Fresnel / rim light
Surfaces reflect more at glancing angles, creating the bright edge or rim you notice around glass, water, and polished objects.
See it
What it is
The Fresnel effect is the rise in reflectivity as a surface turns away from you. Look straight down at water and you can see into it; look across the same water at a shallow angle and it becomes a mirror. On a rounded object, those grazing angles collect around the silhouette, which is why a Fresnel term reads as an edge highlight or rim. Real-time shaders almost always use Schlick's 1994 approximation of it rather than the full equations.
Use it to make glass, water, clearcoat, and polished materials behave more plausibly, or exaggerate it as a graphic rim that separates a character from the background. A photographic rim light is related but different: that is a real light placed behind the subject, while a Fresnel rim is calculated from the camera angle and surface normal.
Gotcha: a thick, equally bright outline around everything reads as an X-ray shield, not reflected light. Keep the response tied to the view angle, break it where the material is rough, and remember that real Fresnel changes reflectivity rather than simply adding colored glow.
Ask AI for it
Add a camera-facing Fresnel rim in a three.js ShaderMaterial. In the fragment shader, compute 1.0 minus the clamped dot product of the normalized view direction and view-space normal, shape it with GLSL smoothstep and pow, and multiply it by a cool rim color. Keep the center of the object unchanged and cap the rim intensity so it separates the silhouette without becoming a solid outline.