Classical Mechanics

[Marni Kahn 1A]

In which instances do light rays act like classical waves?

[Jack Riley 4f]

light has wavelike properties on a macro scale

[Yuri Lin 4H]

Light behaves like a wave in examples such as reflection (reflects at the same angle it hits, which is a property of a wave), refraction (light bends like a wave when passing from one medium to another), and diffraction (going around an obstacle and then spreading out again to form interference patterns, much like waves crossing each other in water).

A good resource on this topic I found is physics classroom: https://www.physicsclassroom.com/Class/light/u12l1a.cfm

[805307623]

Light exhibits particle properties when it interacts with matter. This can be seen in the photoelectric effect. Electrons, however, show wave properties of interference and diffraction when they near obstacles.

[Helen Struble 2F]

We only need to describe light as quantized when we are describing interactions between individual photons and electrons/atoms/m'cules. This is why we need to use the quantized model to explain the photoelectric effect—because it is individual photons exciting individual electrons, it is only the energy of the photon which determines whether a sample emits electrons. The energy of the light wave (which is related to its amplitude) only effects the # of electrons emitted if the individual photons already meet the energy threshold. This makes sense, because increasing amplitude increases the # of photons, and if there are more photons which meet the threshold energy, more electrons will be emitted.