Address confusion related to Principal rays

[sarchang]

Some students were confused when comparing Many Rays to Principal Rays because in Many Rays, rays that don't pass through the optic are straight, but in Principal Rays, rays that don't pass through the optic are still reflected/refracted.

The top and bottom principal rays should be dashed lines to emphasize that they do not represent real light rays like the ones in Many Rays.

[veillette]

To clarify, only the portion of the ray that extends beyond vertical optic line (but without passing through the optic) should be dashed.

In the case of the mirror, the "reflected" ray should be dashed if the ray does not go hit the mirror.

[pixelzoom]

Note that this will influence how #125 is addressed:

• Rays should look like they're passing through things. #125

[pixelzoom]

@arouinfar Does this need to be addressed, or should it be closed?

[pixelzoom]

Discussed with @arouinfar on Zoom, yes we want to do this.

[pixelzoom]

I wasn't totally clear on what is desired here, so I discussed with @arouinfar on Zoom.

Several of the comments above referred to "dotted" lines. Dashed lines are desired, not dotted. I've edited the comments to prevent confusion.

The screenshots below show the scenarios that we need to handle. The red arrows point to the ray segments that should be dashed.

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Convex Lens + Real Image

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Convex Lens + Virtual Image

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Concave Lens + Virtual Image

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Concave Mirror + Real Image

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Concave Mirror + Virtual Image

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Convex Mirror + Virtual Image

[pixelzoom]

@arouinfar proposed that virtual rays would look nicer using dashed line style. I prototyped this, ran it past @arouinfar, and she liked it. So it's implemented in the above commit. The screenshot below shows an example.

This makes it problematic to use a dashed line style for some of the real ray segments. I have some other ideas for how to address that, which I'll work on next.

[pixelzoom]

I don't think that using a different line style for specific segments of real rays is not going to prevent confusion. It didn't help me undertand this. What did help was @ariel-phet describing it to me, particularly the bit about thinking of the optic as extended infinitely.

So I think a better approach is to:

• use the same line style for all real ray segments
• provide teacher tips and other scaffolding to discuss Principal rays
• modify the vertical axis to help support the "infinite optic" concept

I did a quick prototype, showed it to @arouinfar and @ariel-phet, refined it and pushed in the above commit. Screenshot shown below.

[pixelzoom]

So to summarize, the proposal for addressing this issue is:

• Provide teacher tips and other resources related to Principal rays.
• Modify the look of the vertical axis to support those resources.

@arouinfar please review.

[arouinfar]

I completely agree. Thanks for summarizing @pixelzoom. I've also added this issue to the Teacher Tips issue.

The vertical axis in #140 (comment) looks like an extension of the lens, and I'm happy with that look.

Things look a bit off in master (vertical axis only extends below optical axis), however, so back to @pixelzoom to investigate.

[pixelzoom]

@arouinfar said

Things look a bit off in master (vertical axis only extends below optical axis), however, so back to @pixelzoom to investigate.

Handling that problem in #298, so closing this issue.