1. Overview
Light is electromagnetic radiation exhibiting wave-particle duality, serving as a fundamental carrier for energy transfer and information transmission in modern technology.

Its behavior is described by three core models:

• The ray model for macroscopic propagation paths;
• The wave model for microscopic interference and diffraction;
• The photon model for its quantized energy nature.

2.Ray Model
The Ray Model is the most basic optical approximation, ignoring wave-particle duality effects.It describes light propagation as geometric rays along straight paths, grounded in Fermat’s Principle: light travels along paths of extremal optical path length (maximum, minimum, or stationary).Its behavior follows three fundamental laws:
2.1 Law of Rectilinear Propagation
In a homogeneous medium, light propagates in a straight line.
This law explains macroscopic phenomena (e.g., solar eclipses, pinhole imaging, shadow formation) and underpins all geometric optical systems.

2.2 Law of Reflection
When incident on a medium interface, a light ray obeys two rules:

1. The angle of incidence equals the angle of reflection:$\theta_i = \theta_r$
2. The incident ray, reflected ray, and surface normal lie in the same plane.

Both angles are measured relative to the surface normal.This law governs all specular reflection, as demonstrated by mirror imaging of a coin and landscape reflections in a calm lake . 2.3 Law of Refraction(Snell’s Law)
When light crosses the interface between two media with different refractive indices ($n_1$ and $n_2$ ), its path bends according to:
$$n_1 \sin\theta_1 = n_2 \sin\theta_2$$ where $\theta_1$ and $\theta_2$ are the angles of incidence and refraction, measured from the surface normal. This law quantifies refraction and is the fundamental principle behind the operation of lenses, prisms, and other refractive optics. A classic demonstration is the apparent bending of a straight object, like a pencil, partially immersed in water.

3.Wave Model
This model describes light as an electromagnetic wave. The figure illustrates its typical transverse wave characteristics: electric and magnetic field vectors vary sinusoidally in a plane perpendicular to the direction of propagation.
Its core wave properties are exhibited through phenomena such as interference (exemplified by double-slit interference fringes), diffraction (exemplified by single-slit diffraction), and polarization (exemplified by circular polarization). The relationship between the speed of light, wavelength, and frequency is given by the fundamental equation: $$c = \lambda \nu$$ where：

• $c$: speed of light in vacuum,
• $\lambda$ : wavelength of light,
• $ \nu$ : frequency of light.

When $c$ is constant, a longer wavelength corresponds to a lower frequency, and vice versa, reflecting the constraint between the temporal and spatial domains.

4. Photon model
The photon model focuses on the particle nature of light, and its basic unit of energy is called a photon.

Photons are massless particles that carry energy and momentum, described by the fundamental energy relation: $$E = h\nu = \frac{hc}{\lambda}$$

where:

• $h$: Planck constant,
• $\nu$: frequency of light,
• $c$: speed of light in vacuum,
• $\lambda$: wavelength of light.

Photon energy is directly proportional to frequency and inversely proportional to wavelength.

4.1 Photoelectric Effect
The photoelectric effect is key evidence for the photon model. An incident photon transfers its energy to an electron in a material. If the energy is sufficient to overcome atomic binding, the electron is emitted as a photoelectron.

4.2 Photon Shot Noise
Photon shot noise originates from the discrete nature of photons. It is particularly evident in low-light conditions, where the photon flux at the sensor is low.