Light and Dark Spots There's more going on here that you haven't explained yet. On x-ray images, why are there light and dark spots? The light and dark spots come from when you expose a photographic plate to x-rays. The plate starts off whitish, and when x-rays strike the plate, that section of the plate gets darker. That means that different amounts of x-rays are reaching different parts of the plate. Uh-huh. And why should there be these differences? Try this out. That makes sense and explains something else. Anytime there is more matter, whether it's thicker or just has a higher density, fewer x-rays get out. Bones appear lighter on the image because bone is denser than the rest of the body. And in dental x-rays, metal fillings appear white, because the metal is very dense. That's why they put a lead apron on you, to protect your body from the x-rays. If you're trying to x-ray a tooth, there is no need to have x-rays through your entire body.

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