Chapter 5: Nature of Light
This chapter provides an overview of the fundamental properties of light, with particular emphasis on its wave nature, polarization, and principles of geometrical optics.
5.1 Introduction
Light plays a central role in how we observe and understand the biological world—from imaging cells under a microscope to detecting biomolecules using spectroscopy. Techniques such as absorption and emission spectroscopy, optical microscopy, and ultraviolet (UV) and infrared (IR) spectroscopy all rely on the fundamental properties of light.
In this chapter, we explore these essential properties, beginning with the dual nature of light, which exhibits both wave-like and particle-like behavior. We then examine polarization and the basic principles of geometrical optics, which explain how light propagates, reflects, and refracts through different media.
By developing a clear understanding of these concepts, you will gain the physical intuition needed to interpret and apply a wide range of optical and spectroscopic techniques used in modern biological and biomedical research.
5.3 Basic Properties of Light
Light is a form of electromagnetic (EM) radiation. The visible region, detectable by the human eye, span wavelengths approximately from 380 nm to 750 nm (see Figure 1).
In a vacuum (free space), all electromagnetic waves—regardless of their frequency or wavelength—travel at the same constant speed, known as the speed of light: \begin{equation} c \approx 3.00 \times 10^8 m/s \end{equation}
This fundamental result arises from Maxwell’s equations, which describe the behavior of electric and magnetic fields. The relationship between the speed of light 𝑐, frequency 𝑓, and wavelength \(\lambda\) is given by: \begin{equation} 𝑐 = 𝑓 \lambda \tag{5.1} \end{equation}
The electromagnetic spectrum is commonly divided into regions such as radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays, and gamma rays. These classifications are based on how the waves are produced, detected, and how they interact with matter.
From a particle perspective, light consists of discrete packets of energy called photons. Photons are massless particles that carry energy proportional to the frequency of the light. The energy of a photon is given by: \begin{equation} E = h f \tag{5.2} \end{equation}
where \(ℎ = 6.626 \times 10^{-34} J \cdotp s\) is Planck’s constant, 𝑓 is the frequency of the light.
Thus, higher-frequency light corresponds to higher-energy photons, while lower-frequency light has lower-energy photons.
Photons in a pale blue light have a wavelength of 500 nm. What is the energy of this photon in eV?
\(\textbf{Solution}\)