Tag Info

Hot answers tagged

57

You are seeing particles. However there's more to this than meets the eye so I need to explain exactly what I mean by this. Light is neither a particle nor a wave. Instead it is a quantum field. As a general rule while light is travelling it appears as a wave, but when the light quantum field is exchanging energy with anything it does so in quanta that ...


49

Mostly, you see things because they reflect light. They absorb some of it, which gives them their color, but you will also see them, if you shine infrared or ultraviolett light at them. So: Whatever light you shine at them, a large part of this light will be reflected and you can detect this light to "see" matter. Your argumentation therefore seems ...


22

EM radiation is aborbed in two ways: black body absorption electronic/vibrational/rotational excitation All solids absorb due to the black body mechanism, (1),but all this does is convert the incoming radiation to heat and it's difficult to be precise about exactly where the incoming photon hit. Some very basic imaging can still be done, for example ...


19

In addition to the other answers, the visual spectrum of light corresponds to a notch in the atmosphere's absorption spectrum. Image source: http://en.wikipedia.org/wiki/File:Atmospheric_electromagnetic_opacity.svg


9

Apart from the "physics" explanations in the other answers, we must consider biology. To make an "eye" one needs some kind of lens (or, conceptually, a mirror -- evolution found the Galilean telescope, not the Newtonian, but maybe it could have) that focuses the incoming light, and receptors that get activated by these photons. There are (at least) three ...


7

In addition to the answer by Martin, there is also the physical mechanism of detection of light. light with an ultraviolet wavelength can be absorbed to ionize an atom, triggering a chemical reaction; light in the visible region can only be absorbed to trigger excitation and require complex molecules with specific chemical properties to transform this ...


7

The solution boils down to examining what is meant by the term instantly in: "If I turn the light off in a room why does the light instantly disappear" All of the visible photons in the box cannot instantly disappear since the information about the source going out only travels at finite speed $v\approx c_\text{vacuum}$. Once the source stops ...


6

From Wikipedia: Triboluminescence is an optical phenomenon in which light is generated through the breaking of chemical bonds in a material when it is pulled apart, ripped, scratched, crushed, or rubbed (see tribology). The phenomenon is not fully understood, but appears to be caused by the separation and reunification of electrical charges. ...


6

Supposing we could shield ourselves with a perfectly nonabsorbing, reflective shield so that light would perfectly elastically bounce off us, thus preventing high power beams from incinerating us as Anna V's answer validly argues they would. Then the "fundamental" answer to your question is "because light has zero rest mass"; to explain further: the ...


6

It would be physically impossible to be able to "see" light as anything other than a particle (photon). The only time photons, or any other subatomic particle for that matter, can be described as a wave is when we are NOT looking at them.


5

A ray of light is a geometrical line describing the propagation of an electromagnetic wave. The electromagnetic wave is composed of zillions of photons each with a tiny momentum. The momentum is not large enough to sense an impact, it is pico newtons even for a laser beam. Lasers can have very high energy and momentum, but like knives, they cut soft tissue ...


5

There are several terms with precise meaning in physical sciences that have been pulled into common language and misused. Opaque however does not appear to be one of them. According to several etymology sources I just looked up (like Etymology Online for example) it appears to have rather mushy origins in Latin and French meaning darkened or shady. For the ...


5

There are two questions here. The first - "what is the definition of opaque" is terribly broad and depends on the field / context. I will focus on the second: when and how does a body let radiation through? We should really ask the converse question: by what mechanisms does a body stop radiation from going through. I will answer this for different parts of ...


5

For fluorescence to occur, you need to excite at an energy higher than the emission. According to the spec sheet this plastic scintillator emits at a peak of 420 nm - it may be that 400 nm is just not a high enough energy to excite it. I am used to thinking of scintillators as being used with radiation sources - xray, gamma sources. You need a pretty "hot" ...


4

Of course you can. The prism is likely to disperse the light - that is, different colors will be refracted by different amounts. That means you don't just get "a" refractive index, but with careful experimental setup you will get the entire refractive index curve: a different value for every wavelength / color. Setup: a narrow beam of white light incident ...


4

It may be useful to start this explanation from the origin of a light wave: an oscillating charge. Start with the idea that a stationary charge is surrounded by an electric field, then imagine wiggling that charge up and down. Now the field lines will turn to wiggles instead of straight lines. Those wiggling field lines are the electromagnetic waves we call ...


4

Acoustic waves travel through a medium (air, water, metal, etc), there is no known medium through which light travels Both the speed of sound and the speed of light have fixed values regardless of the speed of their source Acoustic waves can be longitudinal (in gases) or transversal (in solids) whereas light is only transversal. You can measure acoustic ...


3

The book is imprecise because there are other types of polarized light. Now, consider a point in the path of the wave, like the green point in animation you mentioned. In that image, the light is linearly polarized so the electric field draws a line at the location of the point. However, there is also circularly polarized light where the electric field draws ...


3

The only requirement for light is that the electric field must be perpendicular to the magnetic field at any given point in time or space. This assumption arises naturally from Maxwell's equations. The most intuitive way of thinking about light is with the picture you included of the light wave. However, you have to imagine an infinite number of light waves ...


3

I get that the sun is producing white light which is scattered threw our atmosphere so that the light of the sun reaching our eyes is yellow. Not very much. When the sun is high in the sky, most would describe the light as "white", not "yellow". That would be more true for a sun low to the horizon. So how come if I look to a piece of white paper ...


3

Adding two stokes vectors does not give you the stokes vector for the combination of the two beams. For example, adding a beam of horizontal and vertical polarization would make a beam of 45deg (linear) polarization. In order to add two beams you would have to come up with a Muller matrix $M_\vec{a}$ for adding $\vec{x}$ to $\vec{a}$. Unpolarized light has ...


3

I think you mean that the refracted ray is at the critical angle. I tried it myself with 1 INR coin. Although the image I took was not from a ray $90^o$ from the normal, it was $1^o-2^o$ lesser. The meniscus of water interfered with my observations. When we look at the coin from the normal, a flat 2d image appears. As the angle between the normal and the ...


3

Macbooks use LCD screens. These have a liquid crystal layer with a backlight behind it. The LCD screen works by changing its opacity i.e. it controls the amount of light that can be transmitted through it from the backlight. If the LCD is not blocking any transmitted light then all the light from the backlight is transmitted and the screen looks white. If ...


3

The light we see with our eyes is electromagnetic radiation, very well modeled by Maxwell's equations. Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. This 3D animation shows a plane linearly polarized wave propagating from left to right. Note that the electric and magnetic fields ...


3

Assuming you're talking about propagation through free space, the beam will be diffracted by the aperture you pass it through (3cm in this case) and that will cause the beam to diverge. The far field angular divergence, $\theta$, is approximately given by the equation for the Airy disk: $$ \sin\theta \approx 1.22 \frac{\lambda}{d} \tag{1} $$ where $d$ is ...


3

Optically speaking, and very simply: An opaque material permits no light to pass through it. A material which passes light but does not pass image detail is called translucent. A material which passes light AND image detail is called transparent. "Passing light" technically means ANY light, but in practice some materials pass so little light that they ...


3

You can get original Fiestaware pieces online or at some antique stores. The uranium glaze is a decent low-level x-ray and gamma emitter that is safe to handle. My orange salt-shaker reads a couple hundred Bequerel on a good Geiger tube held a few centimeters away. You can also order nano-curies of a number of isotopes without any paperwork (in the US, no ...


3

I'd like to add to dmckee's answer PLEASE heed his last paragraph. There are heaps of experiments on the internet that blithely instruct you to take a smoke detector apart. Another good "safe" source is uranium marbles: they are traded by marble collectors and are easier to come by than Fiestaware or Annagr√ľn / Annagelb pieces (and the last two are too ...


3

You surely know that glass is colorless in visible spectrum. But it does absorb in IR and UV parts of spectrum. Here's a sample spectrum of borosilicate glass 3.3, universally called Pyrex glass: source If our eyes could see light with wavelengths around $3\,\mu\text m$, we'd actually see this glass sample as coloured, but, as you can see, the whole ...



Only top voted, non community-wiki answers of a minimum length are eligible