Huygens
Is light a particle or a wave?
That time, that place
Huygens built the pendulum clock, transforming timekeeping at a stroke, and worked out that Saturn is encircled by a ring. On the nature of light, he stood directly against Newton.
Why this question
Newton took light to be tiny particles: shadows have sharp edges, whereas waves would bend around a corner and spread.
Huygens saw something else. Two beams of light cross one another and pass through untouched. If they were particles, should they not collide?
What was found
He treated light as a wave and explained reflection and refraction with it. Light bends entering water, he said, because it slows down in water.
This is the crux. Newton's particle theory predicted the exact opposite: light should speed up in water.
Two theories giving opposite answers means a measurement decides — compare the speed of light in water and in air. Except there was no way to do it. For two hundred years it could not be measured. So nothing was settled, and Newton's authority kept the particle view ahead.
Light was a stream of tiny particles
Beams that cross without colliding, and refraction explained as a wave
In 1801 Young's interference experiment favored the wave; in 1850 Foucault measured light moving slower in water. Then the twentieth century answered that both were right