This is a transparent material (default glass Principled Shader with red color) with a point light behind it:



It doesn't work as I expected. In real life, if I put a light behind a semi-transparent object, the light should be very obvious, right?

I changed the light's type from point to sphere:



All the other parameters are the same. Why do point light and sphere light behave so differently here?

But it still doesn't look realistic at all. I changed the light's color to cyan and increased its exposure:



The light is so strong that the bottom of the glass is lightened by the reflected light. I'm sure in real life we should be able to see the light source through the glass.

raincole
All the other parameters are the same. Why do point light and sphere light behave so differently here?

Pointlights are infinitely small, so theoretically they should be invisible. I would expect the min roughness of the ggx to make it visible though. Perhaps it's being clamped out by the indirect clamp.

Keep in mind that karma clamps indirect light sources to reduce fireflies. You can change the indirect clamp value to some high number to disable it. The clamping is what makes refracted light sources look artificially dark.

raincole
But it still doesn't look realistic at all. I changed the light's color to cyan and increased its exposure:

You've invented anaglyph 3D glasses. Red glass blocks non-red light, cyan light contains no red light, so no light goes through.