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69

A 40W incandescent light bulb has a luminous efficiency of 1.9%. That means only 1.9%, or 0.76W, of the energy consumed by the bulb ends up as visible light. LED bulbs have an efficiency of around 10% - the efficiency depends on the design and can be as high as 15% or as low as 8%. So a 6W LED bulb will produce between 0.9 and 0.48W of visible light. The ...


45

The advertising suggests that the new 6W bulb generates as much light as a 40W incandescent light used to. Most of the energy in an incandescent light bulb is converted into infrared radiation that we can't see or it's at the red end of the spectrum where the human eye is not very sensitive. This is a typical incandescent spectrum and you can see how ...


31

Yes, photons can. See https://en.wikipedia.org/wiki/Radiation_pressure (and photons are certainly massless). PS In fact, any massless particle has momentum (which has a fixed value since they can only travel at the speed of light) and if it is scattered on a body, it changes its own and the body's momentum, which is what a force does.


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You're mixing power needs with luminous effects. According to the advert you posted, that LED bulb consumes 6W (power) to get a luminous flux of 500 lumens (lumen is a photometric unit, like candela or lux): https://en.wikipedia.org/wiki/Luminous_flux On the other hand, an incandescent bulb would need to consume 40W (power) to get the same luminous flux. ...


14

When an atom or molecule absorbs a photon, it enters an excited state; each excited state has a mean lifetime. When the atom or molecule returns to the ground state it may emit a phonon (vibrations), or it may decay through multiple levels; in this case there are multiple photons, with different wavelengths. In the case where the absorbed and emitted ...


9

Every Element/atom has a different electron configuration. This gives the valence electrons unique energy levels and arrangements. When electrons absorb energy they are excited to certain and again unique energy levels. When The energy is released it gives a photon a certain frequency which we perceive as a certain color.


7

It sounds like you know some of the most important summary points about blackbody radiation, but here is a reference on the subject, since I will be talking almost entirely about blackbody radiation: https://en.wikipedia.org/wiki/Black-body_radiation Given any temperature, there is a certain emission spectrum (see ...


6

When light enters glass (or another transparent material), its frequency stays the same and its wavelength changes. In a comment, you say that you are using "color" to mean "wavelength". Well, I think you are using the word "color" incorrectly. According to a normal definition of "color", the color of light does not change when it enters glass. But the ...


6

This phenomena is called Heiligenschein effect. But there are many other names for it. This is generally seen on dew covered grass. The dew drops act like lenses, focusing sunlight on the grass leaves and illuminating them. Read these for more information and pictures: Dew heiligenschein (more images) Optical effects: Heiligenschein Watch this video ...


5

Newton's 2nd Law of Motion gives the impressed force as $F=dp/dt$, so a physical theory for a massless particle exerting a force requires that the particle have momentum, $p$. First we will discuss mass, momentum, the force law, and Special Relativity. In Newtonian physics mass is identified in two ways: by it's inertia, or as the quantity of matter. The ...


5

A lot of things can affect the color of an object. As you've mentioned, absorption plays an important role in determining what part of the visible spectrum gets subtracted from the color your eyes perceive. Optical bandgap arising from the microstructure of materials determines what portion of the spectrum is absorbed. It is closely related to the electronic ...


4

An inverted laser medium can be used to amplify light in general so in principle this is perfectly possible. The output would however inherit the properties of the sun light and you would not get a laser like type of radiation. What you also have to keep in mind is the limited gain bandwidth of typical laser materials. Ti:sapphire is an example of a very ...


4

This is one of the first examples of energy levels for electrons within the atom! If we take the Bohr model, which imagines that electrons circle the nucleus on set orbits Each of these orbits has a corresponding energy. The electrons are more stable at lower energy levels, and thus, prefer to be there. When you provide energy to the electrons (in the ...


4

The material part of a black hole is (classically) compressed into a zero volume area, and almost all of the information of the matter that eventually became the black hole was dissipated away, so the original notion of your question is unanswerable. There IS another sense in which we can think of your question, though. Black holes are known to shine light ...


4

There are three pigments in the human eye cone cells and combinations of their light sensitivities are the basis of our color vision. These pigments are red, green, and blue. The violet end of the spectrum excites blue pigment only. Less-extreme 'blue' perception includes some slight green pigment response. The color that results from red and blue ...


3

For typical laser goggles the color of lens is the color of light that is transmitted through the lens. Thus if the lens is red, it will not protect you from laser beams that are in the red portion of the spectrum. The color you are looking for will be, in some sense, the complementary color; since red-orange-pink are far from blue-violet-ultraviolet, ...


3

Accelerating and decelerating charges produce electromagnetic radiation. Bremsstrahlung : Bremsstrahlung (German pronunciation: [ˈbʁɛmsˌʃtʁaːlʊŋ] ( listen), from bremsen "to brake" and Strahlung "radiation", i.e. "braking radiation" or "deceleration radiation") is electromagnetic radiation produced by the deceleration of a charged particle when ...


3

It is not possible to get a Bose-Einstein condensate of photons in three-dimensional equilibrium. Since the photons have no mass gap and no chemical potential, they can just be absorbed by the walls. In this example, however, the experimenters used a gas that was out of equilibrium, with different effective temperatures for the motion in different ...


3

You cannot simplify the effects of EM radiation on biological systems to simply $E=hf$ because different materials absorb or transmit different frequencies preferentially. $E=hf$ tells us the energy per photon, but it doesn't tell us how much is absorbed by any particular type of cell. It also doesn't tell us the intensity of the radiation (energy per ...


3

Sun pumped laser The two most studied lasing media for solar-pumped lasers have been iodine,1 with a laser wavelength of 1.31 micrometers, and NdCrYAG, which lases at 1.06 micrometers wavelength... The largest solar-pumped laser is currently being operated by a research facility in Uzbekistan. It is a 1 MW solar input power NdYAG type laser, ...


3

Yeah, @EddyKhemiri, as @almagest wrote it isn't just visible light, but rather that all electromagnetic waves moving through a vacuum travel at $c$. Also- just a pet peeve, but remember that this is the speed in a vacuum; it can be slower in different mediums.


2

The reason for these is mostly reflection inside the optics of the camera, both from the surfaces of the lens elements and from the structure of the lens body, diaphragm, camera body and so on. There can be other reasons for artifacts like this: diffraction effects when the lens is stopped right down and effects due to light bouncing around inside the film ...


2

You will want to refer to the blackbody curve, which will have a broad spectrum and a peak wavelength. Consider the sun for a moment. At a temperature of around 6000 Kelvin the peak wavelength of its blackbody curve will appear around 500nm, which is blueish-green light. But that is obviously not the only wavelengths that the sun is emitting. The ...


2

From the view point of information loss we can think about how many degrees of freedom there are. The detector will in general have few degrees of freedom (compared to the source system you are trying to measure) and that is even if its precision is perfect! Now let me try to specifically address the points in your question wether we could see see ...


2

Being shortsighted I needed glasses while growing up, but managed to avoid wearing them by cheating the yearly eye-tests at school. (While the tester was in the teacher's lounge drinking coffee I memorized the two bottom lines of the chart he left in the gym.) My best friend wore glasses and complained constantly that other people made fun of him and I ...


2

The truth of the matter is that we don't know. Gravitons are a theory slowly becoming accepted but there is simply little evidence to support them as a viable theory on how gravity works. Gravity is still just the magical notion that was devised centuries ago. We only know that gravity is not constant and it is the only force not conforming to the theory ...


2

Gravity "bends" light, predicted with theory of relativity and subsequently observed: how does gravity and gravitational waves achieve this effect I conceive the question as one referring to General Relativity (GR) which is an entirely classical theory, and not to a (thus far missing) theory of Quantum Gravity. The point of view of General Relativity is ...


2

Short answer: autofluorescent proteins, most likely. Many biological structures emit light after absorption of light at a different wavelength; this is called autofluorescence. "Auto" indicates that the fluorescence occurs naturally, i.e. without adding artificial sources of fluorescence. For example, melanin (pigment which determines skin color) absorbs ...


2

Air is very sparse compared with liquids and solids, so there's just much less material to block light. Secondly, air is very uniform and homogenous so there are no edges to cause reflections and scattering, except in the extreme case of a explosion shockwave. For the same reason, still water is mostly invisible except at the surface, while frothy water is ...


2

Not all the light shining on the top surface of your glass block can pass straight through the block. Inside the block, total internal reflection occurs at the side faces. The light which was "supposed" to exit through the sides is refracted to the inside. This causes the shadow you observe and a slighly brighter middle section, which is hard to see. In ...



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