Optics is the branch of physics that deals with light and its properties and behavior. It is a vast science covering many simple and complex subjects ranging from the reflection of light off a metallic surface to create an image, to the interaction of multiple layers of coating to create a high optical density rugate notch filter. As such, it is important to learn the basic theoretical foundations governing the electromagnetic spectrum, interference, reflection, refraction, dispersion, and diffraction before picking the best component for one's optics, imaging, and/or photonics applications.
THE ELECTROMAGNETIC SPECTRUM
Light is a type of electromagnetic radiation usually characterized by the length of the radiation of interest, specified in terms of wavelength, or lambda (λ). Wavelength is commonly measured in nm (10-9 meters) or μm (10-6 meters). The electromagnetic spectrum encompasses all wavelengths of radiation ranging from long wavelengths (radio waves) to very short wavelengths (gamma rays); Figure 1 illustrates this vast spectrum. The most relevant wavelengths to optics are the ultraviolet, visible, and infrared ranges. Ultraviolet (UV) rays, defined as 1– 400nm, are used in tanning beds and are responsible for sunburns. Visible rays, defined as 400 - 750nm, comprise the part of the spectrum that can be perceived by the human eye and make up the colors people see. The visible range is responsible for rainbows and the familiar ROYGBIV - the mnemonic many learn in school to help memorize the wavelengths of visible light starting with the longest wavelength to the shortest. Lastly, infrared (IR) rays, defined as 750nm – 1000μm, are used in heating applications. IR radiation can be broken up further into near-infrared (750nm - 3μm), mid-wave infrared (3 - 30μm) and far-infrared (30 – 1000μm).
Figure 1: Electromagnetic Spectrum [View Larger Image]
Isaac Newton (1643 - 1727) was one of the first physicists to propose that light was comprised of small particles. A century later, Thomas Young (1773 - 1829) proposed a new theory of light which demonstrated light's wave qualities. In his double-slit experiment, Young passed light through two closely spaced slits and found that the light interfered with itself (Figure 2). This interference could not be explained if light was purely a particle, but could if light was a wave. Though light has both particle and wave characteristics, known as the wave-particle duality, the wave theory of light is important in optics while the particle theory in other branches of physics.
Interference occurs when two or more waves of light add together to form a new pattern. Constructive interference occurs when the troughs of the waves align with each other, while destructive interference occurs when the troughs of one wave align with the peaks of the other (Figure 3). In Figure 3, the peaks are indicated with blue and the troughs with red and yellow. Constructive interference of two waves results in brighter bands of light, whereas destructive interference results in darker bands. In terms of sound waves, constructive interference can make sound louder while destructive interference can cause dead spots where sound cannot be heard.
Interference is an important theoretical foundation in optics. Thinking of light as waves of radiation similar to ripples in water can be extremely useful. In addition, understanding this wave nature of light makes the concepts of reflection, refraction, dispersion and diffraction discussed in the following sections easier to understand.
Figure 2: Thomas Young's Double-Slit Experiment [View Larger Image]
Figure 3: Constructive and Destructive Interference [View Larger Image]
Reflection is the change in direction of a wavefront when it hits an object and returns at an angle. The law of reflection states that the angle of incidence (angle at which light approaches the surface) is equal to the angle of reflection (angle at which light leaves the surface). Figure 4 illustrates reflection from a first surface mirror. Ideally, if the reflecting surface is smooth, all of the reflected rays will be parallel, defined as specular, or regular, reflection. If the surface is rough, the rays will not be parallel; this is referred to as diffuse, or irregular, reflection. Mirrors are known for their reflective qualities which are determined by the material used and the coating applied.
Figure 4: Reflection from a First Surface Mirror [View Larger Image]