Light, as a core carrier spanning the fields of optical engineering and electronic information, is a key medium for energy transfer and information transmission. From basic illumination and high-precision imaging to fiber-optic communication, laser precision manufacturing, and cutting-edge quantum technologies, the applications of light are deeply embedded in the lifeline of modern technology. In terms of its physical nature, light is a form of electromagnetic radiation that exhibits significant wave-particle duality. In practical applications and theoretical research, its characteristics can be interpreted through three core models: the ray model analyzes macroscopic propagation paths, the wave model reveals microscopic interference and diffraction laws, and the photon model defines its quantized energy nature. This article will systematically analyze the characteristics and principles of light based on these three core models.

Photon Model
The photon model focuses on the particle nature of light, and its fundamental energy unit is called the photon.

(Image source: https://www.robotlab.com/hs-fs/hubfs/Wavelet.gif)

Photons have no rest mass and carry energy and momentum. Their energy formula is $$E=h\nu=\frac{hc}{\lambda}$$ where: h is Planck's constant, ν is frequency, c is the speed of light, and λ is wavelength. The energy is directly proportional to the frequency and inversely proportional to the wavelength (see Figure 3).

(Image source: https://en.wikipedia.org/wiki/Spectrum#/media/File:EM_Spectrum_Properties_edit.svg)

In a vacuum, the speed of light is constant at $3\times10^8 m/s$. This property defines the energy exchange process between light and matter (such as the photoelectric effect, see Figure 4). When the energy carried by incident photons strikes electrons in a material, it transfers its energy to the electrons. When the energy is sufficient to overcome the atomic binding, the electrons escape to form photoelectrons, which is the core process of the photoelectric effect.

(Image source: https://en.wikipedia.org/wiki/Photoelectric_effect#/media/File:Photoelectric_effect_in_a_solid_-_diagram.svg)

In low-light environments (i.e., when fewer photons reach the sensor per unit time), photon shot noise (see Figure 5) has a relatively significant impact on the image.

(Image source: https://commons.wikimedia.org/wiki/File:Photon-noise.jpg)

Wave Model
The wave model focuses on the wave nature of light, treating light as an electromagnetic wave propagating in the form of transverse waves. Light requires no medium for propagation and manifests as periodic alternating changes in the electric and magnetic field vectors. Its core wave characteristics are manifested in phenomena such as interference (e.g., double-slit interference fringes), diffraction (light propagating around obstacles), and polarization (the directional nature of light's vibration). These not only reveal the physical essence of wave phase superposition and diffraction but also clarify the relationship between frequency and wavelength: $$c=\lambda*\nu$$ where: c is the speed of light, $\lambda$ is the wavelength, and $\nu$ is the frequency. The frequency $\nu$ and wavelength $\lambda$ are inversely related, reflecting the constraint between light in the time domain (frequency) and the spatial domain (wavelength).

(Image source: https://upload.wikimedia.org/wikipedia/commons/a/a9/Doubleslit.gif)
(Image source: https://en.wikipedia.org/wiki/Diffraction#/media/File:Wave_Diffraction_4Lambda_Slit.png)
(Image source: https://upload.wikimedia.org/wikipedia/commons/d/d1/Circular.Polarization.Circularly.Polarized.Light_Left.Hand.Animation.305x190.255Colors.gif)

Ray Model
The ray model ignores the wave-particle duality of light (such as interference, diffraction, and quantum effects) and abstracts the complex propagation of electromagnetic energy into “rays” traveling along geometric line segments. Its theoretical cornerstone is Fermat's principle—that light propagates along a path of extremal optical path length (maximum, minimum, or constant). Based on this principle, the ray model unifies the propagation behavior of light in isotropic media. In a homogeneous medium, light propagates in straight lines, which explains solar eclipses, pinhole imaging, and shadow formation. When light evolves at the interface between media, its path strictly follows the law of reflection (i.e., the angle of incidence equals the angle of reflection) and the law of refraction (i.e., Snell's law $n_1 \sin \theta_1 = n_2 \sin \theta_2$).
Light propagates in straight lines:

Law of reflection:

Law of refraction: