of the material are chosen such that the part of the wave that travels through the slab acquires a path difference of λ/2 and at the right end if I keen another convex lens so as to...

As you all know: Planck evidence for a closed Universe and a possible crisis for cosmology https://www.nature.com/articles/s41550-019-0906-9 If confirmed, it would bring back the Big Bounce as a possible hypothesis for the evolution of the universe. This has long-term consequences being that...

Do any animals see in a spectrum other than visible light and infrared?

Hi All In discussing another issue it occurred to me the only experiential evidence I know that normal visible light is EM radiation is Faraday Rotation. I strongly suspect we have a lot more these days. Amusing story. Fermi's wife had a degree in general science that did not cover...

Hi thereLet’s consider a photon of wavelength λ is being reflected across of gap d meters. The photon is a reflected back and forth between the same points on two horizontal sheets that reflect 100% of the light. What happens when the distance is smaller than wavelength? Thanks

I purchased a RadiationD-v1.1(Cajoe) geiger counter off ebay, and attached an arduino nano and a 16x2 display ( I got the instructions off this site https://www.instructables.com/id/Arduino-DIY-Geiger-Counter/). I noticed that the counter was sensitive to UV when I had it out in the sunlight, so...

I was reading (or at least skimming) this paper: https://arxiv.org/abs/2005.10702 in which they seem to be discussing gravitational wave lensing. Is this an analogue of light lensing or is it another subject entirely? I mean, as I understand it, light is bend using gravity (as for...

Has anyone here heard of the the HTLGI festival? Is anyone attending?

The size of light beam is same or different before and after refraction from a medium. If same then why we can not prove from mathematical expression. If not same why?

Hi, Would it be correct to say that at Brewster's angle, all the incident light which has its electric field parallel to the plane of incidence gets refracted, and the rest of light whose electric field is perpendicular to the plane of incidence gets reflected? For example, if the light whose...

I apologize if this question is in the wrong forum section - but I could not find a proper place for what is a basic question or perhaps a nonsensical one. Is the reason why an object cannot travel faster than the speed of light because the object itself is ultimately made of waves? Is the...

In my latest 10th grade physics lesson, we were learning about the refraction of light. I decided to share what I knew about why light slows down in a vacuum, which is, in short, because the electric field of the electromagnetic wave exerts a force on the charged electrons of a medium, which in...

Time as we perceive is not a universal constant. It depends on the speed of the bodies and the distance between them.

My guesses are that if slit is almost closed, then the width is comparable to the wavelength and diffraction will occur. does this means that he will observe the slit S clearly or less clearly? [Picture cleaned up a bit by a Mentor]

Hi, I have a 25w 254nm bulb ozone free UVC bulb (draws about 19watts from my kil-a-wat). This was purchased from amazon, produced in China. This light definitely omits a funny smell, it pulls 19w, and the glow in the dark stickers are illuminated after my test (so I know it is omitting some...

There's an undergraduate physics course at my uni that covers these topics and the course description is: Mathematical descriptions for classical physics: oscillations, mechanical waves, electromagnetic waves, physical optics and thermodynamics. Are there any good studying materials/textbooks I...

so I was recently studying some ray optics and then suddenly a weird question came across my mind I just don't know if it's the correct thread to ask but let's continue anyway So the question is : suppose there is a spaceship traveling at light speed and someone fire a laser inside it what...

Looking for a beginners explanation to the following question: How is Doppler effect separated from the original spectrum of light emanating from a moving body (in astronomy or other physics branches)? If the question does not make sense, here is the reasoning to ask it: If a certain color is...

Light consists of transversely varying electric and magnetic fields. A point charge in the path of a vertically polarized light beam experiences a vertically varying force from which energy can be extracted. In an unpolarized beam, the electric fields of individual photons act randomly in all...

The setup of the problem is shown in the image below. I know that I must add all the contributions of each reflected ray and that its amplitude will be reduced by a factor ##r## each time it is reflected. So after the n-th reflection, its amplitude will be ##E_0r^n##, with ##E_0## the amplitude...

I've been playing around with this for quite some time now this morning but can't get the last bit out. I defined the time functional to be $$T[y] = \int_{x_1}^{x_2} \frac{\sqrt{1+(y')^{2}}}{c(y)} dx$$ which follows from consideration of the time taken to cover an infinitesimal section of arc. I...

The doped a-Si: H layers in a HIT solar cell do not contribute to the photocurrent. The light they absorb (according to their absorption curve below) is lost. For a doped a-Si: H layer at the front side of the cell that is 25nm thick, what percentage of light at 400nm will be lost due to...

In my drawing you can see: A generic bulb covered by a mirrored hollow ball. The hollow ball has a small hole (1 mm diameter). So the complete light will go through this small hole. The lens will bundle the light in a very distant (100 km) focal point. I guess it might be hard to create such a...

I've read a couple of other topics on the Physics Forums about the lumped circuit abstraction requiring that signal timescales (i.e. the period of the highest frequency signal component) be much longer than the propagation delay of the signals though the circuit and that the wavelength should be...