Telescopes: How do they work without using any Energy?

[Dekan]

Suppose you look at the planet Saturn in the night-sky, with your naked eye.

 

All your eye shows you, is a point of light. This point of light, reveals nothing of Saturn's disc, the spots and cloud-belts on the disc, or the rings encircling the disc. These features can't be discerned, because Saturn is so far from the Earth. Say around 800 million miles away.

 

If you wanted to see these features, you could get on a rocketship, take off from Earth, and journey to within say, 8 million miles of Saturn.

 

Then you'd get a view of the disc, belts and rings. Since you'd be 100X closer.

 

But getting this closer view, would require a lot of energy. In the form of fuel expended to take off from Earth. The fuel might be burning chemicals, nuclear fission, nuclear fusion, anti-matter reactions, or whatever.

 

However, if you stay on Earth, and just look at Saturn through a telescope with a magnification of 100X, you get the closer view instantly, and apparently without any expenditure of energy at all.

 

The telescope doesn't seem to expend any energy, or do any "work", in the Physics sense of the word. The telescope (in its simplest form) is just two pieces of unmoving glass: the object glass and the eyepiece. Which just passively transmit light.

 

This strikes my naive brain as hard to understand: that two motionless lumps of glass can produce the effect of an "instant journey" to Saturn. Without any "work" being done.

 

I'll be very grateful for any comments, help or guidance on this. Thanks.

[pwagen]

I think the problem is you're assuming Saturn is a dot in the sky. Unlike far-away stars, the planets actually have a visible area due to being somewhat close. We can't see it with our naked eyes, but it's still there. So what you're doing with the telescope is not building a planet using nothing but a dot. You're simply enlarging an already existing area in the sky.

 

Another thing that can cause a problem is that you imagine us travelling there through the telescope. This would require a very old view of how light works, to be true. A long, long time ago (and I guess some people believe this is the case today), people thought that light was "beams" emitted from our eyes. When it hit an object, we see it through that beam. This, naturally, is wrong. Light is emitted (or reflected) from objects, and then hit our eyes. Rather than us embarking on a journey there, through the telescope, imaging the telescope a tool for capturing the light that was reflected off Saturn.

 

Hope that makes sense!

[swansont]

Reflecting and refracting of light does actually do work on a microscopic scale. But photons have so little momentum that it's a trivial amount. Light simply being absorbed exerts a force of Power/c (reflection is twice that). At a power of a milliWatt (that is, ~10x brighter than the full moon shines on an entire square meter), that's a force of around 3 picoNewtons. Simply changing the direction requires less force. Well within the bonding/structural capability of lenses and mirrors.

 

But take a very strong laser and you can levitate/trap small polystyrene beads

http://en.wikipedia.org/wiki/Optical_tweezers

[md65536]

Then you'd get a view of the disc, belts and rings. Since you'd be 100X closer.

No, magnification is not the same as being closer.

 

The difference is best illustrated with cameras, and the difference between zoom and tracking:

It's also easy to experiment by playing around with the "view angle" in a video game.

 

 

As for "energy"... a telescope's front lens is usually bigger than your eye, and it focuses that larger area into the smaller eyepiece area, which means that it allows more incoming energy to enter your eye than without the telescope.

Edited April 20, 2011 by md65536

[Dekan]

I think the problem is you're assuming Saturn is a dot in the sky. Unlike far-away stars, the planets actually have a visible area due to being somewhat close. We can't see it with our naked eyes, but it's still there. So what you're doing with the telescope is not building a planet using nothing but a dot. You're simply enlarging an already existing area in the sky.

 

Another thing that can cause a problem is that you imagine us travelling there through the telescope. This would require a very old view of how light works, to be true. A long, long time ago (and I guess some people believe this is the case today), people thought that light was "beams" emitted from our eyes. When it hit an object, we see it through that beam. This, naturally, is wrong. Light is emitted (or reflected) from objects, and then hit our eyes. Rather than us embarking on a journey there, through the telescope, imaging the telescope a tool for capturing the light that was reflected off Saturn.

 

Hope that makes sense!

 

Thanks pwagen for your very interesting post#2 - your point about Saturn not being just a dot, is what I was getting at. Although it looks like a featureless dot to our unaided eye, it's actually a planet with a lot of surface features. And rings. But we can't see these features from Earth - unless we use the method of either:

 

1. Leaving the Earth and traveling to Saturn; or:

 

2. Staying on Earth and looking through a telescope.

 

Method 1 requires a lot of energy, but method 2 doesn't seem to. Even though it accomplishes the same result.

 

That's what puzzled me.

 

Re your mention of the old discredited theory of light-beams emitted from people's eyes:

 

I suppose if we weren't humans, but intelligent bats, used to perceiving the world through our sonar-emissions, this theory would seem just plain common-sense!

 

Thanks again for your reply.

 

Dekan

 

Reflecting and refracting of light does actually do work on a microscopic scale. But photons have so little momentum that it's a trivial amount. Light simply being absorbed exerts a force of Power/c (reflection is twice that). At a power of a milliWatt (that is, ~10x brighter than the full moon shines on an entire square meter), that's a force of around 3 picoNewtons. Simply changing the direction requires less force. Well within the bonding/structural capability of lenses and mirrors.

 

But take a very strong laser and you can levitate/trap small polystyrene beads

http://en.wikipedia....ptical_tweezers

 

Thanks Swansont for your reply.

 

I've read through the wikipedia article you kindly cited. While I don't pretend to understand all the maths involved, it's amazing what can be done with light-beams! But don't all these techniques, optical tweezers and so on, necessitate forceably pumping a lot of light in, so to speak?

 

If a lot of energy is being pumped into something, then I can understand that it produces results.

 

The thing about the telescope though, is that it seems to produce results without any energy being "pumped" or "forced" into it.

 

The telescope just passively takes in the ambient light-energy.

 

Yet it produces results - eg, showing Saturn's rings, which you couldn't otherwise see. That's what seems slightly spooky to me.

 

Appreciate your reply,

 

Dekan

 

No, magnification is not the same as being closer.

 

The difference is best illustrated with cameras, and the difference between zoom and tracking:

It's also easy to experiment by playing around with the "view angle" in a video game.

 

 

As for "energy"... a telescope's front lens is usually bigger than your eye, and it focuses that larger area into the smaller eyepiece area, which means that it allows more incoming energy to enter your eye than without the telescope.

 

 

Thanks md65536 for your reply.

 

I've watched the youtube video link you kindly supplied. This makes very clear that magnification is not the same as being closer - if you're only concerned with perspective and the relative position, or displacement of different objects.

 

Magnification by optical means, ie Zoom, doesn't alter the apparent relative position of objects.

 

Whereas physically tracking-in, does.

 

I accept that. But in the case I'm thinking about, which is the amount of perceived detail on a distant object, such as Saturn, I don't think relative position or perspective are very important.

 

In the video, I could perceive the same amount of attractive detail on Shelley's face, whether her face was zoomed or tracked-in.

 

And presumably the same would apply in the case of Saturn's face.

 

However I think the last part of your post has really answered my question.

 

The extra energy which allows more details on a distant object to be perceived through a telescope, comes from the extra energy-collecting area of the telescope object-glass (or mirror).

 

Seems obvious now! (Though what telescopes do still seems a bit magical)

 

Thanks again for your reply - much appreciated.

 

Dekan

Edited April 20, 2011 by Dekan

[Airbrush]

"...The telescope doesn't seem to expend any energy, or do any "work", in the Physics sense of the word. The telescope (in its simplest form) is just two pieces of unmoving glass: the object glass and the eyepiece. Which just passively transmit light."

 

The lenses of a telescope bend light rays and focus them. You look through the eye piece, expending energy doing so. The telescope does a little work and you also work to see Saturn. Both you and the telescope do a little work, applying a little force through a little distance.

Edited April 21, 2011 by Airbrush

[Steobeast 01]

Can anyone please give me some info of South African large telescope and how it works

[between3and26characterslon]

Suppose you look at the planet Saturn in the night-sky, with your naked eye.

 

All your eye shows you, is a point of light. This point of light, reveals nothing of Saturn's disc, the spots and cloud-belts on the disc, or the rings encircling the disc. These features can't be discerned, because Saturn is so far from the Earth. Say around 800 million miles away.

 

If you wanted to see these features, you could get on a rocketship, take off from Earth, and journey to within say, 8 million miles of Saturn.

 

Then you'd get a view of the disc, belts and rings. Since you'd be 100X closer.

 

But getting this closer view, would require a lot of energy. In the form of fuel expended to take off from Earth. The fuel might be burning chemicals, nuclear fission, nuclear fusion, anti-matter reactions, or whatever.

 

However, if you stay on Earth, and just look at Saturn through a telescope with a magnification of 100X, you get the closer view instantly, and apparently without any expenditure of energy at all.

 

The telescope doesn't seem to expend any energy, or do any "work", in the Physics sense of the word. The telescope (in its simplest form) is just two pieces of unmoving glass: the object glass and the eyepiece. Which just passively transmit light.

 

This strikes my naive brain as hard to understand: that two motionless lumps of glass can produce the effect of an "instant journey" to Saturn. Without any "work" being done.

 

I'll be very grateful for any comments, help or guidance on this. Thanks.

 

Light dissipates from a spherical body following the 1/r² rule, the amount of light hitting you from 8 million miles from Saturn is 4 times what it would be from 16 million miles away and many times more than from 800 million miles away. The amount of light reaching you from Saturn is tiny, how big would your telescope need to be to collect as much light as your eye would from 8 million miles? Or how long an exposure would you need to collect as much information?

 

A telescope does not make things closer it makes things appear bigger, much like zooming in on a picture on your computer and you don't expend megawatts of energy doing that.

[davey2222]

I think it is a logical fallacy to compare energy expenditure (i.e. work) between the two methods of observation. Try think about the 'problem' when it is applied to extra solar objects and you will quickly come to realise its logical fallacy. It is like comparing apples to oranges. But kudos to those who attempted to solve the problem directly.

 

The more correct term, in my opinion, is efficiency. Using a telescope is more efficient compared to blasting off from planet Earth and going 'there' or anywhere in the universe for that matter. QED