# Tag Info

0

The speed of c "electromagnetism" is a potential force. Yes we can measure potential forces. We can add more potential to an already potential force. But with the speed of c such a potential is engrained into the fabric of life. The very atoms that hold us together are possible by this phenomenon. The neurons in our brain work in correlation to the speed ...

0

What you want is a lens with a shorter focal length. As a rough number, you can work with a LED to lens distance of twice the focal length of the lens. Note that that is what you are doing with your magnifying glass. It's possible to get lenses with focal lengths less than 10 mm, so you can operate at less than 2 cm separation. Since I assume you don't want ...

0

A lens with more curvature will have a focus point closer to the lens. Unfortunately, they are more difficult to make. Since these lenses need good precision, they are made with glass and poor quality lenses are usually discarded. On the up side, LEDs and sensors are designed to work as a pair and may be used up to (about) one meter, so lenses are not ...

1

Every month there is a new moon. The moon isn't visible on this day. You've seen it before. That's how dark it would be.

3

A force is defined as a change in momentum over time. In the Newtonian limit, this means a mass times an acceleration. But when dealing with things like photons, the formal definition is applied. Photons have no mass, only momentum. Therefore, if a force is applied to them, their momentum can be changed. This can happen in two important ways, a force can ...

1

However it makes sense that gravity can't travel faster than light because of the force-carrying photons Whilst it makes sense that gravity can't travel faster than light, we don't actually know this for sure. What we do however know is that the force of gravity is not conveyed by photons. Even electromagnetic force is not conveyed by photons - hydrogen ...

1

$I_0$ is the intensity of light before it hits a polariser the original intensity of the beam, so called. You need it because you need to compare it to the intensity after it exits the polariser so that you can calculate your fraction of incident intensity. this fraction requested by the problem is $I\over I_0$, but $I$ refers to intensity of light exiting ...

19

Depending on the angle of the sun, the shadow becomes elongated so that it traces out an ellipse rather than a circle. This means that the shadows of the blades must traverse a different distance across the ground but within the same time period, thus giving rise to the periodic variation in velocity. If the sun is at an angle $\alpha$ in the sky (see the ...

1

This is true. If for example you subject Hydrogen gas to a perfectly monochromatic 121.57 nm laser, then all that will happen is that the gas will scatter the light in all directions, glowing without increasing the temperature. Otherwise there are many different phenomena that are involved in the heat transfer of energy by radiation. For example in solids, ...

0

Ultraviolet light is very high energy. When it makes contact with your skin, you can actually break chemical bonds.

1

You should read this page :http://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_principle The Huygens principle explain intuitively why the wave will spread after being "cutoff" by an obstacle, as the spherical sources at the edge will not interfere anymore with the adjacent ones (those being stopped by the obstacle) to form a plane wave. As shown in the ...

2

There are three factors that need to be considered across all wavelengths: (1) the ability of the water droplet to refract and disperse the incoming light, (2) the ability of the eye to sense the wavelength, and (3) the ability of air to transmit it. The visible range we 'see' in a rainbow with our eyes satisfies all three. UV , depending on how short the ...

2

If you apply a constant voltage to a piece of semiconductor, you will not get light out of it. All that will happen is that the electrons that are already in the conduction band will start drifting in the applied electric field, so you will get conduction and some heating. In order to get effective recombination of electrons and holes (i.e. atoms that lack ...

9

engineer already answered it completely, I only want to add that the question is completely valid even if you already know that separation of wavelength occurs. The thing is, some materials are practically opaque or too much transparent (refractive index is equal to that of air and no separation occurs) in infrared and ultraviolet while transparent in the ...

5

Is it possible that rainbows have ultraviolet bands and infra red bands and we are not able to see? Yes, see engineer's answer. As for whether we can see them, take a look at aphakia: "Aphakic people are reported to be able to see ultraviolet wavelengths (400–300 nm) that are normally excluded by the lens. They perceive this light as whitish blue or whitish ...

44

Refraction of light in water droplets, leading to the formation of rainbows, is not limited to the visible range. Experimental evidence, compelling due to its simplicity, is shown in the following images taken by University of College London Earth Sciences professor Dominic Fortes. Check the alignment of the rainbow with respect to the trees in each of the ...

1

No. As has been said, the raindrop is not emitting the light, it is just acting as an optical device that deflects light emitted by the sun. However, the spectral lines you would expect to see in sunlight refracted by a prism will not, repeat NOT, be seen. The mechanism that produces rainbows is very different than the mechanism that produces a spectrum ...

0

...couldn't I use a retarding film of randomly varying thickness to convert a Coherent laser beam into an incoherent laser beam to improve eye safety? Absolutely not. A beam's destructiveness to the eye depends on three things: Energy delivered to retina and the time periods it is delivered over, quantified by the ISO60825 concept of Maximum ...

1

Couldn't I use a retarding film of randomly varying thickness to convert a Coherent laser beam into an incoherent laser beam...? If the film is not changing (in time) you are not changing the coherence properties of beam at all. You can think of putting a slab of something in the way as putting a really bad lens (possibly with no optical power) in the ...

0

In other words, why does light seem to pass through itself without interacting? Because it consists of waves. Anna mentioned photons, but see Wikipedia where you can read about E=hf or E=hc/λ, wherein f is frequency and λ is wavelength. Photons have a wave nature. Imagine minor ocean waves, intersecting. They ride over each other and keep on going. They did ...

1

Beams of light as we observe them are described classically by Maxwell's equations which have been validated over and over again. Optics is a very well studied subject. In this classical framework light is an electromagnetic wave that moves in vacuum with a constant velocity c in all frames and does not need a medium to propagate, in contrast to sound ...

0

In older LCDs with cold-cathode fluorescent lamp (CCFL) which runs along one side of display panel they also use special optical elements to distribute light evenly. Example is given here: http://www.zemax.com/support/resource-center/knowledgebase/how-to-model-an-lcd-backlight

1

There is a diffusion panel located on top of an led array, it essentially is a wide flat fiber optic panel with small holes on the front (Screen side). The LEDs shine into the plate and spread throughout the plate via total internal reflection. When light hits the holes, it comes out. Random thesis paper describing the theory of operation of a LCD screen

2

Not quite that simple because of spectrum analysis. A light source is never "pure". So an orange (type-K) sun would make our colors here different, but bluish would still exist because of the range of wavelengths of the source. But it would be less common.

3

Classically (since rob has done a thorough job on the quantum picture), the amplitude of a light wave is not related to any physical extent. It is not the size of the wave in space, it is the strength of the fields (electric and magnetic). We often draw wavy lines, but if you look closely the transverse axes will be label differently for, say, waves on a ...

2

If you twisted my arm and forced me to assign an amplitude to a single photon, I'd do it this way: The energy density of a classical electromagnetic field is \begin{align} U &= \frac12 \left( \epsilon_0 E^2 + \frac1{\mu_0} B^2 \right) \\ &= \epsilon_0 E^2 &\text{(only for light in a vacuum)} \end{align} where $E,B$ are the amplitudes of the ...

0

I just made a complete answer for how rainbows are formed at What really makes a rainbow happen?. I'm answering here because the current answers say that it involves Total Internal Reflection, which is not true. There are internal reflections, but they are not total. And the light does not exist at just one angle, it covers a wide range of angles. Every ...

2

Coherency of light in practice is not an either/or issue. Any light due to any source has some degree of coherence. Laser light has usually much higher coherence than light of a hot metal filament. Some degree of coherence means, in simple wording, that light waves at one point of space due to different parts of the source behave similarly (they have ...

1

The Lorentz transformations are used to transform between different inertial frames. For example if you and I are in relative motion then the Lorentz transformations convert the positions of spacetime points in my rest frame to the positions of spacetime points in your rest frame. However anything travelling at the speed of light has no rest frame, so the ...

1

the forward land delegate IS approaching the light and the backwardland delegate IS moving away from it Are they? Or are the delegates sitting perfectly still, and the Earth spinning quickly beneath them? Of course we have a convention that the Earth is stationary and the train moves across it, but we also know that the Earth is not stationary - it ...

2

Simply put, relativistic speeds cause for events previously thought of as simultaneous to no longer be simultaneous if the velocity of the reference frame of the event changes relative to the defined observer. The best way to wrap your head around this is to pictorially trace what is happening in space time. The case you describe is v>0 Think of v in ...

2

Light moves at about a foot per nanosecond, or a meter every three nanoseconds. In order to capture it propagating across a room over a few frames, you would need to gather something like a billion frames per second. No consumer camera -- indeed no camera on Earth -- is capable of this. Now there have been people playing with "fempto-photography," but they ...

3

The spectrum of the black body radiation is given by Planck's law. The total amount of radiation is given by the Stefan-Boltzmann law. In principle there is no shortest wavelength, because the radiated intensity remains non-zero at arbitrarily small wavelengths. However, at the low wavelength end of the spectrum the radiated intensity falls exponentially ...

5

They taught me that in high school too (i.e., that matter is "mostly empty space.") Only thing is, it's not true. Solid matter is mostly filled with electrons. Yeah, the mass is all concentrated in the relatively tiny nucleii, but the mass is not what photons interact with, and the mass is not what defines the physical and chemical properties of ordinary ...

1

The speed of light in a vacuum, or c, is 299,792,458 meters per second. Any other medium will slow light down. For example light takes about 40% more time to go through glass than vacuum. But media that would increase the velocity of light would violate fundamental laws of physics and can therefore not exist.

10

Molecules aren't just sums over their constituent atoms. There's many different kinds of bonds which involve different patterns in the overlap of electron orbitals, and which affect the energy levels those electrons can occupy - I'm assuming the QP video you watched explained how "color" relates to electron energy levels. The (hydrogen-like-)atom case is ...

2

There are two ways to look at light, classical and quantum mechanical. Electromagnetic waves given by the classical solutions of Maxwell's equations will have interference patterns as predicted mathematically from the sinusoid form of the solutions. Are we working in the double slit argumentation with destructive interference arguments too? Young has ...

4

The answer is that color is determined by electron transitions between different energy states. Those levels are different in molecules than they are in the component atoms where there is only a central force in atoms, whereas the multiple positive charges in molecules creates a more complex potential field for the electrons to move around within. Molecules ...

40

I'll do that teacher thing and turn your question around back at you. Why isn't the spectrum of the lithium atom just the spectrum of the hydrogen atom plus the spectrum of the helium atom? And, for that matter, why is the helium spectrum not simply two copies, somehow, of the hydrogen spectrum? Why do atoms have unique spectra in the first place? The ...

4

The only thing I can think of being true black would probably be a black hole. As light does not bounce off a black hole.

3

First of all I think a suggestion is mandatory: please don't mix units of measure. You are using lux and lumens, so it's better if you stick to SI units. So distances are measured in meters, areas in square meters and angles in radians. This is meant to help you, not to annoy. ;) Lux measure how much light (lumens) hits a square meter of surface. So if you ...

22

The problem with the suggestion of using polarization is that you now have the reflections off the polarizers to contend with. I think the short answer is "it depends on how 'black' you want it to be". "Truly black" = reflectance of 0. I am quite sure that is impossible - there will always be some probability of light scattering off a surface. All you can ...

1

Your eye is just a lens and a screen. The lens is actually a compound lens, since both the cornea and the lens play a role in focussing the light, and the screen is your retina. So if your setup is initially: Then if you take out the screen and put your eye there you'll get: So you can work out what appears on your retina with exactly the same ...

1

The main question I am getting at is, does time dilation have a refractive index? What I mean is, if I were to shoot a laser past a black hole, would the laser's path "bend" strictly from time dilation not considering the gravitational effects? We don't talk of gravitational lensing for nothing, but IMHO you're getting this back to front Joe. See what John ...

4

Newcomers to relativity tend to regard concepts like time dilation and Lorentz contraction as somehow fundamental concepts from which relativity it derived, but this is not the case. Time dilation arises because the integral of proper time along the worldline of some object will not necessarily match the coordinate time measured by a distant observer. So to ...

1

You would experience no redshift, ignoring gravity and the expansion of space.

0

There are two general classes of polarization. Plane polarization where the electric field is in a plane. Glare reducing sun glasses use plane polarizers because reflections of water and other smooth surfaces are polarized due to Fresnel's laws of reflection. The second type of polarization is circular or more generally elliptical polarization. The plane ...

3

The water droplets that create a rainbow are not emitting the light that you see in a rainbow; if they were, you would see a glowing cloud of consistent color, not a rainbow. The rainbow is formed by sunlight refracting and reflecting through water droplets in the air; the water refracts through the "front" of the drop, reflects off the "back," and refracts ...

1

There's an argument I think briefly mentioned in Aristotles Physics; where he argues that an object travels in a straight line since to veer would require a cause; he actually says for what reason would it move up or down or to the left? In another sense it's an argument from symmetry; actually this is related to Newtons First Law where cause is interpreted ...

1

No, or at least not under normal circumstances. Calculating the gravitational force between light beams turns out to be rather complicated, so let's use an analogous but simpler system. Instead of a spherically symmetric pulse of light consider an explosion that throws out a spherically symmetric cloud of light particles. At any time $t$ the cloud of ...

Top 50 recent answers are included