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Light's behavior can be understood through phenomena like interference and diffraction. Interference happens when two or more light waves meet and combine. Depending on their phase, they can amplify or cancel each other.
Diffraction occurs when light waves bend around obstacles or spread out after passing through narrow apertures. These behaviors are characteristic of waves and are vividly depicted through experiments such as Young's double-slit experiment.
In this experiment, light passing through two closely spaced slits forms a pattern of bright and dark fringes on a screen, demonstrating constructive and destructive interference.
This experimental evidence strongly supports the wave nature of light.

The photoelectric effect provides key evidence of the particle nature of light. In this phenomenon, light shines onto a metal surface and ejects electrons from it.
Classical wave theory couldn't explain why light below a certain frequency, no matter how intense, couldn't eject electrons.
However, the concept of photons, introduced by Einstein, did. He proposed that light is composed of particles called photons, each carrying a quantized amount of energy determined by its frequency: E = hf
Here, E is the photon energy, h is Planck's constant, and f is the frequency of light.
When photons hit the metal, they transfer their energy to electrons. If the photon's energy is enough, it can eject an electron from the atom, explaining the photoelectric effect comprehensively.

The concept of quantized energy means that photons, the particles of light, can only exist at specific energy levels.
This quantization is a pivotal idea in quantum mechanics. The energy of a photon is given by E = hf , where E represents the energy, h is Planck's constant ( 6.626×10^{-34} Js), and f is the frequency of the light.
This relationship implies that as the frequency changes, the energy of the photons also changes. One important consequence of this is the phenomenon of the photoelectric effect, where photons with higher frequencies (and thus higher energies) can cause electron ejection.
This quantized nature helps in understanding many other quantum phenomena, such as atomic emission spectra and the behavior of electrons in atoms.