Evaluate evidence demonstrating that electromagnetic radiation acts as both a wave and a particle depending on the phenomenon (HS-PS4-3).
When light passes through two narrow slits, it creates a pattern on the back wall.
If light were purely a stream of particles (like bullets), we would expect exactly two bright lines on the back wall corresponding to the two slits.
Instead, we see an interference pattern (multiple stripes).
This proves light travels as a wave, diffracting out of the slits and adding/canceling (constructive and destructive interference) before hitting the wall.