# Fermat’s principle

In the context of refraction, Fermat’s Principle accounts for why light follows the specific path it does when bending at the interface between two media and helps to explain why the bending follows Snell’s Law.

Fermat’s Principle states that light travels between two points along the path that requires the least time. In other words, when light transitions from one medium to another (e.g., air to glass), it bends in such a way that minimizes the time taken for its journey.

• In the context of refraction, Fermat’s Principle accounts for why light follows the specific path it does when bending at the interface between two media and helps to explain why the bending follows Snell’s Law.
• Fermat’s Principle states that light travels between two points along the path that requires the least time. In other words, when light transitions from one medium to another (e.g., air to glass), it bends in such a way that minimizes the time taken for its journey.
##### Light and Refraction
• Light travels at different speeds in different media. For example, light travels slower in water than in air. When light encounters an interface between two media, it bends due to the change in speed.
• Although Fermat’s Principle doesn’t rely on the concept of conservation of energy, it’s important to note that the total energy of the light wave remains constant before and after refraction.
##### Fermat’s Principle & wavefronts

Fermat’s Principle and refraction are related to the shape of the wavefront at the leading edge of a wave.

• Wavefronts are typically used to describe the behaviour of waves in classical physics, where waves exhibit properties such as interference, diffraction, and refraction.
• While wavefronts are often associated with the behaviour of light, which can exhibit both wave-like and particle-like properties, they are a description of the wave nature of light rather than individual photons.
• At the quantum level, the behaviour of individual photons is described differently, often using concepts such as wave functions in quantum mechanics.
• The shape of a wavefront depends on the light source, the medium through which light is propagating and the obstacles it encounters.
###### References
• In the context of reflection, Fermat’s Principle accounts for why light follows the specific path it does when reflecting off a surface and helps to explain why the angle of incidence is equal to the angle of reflection.
• Fermat’s Principle states that light travels between two points along the path that requires the least time. In other words, when light transitions from one medium to another (e.g., air to glass), it bends in such a way that minimizes the time taken for its journey.