[DarkKnight (OP)]

Can we detect coloured objects wavelength's by device?

[alancalverd]

Yes. My uncle devised a hand held spectrophotometer to allow blind people to select appropriate items for their wardrobe. AFAIK it is still in production.

[DarkKnight (OP)]

Yes. My uncle devised a hand held spectrophotometer to allow blind people to select appropriate items for their wardrobe. AFAIK it is still in production.

In experiment I positioned a satellite dish pointing towards a blue background, my monitor did not detect 450nm ,why is this?

[Origin]

In experiment I positioned a satellite dish pointing towards a blue background, my monitor did not detect 450nm ,why is this?

Why do you think your satellite dish and electronics would be able to detect and display a blue color on your monitor?

[alancalverd]

There are three reasons why a piece of equipment doesn't do what you expect:

1. It is the last of the Mark I's, which were discontinued for exactly this fault

2. It is the first of the Mark II's, which don't have this feature because nobody used it on the Mark I's

3. It is working, but you don't know how to read the output.

[DarkKnight (OP)]

In experiment I positioned a satellite dish pointing towards a blue background, my monitor did not detect 450nm ,why is this?

What did it detect?  I mean, blue is a lot more frequencies than just 450 nm.
You can't for instance point it at a picture of a banana on your TV and expect the device to actually detect yellow light coming from it. TV screens don't work that way.

In a second experiment , I positioned an orbiting satellite and emitted a 450nm carrier signal. My aligned satellite dish recieved the signal and my monitor detected 450nm , displaying blue. This proving my apparatus was functioning correctly. In the earlier experiment , only white noise was detected , no evidence the blue background was emitting any sort of wavelength.

[DarkKnight (OP)]

In experiment I positioned a satellite dish pointing towards a blue background, my monitor did not detect 450nm ,why is this?

Why do you think your satellite dish and electronics would be able to detect and display a blue color on your monitor?

The same reason I can detect television carrier signals .

[Bored chemist]

I positioned an orbiting satellite and emitted a 450nm carrier signal.

How?

[DarkKnight (OP)]

I positioned an orbiting satellite and emitted a 450nm carrier signal.

How?

I took out my video camera and live streamed a blue background , how else?

[Eternal Student]

Hi.

   As outlined by @Bored chemist,   the ability to move satellites (satellites in space?) does seem impressive.   This is a bit more than a home experiment isn't it?

   As regards the sky,   the modern explanation for why it looks blue is Rayleigh scattering.   This means that it isn't really just blue, it's just less red when you observe from the surface of the earth.  As @Halc suggested much earlier, you might be receiving a wide spectrum of wavelengths and the sky is then mainly white - a mixture of many wavelengths.

    I don't know how your receiver works when picking up a spectrum of wavelengths.   For example does it perform a Fourier Analysis to pick out individual sinusoidal frequencies and how well does it do this?   For a wide mixture of wavelengths the average displacement of the E field at the satellite dish is always going to be close to 0.

Best Wishes.

(A new post was reported before I finished writing this, sorry).    I've just read that -   I'm not sure what it was you were doing.   Perhaps you could explain a bit more.

[DarkKnight (OP)]

Hi.

   As outlined by @Bored chemist,   the ability to move satellites (satellites in space?) does seem impressive.   This is a bit more than a home experiment isn't it?

   As regards the sky,   the modern explanation for why it looks blue is Rayleigh scattering.   This means that it isn't really just blue, it's just less red when you observe from the surface of the earth.  As @Halc suggested much earlier, you might be receiving a wide spectrum of wavelengths and the sky is then mainly white - a mixture of many wavelengths.

    I don't know how your receiver works when picking up a spectrum of wavelengths.   For example does it perform a Fourier Analysis to pick out individual sinusoidal frequencies and how well does it do this?   For a wide mixture of wavelengths the average displacement of the E field at the satellite dish is always going to be close to 0.

Best Wishes.

(A new post was reported before I finished writing this, sorry).    I've just read that -   I'm not sure what it was you were doing.   Perhaps you could explain a bit more.

I did not mention the sky .

Consider a live video feed where the camera points at a blue wall . The feed is being transmitted to an orbiting satellite then relayed back to a satellite dish . The monitor displays the blue wall.

Now remove the camera and point the satellite dish at the blue wall, the monitor displays white noise . No apparent evidence the wall is emitting a constant wavelength.

[alancalverd]

See #5 above, reason 3.

[Bored chemist]

I positioned an orbiting satellite and emitted a 450nm carrier signal.

How?

I took out my video camera and live streamed a blue background , how else?

What you say you did would not do what you say it did.

[paul cotter]

DarkKnight, you appear to be somewhat confused. Your sat dish is expecting wavelengths in the region of 0.•1-0.•01m. Blue light @ 450nm has wavelength of 0•000000045m. In other words your sat dish cannot see blue light or any light. If you aim it at anything other than a source of the requisite transmission all you will see on your monitor is white noise. And when you aim your camera at a blue source you are then sending a carrier at maybe ~0.05m MODULATED with your 450nm signal-you are not sending a carrier at 450nm.

[evan_au]

the monitor displays white noise

This sounds like a very old experiment. We don't use direct analogue modulation any more.

If you point your camera at a blank blue wall, your movie camera's electronics examine the pixels and categorise them as "a lot of blue, a bit of green and hardly any red; this is spread uniformly across the wall (not a gradient); and it's not moving over time". (That's my verbal description of MPEG encoding; other video encodings are available...)
- You transmit this to a TV screen (optionally via a satellite), and the TV electronics decodes this as "a lot of blue, a bit of green and hardly any red; this is spread uniformly across the wall (not a gradient); and it's not moving over time".
- If you point your satellite dish at a random point in the sky, it will be picking up the cosmic microwave background radiation in the GHz region of the spectrum, which is random noise. It will not pick up the colour of visible light, which is around 400 THz.
- If you point your satellite dish at a wall, it will be picking up thermal noise in the GHz region of the spectrum, which is random noise.  It will not pick up the colour of visible light, which is around 400 THz.
- None of this random noise has the highly ordered structure of a modulated carrier, carrying the highly ordered structure of an MPEG stream and encoded with the highly ordered structure of an error-correcting code.
- A modern digital TV would not show white noise - it would detect that the highly ordered input is missing, and display a message like "no signal".

[alancalverd]

And when you aim your camera at a blue source you are then sending a carrier at maybe ~0.05m MODULATED with your 450nm signal-you are not sending a carrier at 450nm.

Except that the carrier isn't modulated with a 450 nm signal either, but a digital code for "blue".

[paul cotter]

Yes, yes you are both correct. I didn't phrase things very well. I haven't worked in communications since 1984 when there was no digital. I was trying to explain how a sat dish could not possibly "see" a blue object. The op was the one who said he had white noise on his monitor and I was trying to stay within his experience. Mea culpa, as is said.

[Eternal Student]

Hi.
    I see what you did now @DarkKnight .
    Don't worry about it too much, when I was just a little bit younger we had VHS tape which could produce video on a thing called a VCR.   I was fairly sure that if you pulled out the tape and held it up to the light you would be able to see still images of the video.   It didn't seem to work but I gave it a good try with bright lights.
   More worryingly, the whole tape looked black.   I might have exposed the film and damaged the pictures on it.    Just to be sure, I left it in a really dark cupboard for a week so that it would recover.
   Just to be extra sure, I left a couple of my sisters toys in the same cupboard with their hand pointing to the tape.  Don't assume I was a total idiot,  I knew there was only a small chance they could fix it but even if they didn't, if my parents tried the tape they would suspect my sister had been playing with it.
 
Best Wishes.

[DarkKnight (OP)]

If you point your camera at a blank blue wall, your movie camera's electronics examine the pixels and categorise them as "a lot of blue, a bit of green and hardly any red; this is spread uniformly across the wall (not a gradient); and it's not moving over time". (That's my verbal description of MPEG encoding; other video encodings are available...)
- You transmit this to a TV screen (optionally via a satellite), and the TV electronics decodes this as "a lot of blue, a bit of green and hardly any red; this is spread uniformly across the wall (not a gradient); and it's not moving over time".

Are you saying that the frequency transmitted is via the camera rather than the blue wall?

[alancalverd]

It would be very difficult for a blue wall, or any other object, to appear simultaneously in half a billion households around the world. Yet, surprisingly, about that number of people  were able to distinguish red and blue shirts during the last World Cup final without all being in the stadium. This rather suggests that the TV camera transmits, and the satellite rebroadcasts, some kind of code related to the color of whatever is in front of it, and the TV receiver converts that code into a distribution of illumination on its screen.

The system has been in use in various forms since 1926.