The rays of the sun are parallel

[Strange]

1 hour ago, michel123456 said:

It is not a difficult question: How are they received?

Well, let's see: they are diverging when they leave the Sun, they travel in straight lines so I guess they must still be diverging when the reach the Earth. What do you think?

[swansont]

Two things:

1. The minor issues here illustrate that there are assumptions being made. One might wonder why we would bother bringing them up. But since there is an obvious misconception at play, it's not clear at the outset whether the assumptions are involved. Sometimes the assumptions won't be valid. You have to check.

2. The main misconception here is one of not understanding perspective and how vision works in this regard. I would say it's far less mysterious than it's being portrayed here and in the other thread, but when you don't understand something, it doesn't seem that way. (it only becomes a simple concept after you understand it)

By the same token, however, it's a bad tactic to present something as inherently mysterious simply because you don't understand it.

 

The short answer to "why at the sunset do we see the sun rays converging towards the sun?" is because that's how perspective works. The important concept here is not about the sun's rays being parallel or nearly so, it's that the rays reaching our eyes from different points are not parallel. There is an angle between different non-radially--aligned points in our vision. The size of an object in our vision is a function of that angle and the distance to the object. The objects looks smaller when they are more distant, because the angle is smaller.

You want more? Then study up on the geometry of vision and perspective.

It's an important concept, because it's why we can have solar eclipses, even though the moon is far smaller than the sun.

[Janus]

Start with two parallel line ( white lines in diagram)  Consider pairs of points along those line (AA and BB)  The distance between A and A is the same as between B and B.  The A point are further from the viewer's eye ( small oval represents the lens and vertical line the retina.)

The red and blue lines are the light leaving these points, passing through the lens and forming an image on the retina.

Such an image will show the two "A"s being further separated than the two "B"s. this is just the result of projecting a 3 dimensional world onto a 2 dimensional surface.

[michel123456]

1 hour ago, Janus said:

Start with two parallel line ( white lines in diagram)  Consider pairs of points along those line (AA and BB)  The distance between A and A is the same as between B and B.  The A point are further from the viewer's eye ( small oval represents the lens and vertical line the retina.)

The red and blue lines are the light leaving these points, passing through the lens and forming an image on the retina.

Such an image will show the two "A"s being further separated than the two "B"s. this is just the result of projecting a 3 dimensional world onto a 2 dimensional surface.

Thank you but for your information I have constructed perspective drawings for more than 20 years (by hand). Usually you put the projection panel on the other side (left of the crossing point) otherwise you get a reversed image (as happen in cameras & inside your eye, other weirdness)

The point is not whether i understand perspective, the point is whether you understand how weird is the world as you see it around you everyday, at any moment, standing still or moving.

One example is the rays of light coming from the sun: try to explain to a kid that when he draws a circular sun and rays all around, that the radial rays are parallel. You will need 12 years of education.

One other example is the constant velocity that we almost never observe as constant (see the other thread). the only way to observe constant velocity is to make a train circulate all around you, and then the mathematician will explain that it is accelerating (because it changes direction constantly)!

One 3rd example is that when you move your head, the world around you changes: we are so used to it that it looks trivial but it is not. We are victims of being embedded inside the geometry. We believe that it is "natural", "evident", "normal" but in fact it is baffling. If you are not baffled then you are not understanding anything. At the beginning of the 20th Century some artists were perturbated by the fact that you cannot represent in painting all the faces of reality. See Cubism. These were not scientist but at least they realized that the way we are apprehending the world is weird.

4 hours ago, Strange said:

Well, let's see: they are diverging when they leave the Sun, they travel in straight lines so I guess they must still be diverging when the reach the Earth. What do you think?

The rays of light that we see are converging into our eyes, see Janus diagram above.

[Strange]

5 minutes ago, michel123456 said:

The rays of light that we see are converging into our eyes, see Janus diagram above.

Not necessarily. That is why we have an adjustable lens. 

[michel123456]

To continue on the same stance: isnt'it weird that following the rules of Relativity, if I change my state of motion (if I accelerate) the measured dimensions of a remote object will change? (see length contraction).

At the end, is it different from the weirdness we are living everyday?

[studiot]

41 minutes ago, michel123456 said:

To continue on the same stance: isnt'it weird that following the rules of Relativity, if I change my state of motion (if I accelerate) the measured dimensions of a remote object will change? (see length contraction).

At the end, is it different from the weirdness we are living everyday?

Yes this is true, have you seen any of the videos about the view of objects that are have relativistic magnitudes of relative motion?

But that is off topic here, no?

What did you or your 12 year old boy make of my contribution?

What actually do you think a ray of light is, and how do you explain its existence?

 

[michel123456]

6 hours ago, studiot said:

 

As a matter of interest, current approximations have the waves of light leaving points on the surface of the Sun as spherically expanding waves, but by the time they have reached Earth, we model them as plane waves.

Plane waves have parallel rays, spherical waves do not.

Eratosthenes thanks you.

54 minutes ago, studiot said:

Yes this is true, have you seen any of the videos about the view of objects that are have relativistic magnitudes of relative motion?

But that is off topic here, no?

What did you or your 12 year old boy make of my contribution?

What actually do you think a ray of light is, and how do you explain its existence?

 

1 is off topic indeed. But no, i haven't seen such videos. I have been told that relativistic transformations are "real", because there is no preferred FOR, and that perspective transformation are not "real" (see other thread).

2 I thank you. The boy is imaginary (and is not 12 year old)

3 is off topic I guess. 

Edited October 9, 2019 by michel123456

[studiot]

2 hours ago, michel123456 said:

3 is off topic I guess. 

Well I think the question is directly on topic since it directly address different models of light propagation and ties in with my spherical v plane wave comment.

Like many complicated phenomena we have many models of the phenomena, partly as our knowledge has increased and partly because we only need our model to be able to provide answers to questions we are interested in.

So the simplest form of optics is called 'geometrical optics' and is really the rules of 'light rays'.

Next up on the list is the wave theory of light.

Then we have relativistic theories.

And finally quantum theories of behaviour.

Each of these theories/models addresses more and more of the observed behaviour of light and must of necessity default to the simpler one in the list where they both address the same properties.

So the wave theory must explain the tenets of geometrical optics

1) Light travels in straight lines

2) Rays originating from infinity are parallel

 

It does so because it works on phase or phase difference of the light wave.

If you can imagine a circular (spherical) wave emanating out from a source you should picture a series of concentric circles around the cource.

Each of these circles is an isophase line - a line of constant phase or a (circular) line where the phase of the light is the same.

Close to the source the curvature is large and the radius small.

The radius is normal to all of these concentric circles and forms the ray.

As the radius becomes very large the curvature of the circles flattens out and neighbouring lines normal to the circles become more and more nearly parallel

Until at infinity they become mathematically parallel.

We take the distance between the Sun and Earth as sufficient to count as condition (2) above then we can say that in wave theory the original spherical waves have become plane waves ny the timethey reach the Earth.

In geometrical optics this condition strictly applies to on axis rays.
However again we make the approximation that rays 'nearly parallel' to the axis are included.

Rays which are substantially divergent from the axis will not be brought to the same image (eg the retina) surface.

[swansont]

15 hours ago, michel123456 said:

To continue on the same stance: isnt'it weird that following the rules of Relativity, if I change my state of motion (if I accelerate) the measured dimensions of a remote object will change? (see length contraction).

At the end, is it different from the weirdness we are living everyday?

Weird, perhaps — as an individual, subjective observation — but also a logical consequence of c being invariant. So in an objective sense, not weird at all, since it's the expected behavior.