Tag Info

Hot answers tagged

41

The eyes are measuring the number of photons of each color that are hitting a given point of the retina – that are coming from some direction. This is a function of time, $f(t)$, for each point. However, when this function is changing too quickly, the eye can't see the changes. Effectively, the eye may also see the average of $f(t)$ in each period of time ...


27

From the wiki article on color vision as an illustration of how photons are absorbed: Perception of color begins with specialized retinal cells containing pigments with different spectral sensitivities, known as cone cells. In humans, there are three types of cones sensitive to three different spectra, resulting in trichromatic color vision. Each ...


16

Photons can be created and destroyed freely, since they don't have charge or mass. Turn on a light, and you create many photons. Any body (made of atoms) not at absolute zero temperature will spontaneously emit photons. They are consumed just as easily. Most any bit of bulk matter will absorb a photon in the electrons on the surface, transforming the energy ...


6

Light from all over the place hits your eyeball fairly randomly. The lens forces light from a specific angle to hit a specific part of the retina. This HowStuffWorks article shows how the mechanics of that work. The only major differences between camera lenses and eyeball lenses is that we can dynamically alter the shape of the lens to focus on different ...


6

The typical extinction for a line of sight out of our Galaxy (but avoiding the Galactic plane) is of order a few tenths of a magnitude at visible wavelengths (it is a factor of 10 less in the infrared and factors of a few more in the UV). This means that the typical attenutation of a signal arriving at the HST from outside the Galaxy is around say ...


6

In 1974 Stephen Hawking published a paper that provides a theoretical basis for the thesis that black holes eventually may radiate away all the mass, light, and other energy they accumulate. Evaporation of black holes has been called Hawking radiation. It takes place so slowly (at least until the black hole shrinks to a small size) that none has been ...


4

You are mixing up two different things. The refractive index is usually defined in terms of the velocity of light: $$ n = \frac{c}{v} $$ where $v$ is the velocity in the medium. However the velocity is related to the frequency and wavelength by: $$ v = \lambda f $$ so: $$ n = \frac{\lambda_0 f_0}{\lambda f} $$ The frequency of the light, $f$, doesn't ...


4

Black is not a color, it is a shade. In physics, we call something "black" when it does not reflect any of the incident light. However, all black bodies radiate. The frequency of that radiation, the black body spectrum, is a function of the temperature of the object and follows Planck's Law: $$B(\lambda, T) = ...


4

It is believed, according to our most tested theory of gravity (General Relativity), that objects, such as material from other stars, particles such as electrons and also photons of light, may actually pass through the Event Horizon of a black hole. For a large enough black hole, a person in a spaceship passing through the event horizon may not notice any ...


4

A photon traveling at speed of light has a lightlike worldline. It has one place of emission and one place of absorption. The spacetime interval between both points is empty (=0), that means that no spacetime is between them. That means, if a photon would experience something, it would experience both points as simultaneous. But there is no reference frame ...


4

Imagine a spring-loaded trap with a hole that's sized such that only a particular size of object can enter the hole and trigger the trap. The molecules involved in vision are like that trap, with a bond having an electron energy gap tuned to the visible frequencies of light, encapsulated in a specialized protein that transforms the absorbed energy into a ...


4

You can't tell directly. But you can look at a bunch of it and notice that it is at the same temperature here there and everywhere in all directions in every single place where your view isn't blocked by some moon planet star or galaxy. And that temperature is quite cold it is hard to get something that cold. And either there are many things with the same ...


3

The hydrogen discharge tubes typically used in student labs are not designed for long-term use. After a couple of years, the tubes leak and air gets mixed with the hydrogen. This causes them to get dim and the weaker lines are almost impossible to see. It has nothing to do with the power supply and everything to do with how new the tube it and how many ...


3

Just a guess, but those rings look to me like Haidinger fringes. When monochromatic light falls on a thin, transparent plate or film, some of the light is transmitted and some gets reflected back, then reflected forward again, from the two surfaces. The reflected light interferes with the transmitted light. Whether the interference is constructive or ...


3

You have picked a very striking picture to illustrate your question, although the northern lights do not always have such a sharp lower border to them. However, let me advance a plausible explanation. The green northern lights are formed high up ($\geq 100$ km) in the Earth's atmosphere, largely by photons at 557.7 nm emitted from excited oxygen atoms. This ...


3

When you say: we can see an individual atom with visible light you need to clarify what you mean by the word see. As you say, there is no problem detecting that an atom is there because we can measure the light it emits, however it will appear as a point source and we cannot use the light to measure its size or shape. To measure the size and shape of ...


3

A short answer is that frames themselves are moving towards the black hole and light moves relative to a frame and hence it can be stuck. Nothing passes through a black hole. Things can enter a black hole, they can't can't exit without going faster than light. Where do things go then? The important part of that question is the word "where" you ...


3

I'm going to interpret your question a bit liberally - you ask for the case where we ignore the Sun; I'm going to go a little bit further and ignore the entire galaxy (and in fact other nearby galaxies) and talk about the cosmic background radiation. The cosmic microwave background gets a lot of attention, but in fact there are cosmic backgrounds at a very ...


2

Is the working principle of light bulb reversible? No. Electric potential energy is converted into thermal energy in the light bulb filament. At a certain temperature range the filament will light up; that is, it will radiate with a wavelenght in the range of visible light. That this process is non-reversible might be clear if you consider some more ...


2

Disclaimer for those who know more: I only talk about spherical black holes for simplicity; rotating black holes are more complicated. Indeed, a planet's gravity bends light and allows you to see a little bit farther; this is an observed effect (though not on planets), and is called gravitational lensing. If you've seen the movie Interstellar, you might ...


2

When gravity is strong enough, it bends light towards the source of the gravity. Roughly true So if you were on a small planet and gravity were to gradually increase, would the horizon rise as well, allowing you to see further? Yes! If so, at some point, could you look up at some angle and have the light go all the way around the planet ...


2

In classical electrodynamics, the process of how much light refracts, passing through the glass, and how much light reflects, is determined by the Huygens-Fresnel principle. This principle, named after Christiaan Huygens and Augustin-Jean Fresnel, is a method of analyzing the wave propagation patterns of light, especially in diffraction and refraction. It ...


2

Nothing special is happening! Think of a black hole as the accumulation of mass which is exceeding a certain limit. The same laws of gravity are applying before and after exceeding the limit. That means: Mass particles keep on being attracted. They are becoming part of the mass of the black hole. Electromagnetic waves will equally be attracted by the mass ...


2

The problem is that images aren't formed by one ray of light, they're formed by countless rays of light! A "bundle" of light from the sun ("bundle" meaning a ray localized in space but containing many many photons) will hit the surface of a blade of grass, and all the photons will disperse in a diffuse manner in every direction. So a bundle of light hits a ...


2

I am hypothesizing based on the information you gave in the question and clarifying comments. The fact that the distance between the "shadow" (or ghost) and the number increases with distance tells me that the shadow is produced by your eye+glasses, rather than the screen itself. Most likely, you are seeing reflections from the front/back surface of your ...


2

Because human eyes and brains are slow, they cannot resolve the motion of the blades, but only see the average of the moving blades and the image in the background (this is actually primarily really due to the slow reaction time of the cones, which is slow, as is demonstrated by the fact that a 24 frames per second video does not appear as single images but ...


2

The answer is that your view or sight is different from the bare images made by the imaging optics of your eye on your retina. A "view" also includes signal processing from the brain that tracks what you fix your gaze on. Light can pass between the blades and form an image of the retina for at least some time. It's true that there are also blades blocking ...


2

You've got your work cut out for you, so this answer can only point you in a direction. This calculation can be difficult because there is a profusion of pieces of terminology with subtly different meanings. Find a book/other source (but for the straight dope I suggest a book) which discusses radiometry and the differences between radiant flux, radiance, ...


2

No one knows what happens inside the black hole (I mean inside the event horizon). But inside the event horizon space becomes unidirectional (like time in real world) and therefore whatever enters into it must hit the singularity inside (at least according to classical general relativity). But again no one knows what happens at that singularity. By ...


2

The reflectance of a typical mirror depends on the metallic coating used, but usually it is usually aluminum or silver in more expensive mirrors. Special optical coatings can be used to reflect or scatter EM waves at specific wavelengths. Here is a photo of a mirror reflecting IR (700nm - 1mm wavelength): Mirrors are also used to focus X-rays (.01nm - ...



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