7 Light and Optics in Biology and Medicine

Light and the limits of optical imaging

Light is electromagnetic radiation. In vacuum, its speed, frequency, and wavelength are related by $c=f\lambda$, and each photon carries energy $E=hf$. Visible light spans approximately $380\text{–}750\,\mathrm{nm}$. When light reaches biological tissue, it can be reflected, refracted, absorbed, scattered, or transmitted. These interactions make optical measurement possible, but they also affect how deeply and clearly tissue can be seen.

At a boundary between materials, Refraction changes the direction of light. The Refractive index $n$ describes how much light slows in a material: its speed is $v=c/n$. Frequency remains constant as light crosses the boundary, so its wavelength changes. Lenses use refraction to focus light, while mirrors and tissue surfaces can reflect it.

Light also behaves as a wave. Diffraction spreads light around small features, and interference occurs when waves combine. These effects help set the Resolution of an optical instrument: features closer together than its resolution limit may appear as one. For a thin lens, object distance $d_o$, image distance $d_i$, and focal length $f$ are related by:

A microscope objective forms a magnified image of a specimen, and an eyepiece or camera enlarges or records it. Magnification alone does not guarantee that nearby details can be distinguished; resolution also depends on wavelength and the objective’s numerical aperture.

Takeaway: Refraction helps focus light, while wave effects and instrument properties limit the detail an optical system can resolve.

From focused light to visual signals

The cornea and crystalline lens focus incoming light onto the retina. The cornea provides much of the eye’s focusing power, while the lens changes shape to adjust focus for objects at different distances. If the eye’s optical power and length do not match, an image may focus in front of or behind the retina, causing refractive errors such as myopia or hyperopia.

The retina converts light into neural signals. Rods are highly sensitive and support vision in dim light. Cones provide color vision and fine detail, especially near the fovea; humans typically have three cone classes with different wavelength sensitivities. In phototransduction, light changes the shape of the retinal molecule in a visual pigment. This initiates a biochemical cascade that changes photoreceptor signaling, and the brain interprets the resulting signals as visual features.

Takeaway: The eye focuses light on the retina, where photoreceptors convert it into signals that the brain can interpret.

Using spectra to investigate biological samples

A spectrometer separates light by wavelength and measures its intensity. A biological sample’s spectrum can show which wavelengths it absorbs, emits, or scatters. Because molecular energy levels depend on molecular structure and environment, spectra can help identify substances, estimate their concentration, or track changes in a sample.

In absorption spectroscopy, a sample absorbs some incident light. For a uniform, dilute solution, the Beer–Lambert relation connects absorbance to concentration and optical path length:

where absorbance is $A$, molar absorptivity is $\varepsilon$, concentration is $c$, and optical path length is $\ell$. For example, measuring how strongly a purified protein solution absorbs at a characteristic wavelength can help estimate its concentration.

In Fluorescence, a molecule absorbs higher-energy light and then emits lower-energy light, typically at a longer wavelength. Fluorescent dyes and proteins can label particular molecules or structures so their locations in cells can be observed. Raman spectroscopy measures small wavelength shifts in light scattered by molecules; these shifts provide information about molecular vibrations and chemical composition.

Takeaway: Spectroscopy uses wavelength-dependent interactions between light and matter to reveal information about biological samples.

Optical methods in research and diagnosis

Different optical methods create contrast in different ways:

• Bright-field microscopy forms contrast from light transmitted through or absorbed by a specimen. It is useful for stained tissue and many prepared samples.

• Phase-contrast and differential-interference-contrast microscopy convert differences in optical path through a sample into image contrast. They can reveal structures in transparent, living cells without staining.

• Fluorescence microscopy detects emitted light from fluorescent labels, making selected molecules or cell structures visible against a darker background. Confocal microscopy rejects much out-of-focus light to improve contrast in optical sections of thicker samples. Fluorescent labels and illumination can affect observations or damage living samples, and labels can photobleach.

• Optical coherence tomography (OCT) uses low-coherence light and interference to measure depth-resolved backscattered light. Scanning these measurements produces cross-sectional or three-dimensional images without cutting out tissue. OCT is widely used to image structures such as the retina, though scattering limits its depth penetration in tissue.

• Pulse oximetry uses differences in how oxygenated and deoxygenated hemoglobin absorb light at selected wavelengths to estimate arterial oxygen saturation noninvasively. Motion, low perfusion, and other measurement conditions can affect the signal because light must reach and return from the target tissue.

Takeaway: Microscopy, OCT, and pulse oximetry turn light signals into information about biological structures or function, but each method has its own signal limitations.

Choosing an optical approach

Optical methods can provide fine detail and can be fast and noninvasive, but visible and near-infrared light generally cannot image as deeply through tissue as some non-optical methods. Scattering blurs images, absorption reduces signal, and illumination itself may alter a sensitive sample. The best method therefore depends on the measurement goal and the tissue or sample being examined.

When choosing an optical method, consider:

• Resolution: Can the method distinguish the features of interest?

• Depth: Can light reach the relevant structures and return with a measurable signal?

• Contrast: Does the method make the target stand out from surrounding tissue?

• Speed: Can measurements be collected quickly enough for the application?

• Biological impact: Could illumination or labeling change or damage the sample?

Takeaway: Optical measurements involve trade-offs among resolution, depth, contrast, speed, and effects on the sample.