Optics is a branch of physics involved in the study of light, its behavior, and its properties. Optics is a vast area of science covering multiple subjects, both simple and complex. It talks about the reflection of light off a metallic surface, image creation, the interaction of numerous coating layers to form a high optical density notch filter, etc. Understanding the basic theoretical concepts governing interference, dispersion, electromagnetic spectrum, interference, refraction, and diffraction is essential to understanding the components of optics, imaging, and photonics applications.
The Electromagnetic Spectrum
Light can be perceived as a type of electromagnetic radiation. It is expressed by the length of the radiation and specified by wavelength, lambda (λ). The wavelength is measured in nanometers or micrometers. The electromagnetic spectrum encompasses all radiation wavelengths ranging from radio waves (long wavelengths) to gamma rays (short wavelengths). The wavelengths most appropriate to optics are the visible, ultraviolet, and infrared ranges.
UV rays are used in tanning beds, are responsible for sunburns, and can be defined in the range of 1-400nm. Visible rays lie between 400-750nm and lie in the part of a spectrum perceived by the human eye and comprise the colors we see in the rainbows. Finally, the infrared rays are defined in the range of 750nm – 1000μm. They are used in heating applications and can be further split into near-infrared (750nm – 3μm0, mid-wave infrared (3 – 30μm0, and far-infrared (30 – 1000μm).
According to Sir Isaac Newton, light is composed of tiny particles. He was the first physicist to propose the idea. A 100 years after his theory, Thomas young proposed a new theory, which explained that light has wave qualities. He carried out the experiment where he passed light through two closely spaced slits. The results show light interfering with itself. This interference could not be explained if the light was purely particle-based and could only have such behavior if it was a wave. Although the light has both wave and particle characteristics, known as wave-particle duality, wave theory contains much importance in the field of optics, and particle theory plays its role in other branches of physics.
Interference occurs when two or more light waves combine and represent a new pattern. Interference is of two types: Constructive and destructive. The former occurs when the wave troughs align with each other, whereas the latter occurs when one trough aligns with the peaks of the other. Constructive interference of two waves results in brighter bands of light, and destructive causes darker bands. If we talk about sound waves, constructive makes louder sounds, and destructive causes dead spots where you cannot hear sounds. Interference is one of the theoretical building blocks in optics. Considering light as radiation waves like ripples in water can be beneficial.
Reflection occurs when a wavefront collides with an object, changes direction, and returns at an angle. According to the law of reflection, the angle at which light approaches the surface (angle of incidence) equals the angle at which light exits the surface, according to the law of reflection (angle of reflection). Regular reflection occurs when all reflected rays are parallel, and the reflecting surface is smooth. The beams will not be parallel if the rough surface, resulting in an uneven reflection. Mirrors are usually thought to represent consistent reflection, but this is dependent on the material and coating used.
Contrary to reflection, refraction is the mechanism where the wavefront changes its direction when passed through a medium. The refraction degree depends on the wavelength of light and the index of refraction of the medium. The index is the ratio of the speed of light in a vacuum to the speed of light in a medium. The index of refraction represents a quantifying effect of light slowing down as it makes its way from a low index medium to a high index medium.
The sparkle we see in diamonds is caused by total internal reflection. Diamonds exhibit a high range of total internal reflection due to their high index of refraction. This property causes them to reflect a wide variety of angles and sparkle. Another example of understanding total internal reflection is fiber optics, in which light enters one end of a glass or plastic fiber and undergoes several reflections throughout the length of the fiber until it leaves from the other end. Fiber optics have limited accepted angles and lesser bend radii as total internal reflection occurs for a critical angle. It dictates the shortest radii that fibers bend to achieve total internal reflection and the most significant angle at which light can enter and be reflected.
The measure of change in the index of refraction of material concerning the wavelength is defined as dispersion. The phenomenon also determines the separation of wavelengths called chromatic aberration. For instance, a glass with high dispersion will separate more light compared to a glass with low dispersion. Dispersion can be quantified by expressing it with the Abbe number.
Dispersion causes chromatic aberration, which is responsible for the rainbow effect observed in prisms, optical lenses, and similar optical components. The phenomenon of dispersion contains a substantial value in physics, such as in the cases of an equilateral prism, which splits light into the component colors. However, in other applications, a system’s performance can be hindered by dispersion.
Thomas Young’s double-slit experiment visualized interference patterns. These patterns can be characterized by the phenomenon of diffraction. This phenomenon usually occurs when waves pass around a sharp edge or through a narrow slit. Generally, diffraction is directly proportional to the distance between the size of the light’s wavelength and the slit’s width or the object encountered by the wavelength. One of the best examples of diffraction can be demonstrated with the help of diffraction gratings. A diffraction grating has closely-packed, parallel grooves that cause incident monochromatic light to diffract or bend. Interference patterns are formed depending on the degree of diffraction.
All the fundamental concepts of the electromagnetic spectrum, interference, reflection, refraction, dispersion, and diffraction, are the building blocks to understanding the more intricate optical concepts. The properties of light explain much about optics—understanding the foundations of optics helps pave the way for how light interacts with imaging, optical, and photonic components.