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# Diffraction grating watch

1. Hi all,

https://imgur.com/a/1jtaX

I’m finding this image confusing - why is the light parallel in one and not parallel in the other?

Thanks
2. (Original post by JacobPowell)
Hi all,

https://imgur.com/a/1jtaX

I’m finding this image confusing - why is the light parallel in one and not parallel in the other?

Thanks
The parallel light waves are wavefronts, the lines are light rays. Essentially, a ray is perpendicular to a wavefronts. It is done like this to make the image less messy.
3. (Original post by Kushala Daora)
The parallel light waves are wavefronts, the lines are light rays. Essentially, a ray is perpendicular to a wavefronts. It is done like this to make the image less messy.
I don’t mean the incident wave, but the two main images, where one looks like both rays meet at P and the other the rays themselves are parallel.
4. (Original post by JacobPowell)
I don’t mean the incident wave, but the two main images, where one looks like both rays meet at P and the other the rays themselves are parallel.
The right side image is just the left side image but zoomed in so the rays appear parallel but they aren't.
5. It's an approximation where 'D' is very much larger than 'd'. Try drawing a couple of triangles and vary both d and D and notice how the rays, r1, r2, become more or less parallel.

In reality they are not exactly parallel, they wouldn't meet if they were!, but it is a fair approximation in a lot of cases.

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