Are the Sun's rays reaching us always 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.