Fundamental Principles of Light: Reflection, Refraction, and Dispersion

Introduction to Optics

Optics is the branch of physics that studies light. Since ancient times, various theories have attempted to explain light and its properties. For example, ancient atomists argued that vision occurs because objects emit images that travel from them to the souls of people, who perceive them through their eyes.

Speed of Light and Refractive Index

The speed of light is finite and varies depending on the medium through which it propagates. The speed of light in a vacuum is represented by the symbol 'c' and has an approximate value of 300,000 km/s. Light reaches its maximum speed in a vacuum; its speed is lower in any other medium.

Fermat's Principle and Rectilinear Propagation

Fermat's principle states that when light travels from one point to another, it follows the path that takes the least amount of time. In other words, the path of light is such that the time it takes to get from one point to another is minimized.

This principle explains why the path of light is a straight line (rectilinear) in a homogeneous and isotropic medium. One of the first optical phenomena observed was the shadow cast by an illuminated object. If the light source is small, the shadow has the same shape as the object with sharp boundaries. If the light source is large, the shadow consists of two parts: the umbra, the darkest region behind the object that receives no light, and the penumbra, which surrounds the umbra and is only partially illuminated. These phenomena are easily explained by the rectilinear propagation of light.

Reflection of Light

When light strikes the boundary surface between two different media, it is reflected. There are two types of reflection:

• Specular Reflection: Occurs when the surface is smooth and polished, like a mirror. The light rays are reflected in parallel directions.
• Diffuse Reflection: Occurs when the surface has irregularities. The reflected rays scatter in multiple directions.

The laws of reflection state:

1. The incident ray, the reflected ray, and the normal to the surface all lie in the same plane.
2. The angle of incidence (the angle between the incident ray and the normal) is equal to the angle of reflection (the angle between the reflected ray and the normal).

Refraction of Light

Refraction is the change in direction that light experiences when passing from one transparent medium to another. The laws of refraction are:

1. The incident ray, the refracted ray, and the normal to the surface all lie in the same plane.
2. Snell's Law: The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. This constant is equal to the ratio of the speeds of light in the two respective media.

Dispersion of Light

Dispersion is the phenomenon where the speed of light in a medium depends on its wavelength. This means that the refractive index (n) of a material also depends on the wavelength of the light. Consequently, when a beam of white light, which is composed of different wavelengths, passes through a material medium like a prism, it splits into its constituent colors.