Light Definition and 1000 Threads

Reactionless drives are impossible. That means that a laser carried on a satellite would produce no thrust. Why, then, would shining that laser on a light sail not produce a net force? Using a fan to blow air on a sail does not work because the fan and the sail produce opposing forces. But...

<Moderator's note: post split off from another thread> OK I'm new here i have read all the questions and comments. the whole reason i did is because of light energy. "More photons packed in a smaller area = higher light intensity in that area". and solar sails work? so why can't scientists make...

How can a mirror reflect 99,999% of incident light? Or in other words what makes a photon exiting a laser cavity so directional? In fact, I never found a really convincing answer. Thank you.

Hello, Just out of curiosity I disassembled an LED GU10 light to see what's inside and found, a rectifier, a few capacities, and a small transformer. It was working fine before, but when I took the plate that has all the LEDs soldered to it and tired to power it on my DC variable power supply it...

I'm studying history of electromagnetism. Here is my question regarding Maxwell's classical brilliant work. I understand these constants (for now) And I understand this: And I understand this: But I need to understand how this math below gets us to the speed of light. I don't understand...

Homework Statement A thin (460 nm) film of kerosene (n=1,20) is spread out on water (n = 1,30). Light hits the (horizontal) surface of the film coming (almost) perpendicularly from above. A diver underwater, below the film. a) Find out the visible wavelength reflected into air that has the...

First, I would like to say any response needs to be explained like I’m 5 years old. A degree in psychology didn’t require advanced math :) That said, here is my question: If any quantifiable number can be squared, does this mean light can also be squared. I read the forum about light being a...

I am a student of physics at a local Junior College in Mendham NJ and am planning on transferring to a 4 year program at the University of Alabama in a year. Iam having a bit of a difficult time understanding general relativity. Why does a photon bend twice as much under a gravitational field...

I keep hearing in these Science Channel programs that the reason why the Big Bang banged (instead of immediately collapsing into a black hole) is that when it banged it made space to expand faster than the speed of light. I'm always puzzled on how sure and certain the speakers look in these...

I've wondered this for a while but not known how to ask the question, If light is a transverse wave, then what is it transverse to? To elaborate, light travels in three-dimensions, radially. To me, this seems analogous to the sound wave, with pulses of pressure moving longitudinally to the...

Homework Statement I am doing an assignment on Polarization of light. I have conducted experiments to prove that reflected light is at least partially plane-polarized, and to prove that Malus' Law is true. Both my experiments have given good results. I now need to find enough stuff to write...

I know that the speed of light is different in different mediums. The speed of light in Cesium as a medium is actually higher than the speed of light in vacuum. How is that possible? Shouldn't it be fastest in vacuum? [Mentors note: this post has been lightly edited as part of splitting it out...

The problem says: A radio station emits electromagnetic wave with a frequency of 100MHz (102*106 Hz). a)What's the energy of this radiation's/glow's photon? (Solved, i found 6,63*10-26 J) b)Compare your calculation with the energy of another visible radiation/glow, with a wavelength of 600nm...

We have talked about light behaving as a wave and a particle. Having trouble understanding what 'mediated' means in this context.

I know the textbook definitions and descriptions of the phenomenon, but I'm hoping for a fundamental WHY. I can see that when a water wave passes an obstacle, the wave spreads out into that object's shadow because the wave's energy is not constrained to any direction and so it will move out in...

I'm a bit confused as to why the speed of light changes in an accelerating ship relative to an onboard observer. In other words, on a ship with a clock at the nose and a clock and observer at the tail, in an accelerating ship, the clock at the nose will tic faster. The reason (according to a...

Hi, I hope that it's not wrong to ask for this kind of help in this forum. If you don't mind, could you please help me with that name which the presenter says around 46:55 in the video below? One name he says is Cassini but the other name which he says something like "Delawear" I couldn't have...

Is anyone expermently measured two way speed of light from moving source using time of flight?

Hello, I have a physics question that I am hoping the forum can answer. I have lots of them actually, but I would like to start with the one question and go from there. Ideally the answer to the question should be based upon current accepted physics theory. The Question: How do objects...

Destructive interference is excellently demonstrated in Young's double slit experiment, where dark regions are formed due the waves being out of phase. However, what really confuses me is that unpolarised light has intensity. Assuming we had perfectly unpolarised light, as in where the...

Hi, I'm studying physics and was wondering about the light meter I used. It was set to 2000 so I could get readings, instead of 2 because the readings wouldn't show. Does this mean my values I get from the light meter is in mLux (milli) or just Lux?

Romer measured speed of light using moon's of Jupiter but he got value with a error of 26%. Is anyone did same experiment in modern era? And what value they got?

https://www.cockcroft.ac.uk/wp-content/uploads/2015/10/gratus_RelWorksheet.pdf Refer to diagram at top page 8 showing light signal reflected (Event A) back to rocket (Event C) on t'.axis. If the space axis x' were drawn showing the position of rocket when the light signal was returned from...

Someone asked an intriguing question: Does light passing close to a massive body experience the Doppler-equivalent of slingshotting? i.e. blue-shifted or red-shifted, depending on its path? (I guess the light would be acting as the third body in a three-body system?)

In wikipedia I found: If what I underlined is correct, it means that from one incident photon we will get at the other end countless similar photons, as the "shaken" charges radiate "their own electromagnetic wave" (photons, right?). This sounds like light amplification, but the explanation...

Hello, because of the "kinetic isotopic effect", https://en.m.wikipedia.org/wiki/Kinetic_isotope_effect can the ratio of the common isotopes of N, C, P, H, O, S determine the position of an individual in the food chain? (eg, have carnivore animals higher proportion of light isotopes than...

I'm trying to optimize my vein viewer with LED lights but I'm not sure which factors affect the penetration of the light through the skin. (I'm following this guide https://www.instructables.com/id/How-to-make-an-affordable-Vein-Finder-for-use-d/) I can say those factors are the angle of the...

Using Newton's equation for gravity and assuming a corpuscular theory of light, one can calculate the angle that light would bend in a gravitational field. General relativity predicts a bend that is twice as large. In the Newtonian limit of GR (which includes weak gravity), does the GR...

Well maybe. The International Fencing Federation (FIE) ishttps://www.msn.com/en-us/sports/more-sports/fencing-body-interested-in-frances-embrace-of-lightsaber/ar-BBTOQ6y?li=BBnba9I progresses. Apparently this sport is popular in France and has caught the attention of the FIE, How long will...

Homework Statement Using quantum mechanics, what happens when a photon of light hits glass? Homework Equations Momentum= Mass x velocity The Attempt at a Solution I am not sure of my answer but suffice to say the following: Assuming the collision between the photon particle and those in the...

What is exactly hitting the retina for us to perceive white light in a room radiated by light from a lightbulb? The light from the lightbulb caused by heat radiates wavelengths randomly through the space around it. But how could it be that the superposition of these random frequencies and...

<Moderator's note: Moved from a homework forum.> Calculate the required voltage to produce a electric arc between 2 iron nails (distance: 3cm). I´ve read in the internet that you need 1 kV per mm. But how can I calculate this value, that I need 1kV per 1mm?

Hi all. I have a question about something Nima Arkani-Hamed said in his lecture on space-time about space contraction near light speed. I included a link to the lecture at the point where he refers to contraction of two space ships with a 'cable' between them, they are accelerating towards the...

For example, why does blue light refract more than red light in a prism?

I've read about light slowing materials, and I'm wondering if you could slow down light enough, could it be used to say store light from the sun during the day so solar panels could work at night? Or would it not work because the process of slowing light down gradually absorbs some of the...

O.K. I'm new here so if this has been answered I don't know how or where to find the answer. I'm not a scientist just a mechanic so if you could explain this simply maybe i can understand it. Maybe ELI5. Light I see in the sky has been traveling for a long time and "is here now". As far as I...

EM waves can propagate through empty space, but there is also the EM field. Is this field really empty space or is there something that exists that the light wave disturbs, like some kind of fluid? In areas of space where values of the field is 0, is this like empty space or more like calm...

It is commonly said that the speed of light when traveling inside materials is lower than that of light in vacuum, but I don't understand how this can be true. It is the same light traveling, so how can it act differently? Does light appear to be slower in materials because it is not following a...

Homework Statement In a physics lab, light with a wavelength λ0 travels in vacuum from a laser to a photocell in a time t1. When a slab of glass with a thickness d is placed in the light beam, with the beam incident along the normal to the parallel faces of the slab, it takes the light a time...

I have been looking online and it is stated to be 1nm, but these are posts around 6 years old: https://www.azonano.com/article.aspx?ArticleID=3662 https://www.researchgate.net/post/Whats_the_measurement_limit_of_dynamic_light_scattering I am wondering if the limiting resolution has improved...

Play from 3:00 It states that we will age slower if we were traveling at the speed of light compared to being on Earth. Would we not age the same but just be further away hypothetically? Next it states that if we travel away from a clock at the speed of light at 12:00, we would always see...

hello : i have a small question , can we use multi layer thin films to increase absorption of infrared spectrum , on certain surface , ? taking in mind that we use the same kind of thin film for each layer . is that possible ? best hagop

Homework Statement In the Fizeau's Experiment to determine the speed of light, let the gear have N teeth, the frequency of the rotating gear being f, the distance traveled by the light beam/ray L (distance b/w the gear and the mirror) and let there be n eclipses(blocking of the light beam)...

I just found this on the internet. It reminded me of a joke I once made: https://www.physicsforums.com/threads/science-jokes-p2.847743/page-8#post-5603278 It's a nice illustration about the "relativity" of light - kudos https://www.linkedin.com/in/james-o-donoghue-astronomer/

An intense beam of high-energy X-ray photons (violet) hits two adjacent iridium atoms (green) in the crystal. This excites electrons in the atoms for a short time. The atoms emit X-ray photons which overlap behind the two iridium atoms (red) and can be analyzed as interference images. Credit...

Hi Everyone, I'm doing research in Implanted Devices in Human Body (Sensors, Hearing Aid, Pacemaker) for my college course. I've started with FitBit that uses the PPG Principle. But I'm struggling to understand why the FitBit uses both Green and IR LED whereas the sensor at the receiving end is...

Suppose I have two apples, one is white and the other is black. Both apples are exactly the same except for their colors. The black apple is darker than the white apple because it absorbs more energy than the white apple, correct? Would that mean that the black apple would have a higher energy...

Hi. I've recently conducted a Franck-Hertz experiment with mercury. I was able to see bluish glowing regions just as here: However, theory predicts 254 nm, which is far below visible. Are there other energy levels at play here? Wouldn't that mess up the 4.9-V-spaces of the drops in the current...

Hi, The speed of light is given c=1/√(ε0μ0) where ε0 is permittivity of vacuum and μ0 is vacuum permeability. The permittivity and permeability of a material is given as ε=εrε0 and μ=μr)μ0 respectively where ε is absolute permittivity of the material and εr is relative permittivity of the...

When I think about the structure of an atom and its tightly bound subatomic particles, it is a different regime than the 'world of molecules' and their motion. The early theory of light emission (Bohr, etc) described the emission of light as a phenomenon associated with the transition of an...