# Tag Info

0

White light does not have a single wavelength but instead is described by a spectral distribution, which is not uniquely determined. There are many light compositions that can pass as white. The spectrum of the white light from the three LEDs combined is just the sum of the individual LED spectra, weighed with the proper intensity. Likely, your LEDs are also ...

1

As R.W. Bird said, our 2 eyes give us a perception of distance for not too far objects. Our brain uses that information and the apparent size to estimate the real size. But it doesn't work for distant objects. The moon and the sun have almost the same (apparent) size. When guided only by perception, we are completely unable to estimate their real sizes.

2

One eye perceives the angular size of an object. With two eyes we can get an estimate of the distance to the object. In combination these can let us estimate the actual size. Our estimate of size will often be dependent on the context in which the object is observed.

1

You can't filter the light to add additional wavelengths (filters could only remove the blue wavelengths and then you'd be left with nothing else) but a completely different approach is extremely common and blue LEDs are actually the basis for white LEDs typically. Phosphors are used to absorb the blue light emmited by the LED and this puts the phosphors in ...

0

It totally depends on the type of incoherence. If the source is spatially incoherent, it will spread by an angular amount corresponding to the angular size of the source as seen from the final beam- forming element, plus the amount that a beam formed from a spatially coherent source would experience. Temporal coherence doesn't really matter in your ...

3

In 1856 with Rudolf Kohlrausch (1809–1858) he (Wilhelm Eduard Weber) demonstrated that the ratio of electrostatic to electromagnetic units produced a number that matched the value of the then known speed of light. This finding led to Maxwell's conjecture that light is an electromagnetic wave. This also led to Weber's development of his theory of ...

1

The speed of light was measured before Maxwell with the Fizeau-Foucault apparatus: 1848-1849 by Fizeau: $315000$ km/s 1862 by Foucault: $298000$ km/s 1872-1876 by Cornu: $300400$ km/s Around the same timer (1861-1862) Maxwell set up his equations for the electric and magnetic field. By solving these equations he could predict the existence of ...

3

It was based on some experimental as well as theoretical measurements. Maxwell calculated the speed of his so called em waves for vacuum at that time using the formula derived from his equations $$V^2 = \frac{1}{\mu \epsilon}$$ The value which he got from the above equation was $3 × 10^8 /s$ and this value was very close to the experimental measurement of ...

3

It's almost certainly black mold/mildew. It usually appears where humidity is higher than normal.

0

The answers are different because you have, in the course of two successive approximations in your second method, unknowingly assumed that θ/2 = θ. It can be shown that the exact path difference is a tan(θ/2). In approach one, the path difference is a(tanθ)/2. While that in that 2nd one is a sinθ, which is way off compared to the first. Clearly the first is ...

2

I think you are confused due to the terminology 'virtual' here. 'Virtual image' here does not mean there is no image, it simply implies that the image is not where it appears to be. If you are looking through a transparent material, you also see an image but this time, the object is actually at the place from where the light appears to come and hit your eyes....

0

What I have learnt in school is that, we are able to see images behind a mirror(virtual image) because our brain assumes that the diverging rays that our eyes see, are coming from a point behind the mirror Not necessarily. Images are images independent of what the brain is doing. It's not like our brain has some mechanism to first determine if an image is ...

1

A camera doesn't "know" if a source image is real or virtual. As long as the rays from each point in the source image are not converging, and aren't diverging from a point too close to the camera lens, the camera can form a real image on the film or sensor array.

2

In these calculations, the light is modeled as an ordinary particle whose speed happens to be $c$ either at infinity or at the point of closest approach. The speed isn't (and can't be) constant on the whole orbit. The paper that you linked sets the speed to $c$ at closest approach (see the text after equations 5 and 7). There's really no adequate Newtonian ...

0

As I vaguely remember, Newtonian gravity does not have any affect on light. Einstein's General Relativity was the first prediction about gravity bending light, and this was confirmed a few years later by seeing the effect in a telescope during a full solar eclipse.

2

It is easy to construct situations in which light does not take the "fastest" route. For example, set up a laser pointer to send a beam across the room. Put a mirror in its path and deflect it to a second mirror, then tilt the second mirror to direct the beam to the point on the far wall where the beam would hit if all the mirrors were removed. ...

1

Good question and good answers. Worth adding that also the electric field energy and the magnetic field energy are going to be exactly equal in such plane EM wave. It is just the traditional definitions of different units for E and B that make B seem thousands of times smaller. Since the energies are the same it is good they both get represented fairly in ...

4

The atoms in the Sun's atmosphere that are excited by absorbing photons will indeed de-excite by emitting photons. But an emitted photon won't usually be emitted in the same direction that the absorbed photon was travelling. So if the absorbed photon were travelling towards the Earth, the emitted photon almost certainly won't be. Hence the dark absorption ...

1

I drawed two different sized lamps that illuminate a screen trough an obstacle with two holes. I hope I succeeded in making its self explanatory, if not: I have drawn the "limit photons" for each hole. By that I mean the two photons between which all the other photons are that went trough the same hole. You can see that for a larger (and large ...

2

Mass is defined in relativity by $m^2=E^2-p^2$, where $E$ is the mass-energy and $c=1$. A ray of light or an EM plane wave has zero mass. However, mass is not additive, and a collection of light rays, or a more complicated wave pattern can have nonzero mass. It's not really correct to argue about whether light has mass based on gravitational effects as it ...

2

Final edit. If it is the first time you read this go to the bottom, where I have summarized the arguments about classical electromagnetism modeling light, and mass in special relativity, replying to the title. Here once again is an example of how light is an emergent phenomenon from single photons. This is a double slit experiment one photon at a time: ...

2

A very bright, very brief flashes of light are all that's needed. Let's say that the bullet moves at 2 km per second. That's 2 nanometers per nanosecond. There are lots of lasers that emit pulses of a nanosecond or less. A bullet is, say, 2 cm long and you would want something like 10 "frames" in a movie during the time that the bullet moves a ...

1

While a bullet may have its image captured by high speed cameras, its speed is normally just too great to be seen by the naked eye if you and the gun are at rest with each other. Even using shutters or strobes the bullet would have high a speed and too great of a space between frames for the naked eye to observe it. As for a spinning hub, either a properly ...

1

One of the things I learned from Heisenberg's Uncertainty Principle is that when you try to observe the position of a microscopic particle, you have to use a light of smaller wavelength (which implies high energy), and that causes a large change in the particle's momentum. Be careful here. Don't mix up the HUP with the observer effect. The HUP relates ...

0

For light (photons) $E = pc$ and $p = \frac{h }{\lambda}$we can see that photons of higher momentum have higher energy, but momentum is inversely proportional to wavelength. In decreasing the frequency of the light, we are using light of higher wavelength since $c = f \lambda$. This diminishes the light's ability to probe a material, meaning smaller detail ...

0

In a pinhole camera, the photons/waves strike a scene, self-modulate, and then organize contiguously into a 2D image. Where is the template stored for the image if the brain is a 1d processor? The question is concerned with the nature of informational dimension vs. spatial dimension. The material world cannot account for the phenomenon of spatial dimension ...

0

Assume Light is a being. Light doesn't carry information. All it does is, just pass through, if the object it hits, allows it to pass through or it gets reflected back. Example 1 : Infrared Laser in Scanning barcodes. Barcodes are nothing but alphanumerics shrinked in size. When infrared laser is allowed to hit it. Each letter in that alphanumeric ...

0

Do you want to use a material in solution or a solid ? Maybe, organic dyes (2-Aminopurine in water) can be useful for your purpose. Never used in my case, but it seems to be well documented here. In solution, if your concentration of dyes is low enough, you can expect a fluorescence lifetime similar to the literature. In solid, it's more complicated in ...

0

How does light, which is an electromagnetic wave, gets encoded with the information about the object? There are several stages to this. Initially there has to be a light source emitting photons. This can be the object itself, but is more likely to be a separate light source such as the sun or a light bulb, usually a 'white' or 'wide spectrum of wavelengths' ...

0

Short wavelength (blue) light is hard to make because it is hard to make. It requires a large amount of power. shifting the peak wavelength an equal nanometer interval requires exponentially increasing quantities of power as the wavelength gets shorter. Filaments can't do it, the necessary power melts them. you do get vaguely tinted "blue" light ...

3

So you have 2 initial vectors: $\vec k$ is light's wave vector and $\hat n$ is the normal to the reflecting surface. The final wave vector can be some combination of: $$\vec k' = a\vec k + b\hat n + c(\vec k \times \hat n)$$ where the prefactors can be combinations of numbers, and available scalars such as: $$0, 1, k^2, \vec k\cdot \hat n, ||\vec v \times ... 1 When you receive a beam of light, you can measure five things: the direction it came from its wavelength distribution (roughly, its color) its intensity (how bright it is) its polarization (an aspect of light we typically can't see) what time it arrived (or how aspects 1–4 change in time) This is all the information you have. When our eyes receive light, ... 3 Imagine an alien drops a transparent artifact nearby. Not knowing what it is, or whether it's dangerous, you decide not to try to walk up to it and touch it. All you know is that you can see through it, and thus, you don't really know what its outline looks like. However, you have several children and lots of plastic balls. So you give each child a ... 10 The light that is seen in the photo is white light from the solar photosphere that is Thomson-scattered from free electrons in the corona into the line of sight (see diagram below). The hot, optically thin corona, hardly emits any visible light itself (just a few isolated emission lines, the brightest of which is in the green at 530 nm, and almost no ... 0 Your other questions have been beautifully answered by other users and I will try to explain the second part of your question. Also if light hits an object and then another one on the way to our eyes, does it only carry information from the very last interaction it had ? Where does the information due to all previous interactions go (if it is indeed) ? For ... 5 It's unreasonable to expect that any object with approximately-blackbody emission spectrum will have violet color at any temperature. Actual Planckian locus spans colors from red through orange to yellow, white and finally sky-blue, with the latter color achieved at extremely high temperatures. Now, in addition to this, generally photos can't be relied on ... 15 When a photon hit the retina, it only has two pieces of information: Its wave length and its position/direction. That is all. But it is not alone. We are bombarded with billions of photons every second and the pattern these photons make is where the information is hiding. And we have a brain that is pretty good at figuring out these patterns. Let's say a ... 0 Well after reading more on it, I realized the third condition I mentioned is right, but my interpretation of comparable was wrong. I assumed that comparable meant the values of aperture size and wavelength have to be close to each other. But what the condition was actually stating was that the order of λ and d are comparable (and since λ is of the order 10^{... -1 No we can not because shadow is formed on opaque objects. 5 I think that trying to think in terms of light "getting encoded with information" is a confusing and excessively complicated way of thinking about things. Suppose I'm standing next to a window, and there's a lamp on the other side of the window. When light from the lamp encounters the glass, what happens? If you wanted, you could describe what ... 27 You talk about light as if it were a person carrying a clip board writing down things on its way to you. It is a physical phenomenon that gets affected as it propagates. Depending on the various processes that it goes through before it reaches your eye, its amplitude,polarisation, frequency (or wavelength), pulse time etc. get affected from which we can ... 4 Let me first deviate to radio&television: a radio wave with constant frequency does not carry information, since it is absolutely predictable on the basis of a few parameters: its amplitude, its frequency and its initial phase:$$ X(t) = A\cos(\omega t +\varphi) $$The information is encoded into the wave by modulating these parameters, i.e. by changing ... 4 Light may carry information defined by which frequencies of the light spectrum it has. For example, the colour of an object is information carried by light. White light from the sun is actually many different wavelengths combined to create "the colour white". These wavelengths can teach us about: What object it reflected off of What created this ... 0 Yes, it was. Spectral radiance of black body with temperature of 3000\,K in a visible EM frequency range is not zero, as can be seen by Plank radiation law : If you want to find total power emitted per solid angle \Omega in visible wavelength range- you need to integrate Plank law's spectral radiance over that wavelength range like :$$ P_{~\Omega} = \...

0

The reason is physiological, not physical. In short, human retina react differently on different wavelengths, and that varying reaction of the retina causes varying perception of color in the brain. The retina can distinguish between different wavelength becuse it has several types of light-detecting detectors: one that reacts most strongly with long-...

0

The notion of apparent size that you're using here doesn't match up with the usual notion of image size in optics. By your definition, an object like a light bulb or the sun has infinite apparent size since it illuminates your entire sensor panel regardless of the panel's size. It would get dimmer as it moved away, but not "smaller". Your laser ...

Top 50 recent answers are included