B Are the Sun's rays reaching us always parallel?

[ZapperZ]

Wow!

This was a question that was asked at such an elementary level, but it has taken off in a way too-complicated direction.

To the OP: It is considered to be paralleled in the same sense that we consider Earth to be "flat" and "g" to be a constant for most things done on the surface of the earth. It means that if you use the light from the sun in the optics experiments at your level, the result will be practically identical to those that I solve mathematically for "parallel" light. The "wave fronts" of light coming from the Earth are considered parallel by the time it gets to earth.

Think of it this way. If you drop a pebble into water and look at the circular wave fronts, how would they look as the move further and further away from where they are created? The father away they go, the "flatter" they will tend to appear until at some point, their curvature will no longer be significant. The wave fronts will now appear as if they are parallel and moving in a straight line at all points.

It doesn't take light just from the sun to be this way. In my intro physics labs, it is enough that the light source is at one end of the room. Our basic optics experiments give accurate-enough results if we assume that the light source is "infinitely" far away so that the wave fronts are parallel.

Zz.

 

[Shane Kennedy]

If you look at the sun (through a filter), rays from the left side enter your eye, and rays from the right side enter your eye. Those rays are not parallel. Practically, they radiate in all directions from the sun. Only from our perspective are they almost parallel.

 

[sophiecentaur]

Those rays are not parallel.

All rays from one location on the surface can be treated as parallel with no error. But rays from different locations are definitely not parallel. A half degree of difference in arrival angle is very significant. I reckon a high performance racing engine with bores with half a degree of taper would not last long. The poor rings would be knackered pretty soon. Half a degree is like a Barn Door!

 

[russ_watters]

Wow!

This was a question that was asked at such an elementary level, but it has taken off in a way too-complicated direction.

Agreed! I was shocked this thread made it to page 2!

This is not a situation where we can say 99% of the time they can be assumed to be parallel because there are a lot of situations where the fact that they are not parallel is important. But it isn't that difficult to differentiate.

 

[Eric Bretschneider]

OOOOOOOKKKKKKKKKKKK - another bad science textbook.

In order for the rays to be "effectively" parallel you need a point source that is a long ways away or a laser.
The fact that the sun is a source that is about 0.5 degree across means that light from the sun will diverge at about that same angle. If sunlight were parallel then the path of a solar eclipse would about 2100 miles wide not 100-200 miles wide.

The surface of the sun is a Lambertian emitter. Each part emits light in all directions with an intensity proportional to the cosine of the angle to the surface. I think that's the problem with the textbook is that it assumed light from the sun is emitted perpendicularly to the surface.

Lasers are nearly parallel beams, but have a small divergence. For practical purposes the light from a laser is parallel.

 

[ZapperZ]

For practical purposes the light from a laser is parallel.

But that's the KEY point here. At the level which I gather from the OP's post, a "sunlight" can be considered, for practical purposes, as having flat, parallel wave fronts! If you use sunlight to get the focal length of a convex lens, simply measuring the where the focus image is, for all practical purposes, will give you an accurate-enough value to equate to the focal length.

Zz.

 

[russ_watters]

OOOOOOOKKKKKKKKKKKK - another bad science textbook.

In order for the rays to be "effectively" parallel you need a point source that is a long ways away or a laser.
The fact that the sun is a source that is about 0.5 degree across means that light from the sun will diverge at about that same angle. If sunlight were parallel then the path of a solar eclipse would about 2100 miles wide not 100-200 miles wide.

I'm not sure I agree that it is a bad thing. We tend to be overly precise here because of differences in the work and audiences we deal with versus our members. Precision requires extra words that can detract from the message one is trying to convey. As such, it is often beneficial to skip listing caveats/qualifiers when they aren't necessary because they clutter-up/distract from the desired thought process. It is a difficult balance to make.

That said, it wouldn't have been too difficult to add in a one-word qualifier like "effectively" to the problem statement.

 

[Eric Bretschneider]

I'm not sure I agree that it is a bad thing. We tend to be overly precise here because of differences in the work and audiences we deal with versus our members. Precision requires extra words that can detract from the message one is trying to convey. As such, it is often beneficial to skip listing caveats/qualifiers when they aren't necessary because they clutter-up/distract from the desired thought process. It is a difficult balance to make.

That said, it wouldn't have been too difficult to add in a one-word qualifier like "effectively" to the problem statement.

My issue with a textbook that doesn't say approximately parallel or something to that effect is it becomes an ingrained fact of information that shouldn't be questioned. We don't need to distill science down to nice sound bites that are easy to remember. I occasionally tutor students who are adamant about "facts" that they were taught and I spend a lot of time "proving" what I shouldn't have to in order to help them.

The issue with simplification issue in history is even worse. Most historical events were the result of dozens of interwoven factors. Over the years the list of factors has gotten shorter and shorter to the extent that "modern" textbooks give only one or two factors for the cause of a series of events and not the whole picture. It starts innocently enough, but successive iterations . . .

 

[Anachronist]

The rays from OUR sun aren't parallel, which is why shadows have fuzzy edges. The thickness of the edge of the shadow results from partial obscuration of the sun at different angles (some rays hitting the shadowing object while other rays miss it).

However, the rays hitting our planet from OTHER stars can be considered parallel, because stars are basically point sources of light.

 

[gary350]

If you put 2 lens at the focal point of the concave lens you get a beam like a laser beam. Been there done that.

 

[jbriggs444]

If you put 2 lens at the focal point of the concave lens you get a beam like a laser beam. Been there done that.

A concave lens? That won't focus. That'll disperse. In any case, the image of the sun will not magically turn into a point and will not collimate like a laser.

 

[Drakkith]

The rays from OUR sun aren't parallel...

This is half wrong, as has already been pointed out multiple times in this thread. The light rays from any single point on the Sun are extremely close to parallel, which is exactly what the OP's book is talking about when it says Sunlight is parallel.

 

[B_iswas]

1. They’re near parallel — but not parallel. sure, perspective intensifies their otherwise minuscule and imperceptible angle, but they’re not emanating from an infinite planar light source… they’re radiating from the sun.

2.
Anyone can see the angular width of the Sun so that isn’t of interest, but from any part of the Sun the light is virtually parallel.
however, the Sun’s rays received by Earth are converging.

At the time, the edges of the shadows very clearly diverge (and get blurry). This is due to the Sun appearing ~0.53° in the sky. Just think about it, the light from the far edges of the sun are very clearly NOT parallel with each other.

 

[A.T.]

Sun’s rays received by Earth are converging.

The shadows in that picture converge due to perspective (if that is the Sun and the poles are on Earth).

 

[russ_watters]

The shadows in that picture converge due to perspective (if that is the Sun and the poles are on Earth).

He's referring to the fact that each shadow has sides that are not parallel, not the perspective between the two...

...though I still disagree that the answer can be stated so simply. From a point on each "side" (limb) of the sun emerge photons/rays covering about 180 degrees of arc; diverging. This enables some of the rays to converge when considered in certain ways from earth. So I would say the statement "the Sun’s rays received by Earth are converging" is oversimplified at best.

 

[Drakkith]

Anyone can see the angular width of the Sun so that isn’t of interest, but from any part of the Sun the light is virtually parallel.
however, the Sun’s rays received by Earth are converging.

This is incorrect. Converging and diverging light refer to light that comes from a single point, not from light that is emitted from different points. The Sun's light is diverging as it spreads out into space, not converging.

Since people don't want to read anything that's already been written or make sure their information is correct, I think it's time to close this thread.

Thread locked.