Sometimes we see these small translucent shapes moving by when the eyes are open. When the eyes are moved they seem to follow that movement, but with a certain delay, as if they are floating in a thick viscous liquid. What is this phenomenon called?
28801
I have noticed that if you look directly at tubelight (I have only white ones in my home) and close your eyes little, you see little random shaped colourless things sometimes with slow speed and sometimes with falling down.   They are shaped like circles with something in between and in chains of random length.   My father has also seen them. So, I am sure I am not imagining things. What the hell are they?
92359
My possibly mistaken understanding is that  changes with time, whereas a  is, well, constant.  What about gravitational clumping? Detecting relative motion?
119397
Observers at rest in gravitational fields will see infalling light signals as blueshifted.  Do inertial observers in free fall in a gravitational field see the same shift?  If someone is standing on a cliff at rest in the gravitational field and then steps off, do they instantly see a change in the blueshift as a result of being in free fall in an inertial reference frame?
5991
As I learned today in school, my teacher told me that when light enters a glass slab it slows down due to the change in density and it speeds up as it goes out of the glass slab. This causes a lateral shift and the light emerges out from the point different than that from where it should have actually emerged from.  Okay so what I mean to ask is, when light enters point A on glass slab and emerges from point C why does the light speed up? Where does it get the energy it has lost when it entered the glass slab?  P.S.: Also, if I place a very very very large glass slab and make a beam of light pass through it will the light never come out as all the energy was lost in place of heat?
77988
I often read that people believe that human vision has an inherent frames-per-second rate (FPS) that causes stroboscopic effects - such as seeing the spokes of a rotating wheel apparently rotating at a different speed or appearing stationary when moving. For example: in a Physics.SE answer to   The human eye-brain visual refresh rate has an &quot;effective frame rate&quot; of around 30fps. Have you never watched the rims of the wheel in a car next to yours? I can clearly recall many times watching as the wheels sped up how it appears to stand still then move backwards.  This surprises me as I would expect there is no synchronisation of neuron firing in the retina, that neuron firing rates would vary widely depending on light levels and that the brain has no need to process the continuous signals in fixed cycles or in cycles whose length is invariant. I believe this question, or a good answer to it, would differ from  Is human vision in any way subject to a fixed frame rate of 30 FPS?
112054
This can happen during the rain, or when pouring water. Sometimes small (1-2 mm) droplets of water float on the surface for some time (a few seconds) before coalescing with the big body of water. A very refined example of this phenomenon is  experiment, captured by a high-speed video camera.   study explores it in detail, but the explanation for this behavior seems implicit.  So how can this behavior exist at all? I imagine that the forces of gravity and surface tension act on the droplet in the same direction; so what supports it from below (or even makes it bounce, like in the video)? Is it the trapped air that escapes very slowly to the sides? If yes, why is it so slow?
21907
Most of us have had the experience of stumbling over a new fact or bit of knowledge and then finding several more references to it in the near future.  For example, you see a strange word which you're forced to look-up and then the next day the same word appears in the headline of your local paper.  There's a term for this phenomenon but I've been unable to track it down.  Can anyone help?
33727
What causes the surface of the lake to appear darker in some places?   Firstly, I know that it's not fishes that cause it. Secondly, the dark surfaces move from time to time and thus are not associated with the depth of the water or the structure of the lake bottom. Lastly, I may say they are due to a kind of ripple distortion. What causes this distortion and what is its mechanism?  Please, provide more explanation than "the wind", for example.  In this photo, look at the far area; it's darker somehow.
29889
An animation of an object appears under something small or invisible to camera
37590
Greatings!  I'm trying to understand special relativity and have one question bugging me.  In almost every book or video about the subject there is a thought experiment with moving light clock. I hope I need not elaborate on the sutup and the outcome of the experiment.  So the question is this: When a light source on the MOVING light clock emits a photon of light the path of the photon is triangular.   How this can be?   I thought since light speed is constant in every reference frame the movment of the "emmiter" can't affect the movment of the light so light should shoot right up (from the point of stationary observer) and thus move backward and up from the point of moving observer.  So you see my logic is: light speed is invariant => you can't make it move UP &amp; FORWARD only UP. i.e. you can't change it's direction once it's emited in UP direction. Why the hell light moves sideways then?  And I'm puzzeled. There is flaw in the logic but I don't see it.  Thanks for help.
35585
I know this, but I can't recollect at all.   What is this thing called?   They are usually centered in a book or writing and I'm certain they have a name but I can't remember for the life of me. They also come in all shapes, colors and sizes.
116189
Suppose I put a mug in the water and then tilt it so that the water gets inside. It then floats due to buoyant force. But if I try to pull it out then a lot of force is needed. That's due to suction? What exactly happens at the moment when the force I apply is enough to pull the mug out? And what causes the loud pop sound when that happens?
99424
Sometimes you see a video of a fan or a propeller spinning very fast, and the apparent rotation is distorted. The fan can appear to be moving in the opposite direction of rotation, or even stationary. This optical illusion is known as the wagon-wheel effect. If I understand correctly, this occurs because the camera framerate is slower than the frequency at which the fan makes one rev, so stroboscopic effects should be expected.   Does the wagon-wheel effect occur only in videos, or can we observe it just with the eye?
19669
I've been trying to create an animation where an object blinks on and off.   For example, I have a Cube and I set a keyframe on it's Restrict Visibility icon (the "eye" icon) with I, then I toggle off that "eye" so my cube doesn't show in the Viewport and again I press I to record that keyframe. Then, when I preview the animation in the Viewport, I see my cube appears and then disappears like I want, but when I render it, the cube does not disappear at all.  What am I doing wrong?
5490
I'd like answers both in the more intuitive side an on the more precise side.   Thinking of water as "cubes" of water, for example, would allow pressure in the z axis to be independent of the y or x axis. Choosing other "shapes", like a  triangular prism, yield different results. Will this fact, then, be dependent on the format of the molecules of the liquid ?  I've heard of the rotational symmetry of liquids. What is its precise statement ? Why is it true ?
85305
In many pieces of literature, light is said to travel like a "wave". Does this mean the light literally propagates through space like a wave as in up and down and so on or does light move linearly through space? Also, if it move linearly, why do we say photons are a wave then?
119732
is a video game created by the MIT Game Lab which allows users to experience what it would be like if the speed of light was closer to normal walking/running speeds and thus experience relativistic effects in an otherwise familiar environment.  From the site:     A Slower Speed of Light is a first-person game prototype in which players navigate a 3D space while picking up orbs that reduce the speed of light ... [allowing] the speed of light in the game to approach the player’s own maximum walking speed. Visual effects of special relativity gradually become apparent to the player [as more orbs are collected]... These effects, rendered in realtime to vertex accuracy, include the Doppler effect (red- and blue-shifting of visible light, and the shifting of infrared and ultraviolet light into the visible spectrum); the searchlight effect (increased brightness in the direction of travel); time dilation (differences in the perceived passage of time from the player and the outside world); Lorentz transformation (warping of space at near-light speeds); and the runtime effect (the ability to see objects as they were in the past, due to the travel time of light).   When I play the game and collect enough orbs for relativistic effects to really start becoming significant something happens that I didn't expect: as I accelerated towards objects they appeared to move further away from me (despite length contraction definitely being taken into effect, since I was able to get to those objects faster over a given distance than had I not collected any orbs).  Luboš Motl in  says that this is due to:     the shrinking of transverse directions if you're moving forward (or their expansion if you move backwards) which makes object look "further" (optically smaller) if you're moving forward. Because of this shrinking, you may effectively see "behind your head". You also see things how they looked like some time ago.   Based on my understanding of the Lorentz transformations the directions transverse to one's motion aren't effected ($y'=y$ and $z'=z$ if all motion is in the $x$ direction).  So, what exactly causes this effect that I see in the game and Luboš Motl explains?  Am I misunderstanding the Lorentz transformations?