What is invisibility?

[Yash@17]

Does speed is directly proportional to transparency?

So why we can't see a moving flap of ceiling fans!

[Ghideon]

21 minutes ago, Yash@17 said:

Does speed is directly proportional to transparency?

No, transparency of everyday objects such as fan blades is not affected by velocity relative an observer

23 minutes ago, Yash@17 said:

So why we can't see a moving flap of ceiling fans!

I think it can be explained like this: Lets say that the fan blades cover 50% of the circular area of the fan. 50% is openings between blades. Start the fan and let it rotate fast so that eyes cannot track the movement of the blades, it looks like a blur. Now think of how eyes works. Simply the eyes register photons and by averaging over many photons an image is created and interpreted by the brain*. Noe look from below the fan towards ceiling. Since the fan blades are spinning fast the photons coming from the fan blades and also from the ceiling between fan blades. Since the blades are 50% of the area there is approximately 50% photons from fan blades and 50% photons from ceiling reaching the eyes of the observer. Result would be that brain interprets photons as coming from something like a half-transparent disc. For a fan with a color similar to ceiling and thinner blades the fan could look almost invisible or almost fully transparent.

 

*) very simplified, hopefully correct enough for the context

 

[zapatos]

14 hours ago, Yash@17 said:

Does speed is directly proportional to transparency?

So why we can't see a moving flap of ceiling fans!

I saw a plane flying overhead and could clearly make it out, even though it was moving much faster than fan blades.

[Yash@17]

4 hours ago, zapatos said:

I saw a plane flying overhead and could clearly make it out, even though it was moving much faster than fan blades.

But it is not in circular motion!!

 

18 hours ago, Yash@17 said:

Does speed is directly proportional to transparency?

So why we can't see a moving flap of ceiling fans!

 

17 hours ago, Ghideon said:

No, transparency of everyday objects such as fan blades is not affected by velocity relative an observer

I think it can be explained like this: Lets say that the fan blades cover 50% of the circular area of the fan. 50% is openings between blades. Start the fan and let it rotate fast so that eyes cannot track the movement of the blades, it looks like a blur. Now think of how eyes works. Simply the eyes register photons and by averaging over many photons an image is created and interpreted by the brain*. Noe look from below the fan towards ceiling. Since the fan blades are spinning fast the photons coming from the fan blades and also from the ceiling between fan blades. Since the blades are 50% of the area there is approximately 50% photons from fan blades and 50% photons from ceiling reaching the eyes of the observer. Result would be that brain interprets photons as coming from something like a half-transparent disc. For a fan with a color similar to ceiling and thinner blades the fan could look almost invisible or almost fully transparent.

 

*) very simplified, hopefully correct enough for the context

 

 

[swansont]

21 hours ago, Yash@17 said:

Does speed is directly proportional to transparency?

So why we can't see a moving flap of ceiling fans!

You have to separate the inherent behavior from our ability to perceive. The eye's response has limitations, which is why we perceive a series of still images as motion (i.e. it's why we can appreciate movies)

So you need to distinguish "People can't see it" (i.e. invisible to the naked eye) from "it can't be seen/detected" (and can sometimes be applied to specific wavelengths/frequencies of EM radiation, e.g. "invisible to radar")

Are atoms invisible, just because they're small? Depends on which definition you use.

[zapatos]

7 hours ago, Yash@17 said:

But it is not in circular motion!!

It is also not made of wood. What does that have to do with anything? You asked if "speed is directly proportional to transparency".

[koti]

On 9/5/2019 at 3:21 PM, Yash@17 said:

What is invisibility?

Invisible to humans would mean either the ability to absorb 100% of visible light in an object or reflect 100% of light from an object in a way so it appears invisible through perfectly adapting to its surroundings. 

Edited September 6, 2019 by koti

[MigL]

There is definitely a relation between something being invisible, and your eyelids being closed.

[Art Man]

Invisibility has two explanations. Is it visible to your eyes? Does light slip around it without reflection?

[Ghideon]

50 minutes ago, Art Man said:

Is it visible to your eyes? Does light slip around it without reflection?

Good points. Two more that I see as separate cases: Is it completely transparent? Is it camouflaged so that it is indistinguishable from it's surroundings? 
 

Edited September 11, 2019 by Ghideon
grammar

[Sensei]

On 9/6/2019 at 6:36 PM, koti said:

Invisible to humans would mean either the ability to absorb 100% of visible light in an object

No. What you described is black surface..

On 9/6/2019 at 6:36 PM, koti said:

or reflect 100% of light from an object in a way so it appears invisible through perfectly adapting to its surroundings. 

No. You need to have object with refractive index = 1.0. Without any reflections.

Like it is showed on this video:

There is liquid with R.I. the same as glass, therefor glass placed inside of liquid is invisible to human eye.

 

 

[swansont]

6 hours ago, Ghideon said:

Good points. Two more that I see as separate cases: Is it completely transparent? Is it camouflaged so that it is indistinguishable from it's surroundings? 
 

Also, does it radiate, e.g. give off blackbody radiation?

[Art Man]

8 hours ago, Ghideon said:

Good points. Two more that I see as separate cases: Is it completely transparent? Is it camouflaged so that it is indistinguishable from it's surroundings? 
 

Is it tiny like an atom? Is it only visible at infra red?