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Thanks. I think I got most of what you described.

If I understand it correctly:

1) a point / pixel in the scene (as viewed by the eye) sends out a cone of rays, and the final color of this pixel is a combination of what those rays hit. This is the ray casting process, the reverse of light traveling.

2) the overall picture of the scene is the combination of pixels each calculated by the above ray casting process.

Am I right?

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Yes. The problem with working backwards is that some optical calculations have probability elements. A photon that hits a half-silvered mirror has a 50% chance of (specular) reflecting and a 50% chance of transmitting.

So for ray-tracing, you calculate along both paths and give 50% weight to each. Every time a ray hits a triangle in the scene, the material properties determine how the various components sum up to determine the color of the pixel.

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