# Tag Info

41

For an object to be transparent, the light must be emitted in the same direction with the same wavelength as initially. When light strikes a brick, some is reflected in other directions, and the rest is re-emitted in longer, non-visible wavelengths. That is why a brick is opaque to visible light. Some materials we consider transparent, like glass, are ...

29

If I understand you right, you're referring to the phenomenon seen in this picture (from the first Google hit), that near the horison the color of the sky is more light-blue (not exactly white): Rayleigh scattering The scattering in the atmosphere is for a large part Rayleigh scattering off of nitrogen and oxygen molecules, which are much smaller than ...

23

When you say light bends I assume you are talking about refraction i.e. the change in the angle of the light given by Snell's law. You ask: If I'm running straight, and I get slowed down, shouldn't I still be running straight? but suppose one foot get slowed down while the other one didn't. In that case you would turn in the direction of the foot that ...

15

You say: For a non-transparent object like a brick, when the light is absorbed by an electron it will eventually be re-emitted. but this isn't true. In a solid the excited state can decay by transferring energy to lattice vibrations instead of emitting a photon. This means the energy of the incident photon is converted to heat and the photon is lost ...

14

To augment Rennie's answer with a graphical representation let me post this diagram: Imagine, if you will, not a single beam of light but a series of wavefronts. When part of the wavefront slows due to a different density, the wavelength also compresses, thus introducing the characteristic bend.

11

It's because you're not looking far enough. From personal experience, it takes at least 10 km of atmosphere to build up a really obvious blue (see, for example, this picture), and if you're not in hilly country, the horizon is only 5km away. In contrast, most of the sky has distances to space on the order of hundreds of kilometers.

6

Yes, light can be brought to a complete halt under the right conditions. To understand how this happens you need to understand what is going on when light slows down in a medium. Light is an oscillating electromagnetic field, and when it passes though anything that contains charged particles (i.e. any matter made from electrons and protons) the electric ...

5

Well, the Earth is kind of blue when viewed from far away: In the picture above, you see that the "edge" of the Earth is more bluish, which is due to the reason you give in your answer; here we see through a deeper layer of the atmosphere, so we see more blue light. But the light that does reach Earth's surface is reflected back through the atmosphere, ...

4

Now there is a light ray moving outward at the speed of light. I'm afraid that isn't the case; within the event horizon of a Schwarzschild black hole, the radial coordinate is timelike and so, moving 'outward' toward the horizon is as impossible as moving 'backward' in time. This plain to see in the Kruskal–Szekeres coordinates: Image credit See ...

3

Because in the retina of human eyes there are light detector cells of two kinds: rods and cones. Cones "see" colors but are not very good with low brightness. Rods don't detect colors but are better than rods in low brightness situations. Thus at night cones don't function (they are for bright light) and we see only with rods which don't differentiate ...

3

Two reasons: The scattering separates red/orange and blue in different directions. At sunset you'll see the red parts that are missing from the blue skies by day. This isn't noticeable for objects close by, because those objects surround you. The blue from some objects mixes with the red from others. Secondly, there is a lot of air between you and the ...

2

The answer to your question can be found in the description of the engineering of the monument You can read the whole story at that link; I will just quote the most pertinent statement: Using the statistical mean of the 100-year data, the altitude and azimuth angles for the structure were adjusted to provide time/error fluctuation of plus or minus 12 ...

2

Building on prior answers, the facts are: Color is determined by the energy of the EM Wave that reaches your eyeball. Energy is defined as $E = hf$, where $h$ is Planck's constant and $f$ is the light's frequency. Thus, the color of an EM Wave is defined by its frequency. In other words, measuring the frequency of an EM Wave is sufficient to identifying the ...

2

The Earth does look blue, even continents, from satellite. (Still, in normal direction the amount of atmosphere is not that large). Almost all the satellite photographs that you see are retreated to improve the contrast and restore the white balance. (and many other issues such as band-variations of illumination, hot spot, etc).

2

So yes, if you compute the Poynting vector (energy flux density), $\vec E \times \vec H$, for an exponentially decaying evanescent wave, you indeed find zero time-averaged energy flux perpendicular to the reflecting plane. Ask you say, this leads to a conundrum --- how do evanescent waves transfer energy across barriers? For sure, we know they can transfer ...

2

(1) it's the energy in the sense that the photon oscillates at a certain frequency. (2) i'm not sure you can physically explain a light wave. More light is just more photons, more energy is photons with higher frequencies. (3) when it comes to the particle nature of light the photon has a frequency. It frequently oscillates through positive and negative ...

2

For better clarity, let's define the following: Axial direction = the direction the person & light beam are drawn into the BH. Radial direction = the direction perpendicular to the axial direction. If we, looking in the same direction as the person & light are being drawn into the BH, watch the light beam as it is drawn into the BH, we will see the ...

2

What you are confusing here is speed and velocity. Light speed is constant, but the velocity, which takes into account the direction as well as the speed is not. As an example of how something can accelerate without changing speed, consider the case of circular motion, where the acceleration of an object moving at a speed $v$ in a circle of radius $r$ is ...

2

I think a possible analogy would be to imagine that the singularity is a waterfall. By emitting light, you are trying to send a signal upstream using a tame fish. Outside the event horizon the fish is able to make headway against the current. But the river flows so fast within the event horizon as it approaches the waterfall, that your fish ends up going ...

2

If you actually look at (atmospherically) scattered light, you will see that even at ground-level, there is a distinct blue tint. How do you see that, you ask? Any old shadow will do - find some building that throws a nice big shadow, make a photo with a decent camera, and analyze it thoroughly - you'll find that there's indeed a blue tint, as expected. Why ...

2

The equations that describe how light, and all other electromagnetic radiation, works have a couple of constants which mean that the speed of light is constant. The equations don't depend on where you are, what temperature it is, which way you are looking etc so we assume they apply everywhere. It is entirely possible (although experimentally unlikely) that ...

2

This question has been asked and answered (by me) on Astronomy Stack Exchange: It's brighter on Pluto than you think. NASA developed a tool called Pluto time, which tells you when at your place the ambient light conditions are similar to the ones on Pluto. This occurs when the Sun is only 2° below the horizon! That's quite shortly after sunset, ...

1

Dispersion is not the only one phenomenon, where photons of different energy are acting different. In the case of reemission of photons in materials as well as in the case of dissipation of photons in materials the visible for us result is a transparent material like glas. But due to this two effects the speed of light in materials is different. As you see ...

1

The manufacturer's of Pyrex glasses use a special technique to make laboratory glasswares. They call it amber colouring. They do this by spraying a special mixtureon the outer layer of the glassware whose exact combination is not known, and not needed for present purposes. All we need to know is it's properties and why do they do it. Why do they do it? ...

1

This Q boils down to How bright is Earthshine? According to the usual source Oceans reflect the least amount of light, roughly 10%. Land reflects anywhere from 10–25% of the Sun's light, and clouds reflect around 50%. So the amount of sunlight reflected (i.e. albedo) depends on what part of the earth is facing your observer and how cloudy it is. ...

1

This depends on the reflectivity of the objects and the size in the sky. The visual albedo of the moon when near (but not at) full, is about $0.12$. Earth's is around $0.39$. Being in low-earth orbit, the actual amount will vary based on the terrain and atmosphere. If it's overcast below, the value could be much higher. You can assume that a given area ...

1

Take a light-source spreading over the visible spectrum (e.g. light bulb, sun light) and decompose this light with a prism. You can as well wait for the next rainbow or make it yourself on a sunny day.

1

Can I see the edges of the visible spectrum at home? The edges of the visible spectrum do not exist. They are like the edges of the audible spectrum which you can try to find using this Online Tone Generator. Can you identify with precision the lowest and the highest frequency you are able to hear?

1

The star emits red light because it has a surface temperature somewhere around 3,500 Kelvin. Our sun has a temperature of about 6,ooo Kelvin so it emits yellow/white light. Like any black body radiation, the temperature determines the frequency of the emitted photons. As for the gravity potion of your question, I may be wrong but I think you talking about ...

1

Be careful with the terminology. The emission of light depends from the temperature of the emitting body. The bandwidth goes from infrared to red and blue to ultraviolet than higher the temperature. Red and blue shift means, that the well known spectrum from elements like hydrogen is shifted to the blue or red side. And yes, this happens due to the doppler ...

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