Propulsion Disk, wouldn't have an effect at all really.  We're talking 1 atom per cubic centimeter, some cases less, some cases more but there isn't enough for it to be a concern unless you are moving at relativistic velocities.

Propulsion Disk, wouldn't have an effect at all really.  We're talking 1 atom per cubic centimeter, some cases less, some cases more but there isn't enough for it to be a concern unless you are moving at relativistic velocities.

Thanks Doc!

Why we see depth when we look at a mirror even its closed to the eye? (I know this has nothing to do with space, but it has with light)

And how do I edit to remove those codes?

And how do I edit to remove those codes?

As a newly registered user, you won't have editing capability nor permission to create new threads, as per Forum rules, until you reach 10 posts in existing threads. In the meantime, I suggest to write those in plain text, because BBcode was disabled as well for new users, thanks to spambots.

OK, thank you. I`m really noob. So I should put the font in white.

Why we see depth when we look at a mirror even its closed to the eye? (I know this has nothing to do with space, but it has with light)

All science questions are welcome here!   

When you look at an object in a mirror, you see light rays which came from the object and reflected off of the mirror to reach your eyes.  But your brain doesn't know to trace back the reflection.  It instead traces the rays back across the opposite side of the mirror, until they converge at what we call a "virtual image".  "Virtual" because there are not actually light rays going on that side.



Because the mirror surface is flat, the geometry works out such that the distance between the mirror and the virtual image (labelled "d_i" above), is exactly equal to the distance between the mirror and the real object (d_o).  The image is also upright and neither magnified nor demagnified -- instead it is just flipped horizontally.  So it looks very convincingly like there is a mirrored version of the world on the other side.

What if we look at mirrored surfaces that are curved?  We can apply the same ray-tracing technique to understand the appearance of images formed by them.  For example, a convex mirror (like the outside surface of a metal sphere) will produce virtual images which are upright and demagnified, and a shorter distance behind the surface:



Concave mirrors can produce a more complex variety of images.  Objects which are very close to the mirror will produce virtual images on the other side which are upright and magnified, but objects that are far away will produce real images on the same side as the object, and upside down.  Real images can be projected onto something, like if you held up a sheet of paper there then you would see the image projected onto the paper.  You can play around with a simulation showing these ray tracings for a convex mirror here through Pearson Prentice Hall, and there is also a great summary of these concepts for both mirrors and lenses on Hyperphysics.  

Optical instruments like microscopes, magnifying glasses, and telescopes also use these ray tracing principles to allow you to see very small or very distant objects more easily.

A dumb related question from somebody who never went deep into physics.  When electromagnetic waves meet a new medium they can become absorbed (energy taken up by the electrons), pass through or get reflected if the refraction is great enough (or a mix of those).  Is that a complete understanding?  Since the refraction depends on both the medium left and entered, is it easy to make something that is a mirror in air but transparent in water?  Or even a mirror in space and transparent in air?

I hesitate to say it's completely understood, but it's definitely well understood and much of it is encapsulated in the Fresnel equations.  I once derived these in an electrodynamics course, which was a neat experience although quite time consuming.

Anyway yes, it is possible to do interesting and weird things like that.  For example you can make a lens out of some plastic that has the exact same refractive index as water (n=1.33), and if you place it in water it will become invisible because the plastic and water are no different as far as the light is concerned.  This is also related to why your vision gets blurry under water.  The difference in refractive index between your eye and water is less than between your eye and air, so when underwater your eye is unable to focus the light enough to form an image on the retina.

Turning a mirror transparent by changing the surrounding medium is a bit more complicated.  Reflection from a typical mirror is actually not because of the glass, but by a metal film (the glass just protects it), and the high reflectivity happens because of how light interacts with free electrons within the metal.  The incoming wave gets absorbed very quickly by driving the electrons to oscillate, and they also re-emit the wave backwards.  (This also means light does penetrate metal to a very shallow but nonzero depth, which is why sufficiently thin metal films are transparent, like the gold coated face plates the astronauts use).  Anyway I don't think it's possible to turn this sort of mirror transparent by changing the medium since the wave will still interact with the metal in the same way.

However, we can consider the reflection from something like glass instead.  It is possible to completely negate this reflection by coating it with a carefully designed layer of the correct thickness and refractive index.  This is more weird the more you think about it. The glass by itself both transmits and reflects light, but by adding something in front you can make it reflect less, and therefore transmit more?  This actually has useful applications with coatings that improve the performance of camera lenses.

Here's another fun thing. Have you ever held a prism and noticed that at certain angles you cannot see anything behind it, and it's like the inside surface acts like a perfect mirror?  This is total internal reflection.  That in itself is interesting and the basis of fiber optics, but not too weird or unknown.  What's weird is that if you press your thumb up against the back surface where it is acting like a mirror, then you can see just your thumb print against it.  (Try this if you've never seen it).  This effect is called "frustrated total internal reflection".

Why is it weird?  The light was being perfectly reflected inside the prism, except now you have your thumb against it with a very thin gap between the two.  The size of the gap is crucial.  If it is thin enough the light wave hits the inside edge of the prism and then tunnels across this gap and into your thumb.  Exactly like the quantum tunneling of electrons across a barrier (in fact it is the same physics)!  Because the gap is thin enough for the light to tunnel across, it no longer reflects, and this is why you can see your thumbprint against what would otherwise act as a perfect mirror.

Why we see depth when we look at a mirror even its closed to the eye? (I know this has nothing to do with space, but it has with light)

It seems that a lot of people don't understand spacetime, let me walk you through this. simply all light travels at the same speed (almost) but some photons start farther away than other photons. the way our eyes work in this scenario is they act like two altimeters that measure how long it takes for the light to travel to your eyes, and that's why we see depth! For mirrors it's the same thing pretty much, the light just gets reflected or bounced back to your eyes, this process doesn't effect the time it takes for light to get to your eyes because some photons hit the mirror first and therefore still get to your eyes first. By the way, the reason things are farther away still aluminate right when you look at them is because light is still extremely fast, but not fast enough for your eyes not to see depth.

the way our eyes work in this scenario is they act like two altimeters that measure how long it takes for the light to travel to your eyes,

Your eyes do no such thing (otherwise you could distinguish how far away different stars are just by looking at them).  Using light travel time also does not explain why images appear at different distances depending on the curvature of the mirror, while keeping the object distance the same.

the way our eyes work in this scenario is they act like two altimeters that measure how long it takes for the light to travel to your eyes,

Your eyes do no such thing (otherwise you could distinguish how far away different stars are just by looking at them).  Using light travel time also does not explain why images appear at different distances depending on the curvature of the mirror, while keeping the object distance the same.

Then how does parallax work? I always thought light travel time was how we see depth. (PS) I said LIKE altimeters not exactly.

By geometry.  Each eye sees rays from a nearby object coming from slightly different directions compared to more distant objects, which is why if you close one eye and look at your finger in front of the background, and then switch eyes, the location of your finger appears to change.

Again if it instead worked by light travel time, then you could easily tell that the stars are millions of times more distant than the planets just by looking at them.  Yet they seem equally far away -- since your eyes are only a few cm apart, the difference in parallax angle for such distant objects is very small.

Another principle is that a photon does not contain any information about how long it existed or how far it traveled.  (Unless you count cosmological redshift, but that only happens over really big distances).

By geometry.  Each eye sees rays from a nearby object coming from slightly different directions compared to more distant objects, which is why if you close one eye and look at your finger in front of the background, and then switch eyes, the location of your finger appears to change.

Again if it instead worked by light travel time, then you could easily tell that the stars are millions of times more distant than the planets just by looking at them.  Yet they seem equally far away -- since your eyes are only a few cm apart, the difference in parallax angle for such distant objects is very small.

Yeah... I should probably stop trying to answer questions from now on, because whenever I try, you guys always have to correct me. So i'll just stop it so you don't have to keep doing this over and over again.

Propulsion Disk:  It's okay.  I appreciate that you want to help explain things!   There is some skill for knowing how confident to be in an answer or not, which comes with experience, and yet still is never perfect.  Misconceptions especially can be tough.  We all make mistakes, the best thing is to learn from them!


Aside:  Just for fun, let's compute how far your depth perception works by parallax:

The average distance between a human's eyes is about 63mm (and slightly more for men than women).  The minimum angle that the human eye can resolve is also around 1 arcminute (1/60 of one degree).  

The distance at which the shift in angle due to a 63mm separation of your eyes is 1 arcminute is:

[tex]d = \frac{63mm}{tan(1/60deg)} \approx 220m[/tex]

So beyond a few hundred meters it is impossible to discern distances directly with your eyes using parallax.  You can however gain more distance perspective if you are moving (like driving down a highway) as your motion makes things shift against the background.  Having an intuition for the true size of objects also helps with judging distances, though this is also easier to fool.


If we instead set the angle to 1 arcsecond (1/3600 degree), and the baseline distance to 1AU, then we recover the definition of the parsec used in astronomy, which is about 3.26 light years.  1 parsec is the distance at which the parallax angle for a 1AU shift in perspective is 1 arcsecond.