Phase-contrast microscopes are the oldest and simplest type of microscopes that create high-contrast, high-resolution images without staining by altering the wavelengths of light rays passing through the specimen. They utilize refraction and interference, causing structures that refract light to appear dark against a bright background of unrefracted light, while structures with different refractive indices appear in varying levels of darkness.

Phase Contrast Microscopes

Differential interference contrast (DIC) microscopes, also known as Nomarski optics, are instruments that utilize interference patterns to enhance the contrast between various features of a specimen. They operate by creating two beams of light with different directions of wave movement, known as polarization. After these beams pass through either the specimen or a specimen-free area, they are recombined. The specimen's characteristics alter the interference patterns formed during this recombination, which produces high-contrast images of living organisms with a distinct three-dimensional appearance. Because they do not require stains, DIC microscopes are particularly valuable for distinguishing internal structures within live, unstained specimens.

Differential Interference Contrast Microscopes (DIC)

Because light waves have peaks and troughs, they can cancel each other out when they are out of phase, a phenomenon known as destructive interference. In phase contrast microscopy, when light wavelengths are out of phase, the wave troughs cancel out the wave peaks, causing structures that refract light to appear dark against a bright background of only unrefracted light.

Destructive Interference

Interference is a phenomenon where light waves interact with one another, creating complex patterns of motion much like the interacting ripples from two pebbles dropped into a puddle. These light waves can interfere in phase, adding their amplitudes together, or out of phase, canceling each other out.

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How we perceive light depends on how it is reflected, refracted, and absorbed or transmitted through a material medium.


Light Perception

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Microbiology covers the scope and sequence requirements for a single-semester microbiology course for non-majors. The book presents the core concepts of microbiology with a focus on applications for careers in allied health. The pedagogical features of the text make the material interesting and accessible while maintaining the career-application focus and scientific rigor inherent in the subject matter. Microbiology’s art program enhances students’ understanding of concepts through clear and effective illustrations, diagrams, and photographs.

Microbiology is produced through a collaborative publishing agreement between OpenStax and the American Society for Microbiology Press. The book aligns with the curriculum guidelines of the American Society for Microbiology.

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Microscopy relies mostly on refraction, which is the change in direction of a light ray because it crosses into a new material.

Perception in Microscopy 

Reflection is an interaction that occurs when a light wave bounces off a material rather than being absorbed or transmitted. For example, a red piece of cloth appears red because it reflects red light to our eyes while absorbing other colors of the light spectrum.

Reflection of Light

Absorbance is an interaction that takes place when a material captures the energy of an incoming light wave. This absorbed energy may be retained or sometimes later re-emitted, as seen in materials like glow-in-the-dark plastics.

Absorbance of Light

Transmission, or transmittance, occurs when a light wave successfully travels through a material, such as light passing through a glass window. The amount of light that is transmitted depends on the material's properties, dictating whether it is transparent or opaque.

Transmission of Light

Transparency and opacity are properties that describe a material's ability to allow the transmission of light. A material with high transparency and low opacity is thinner or clearer, allowing a large proportion of light to pass through it, whereas an opaque material restricts light transmission.

Transparency and Opacity

Diffraction is a phenomenon where light waves bend or scatter as they interact with small objects or openings. The effect of diffraction is more pronounced when the object or opening is smaller relative to the light's wavelength, and diffracting waves often interfere with each other.

Diffraction of Light

Wave Interference

Refraction is a behavior exhibited by light waves where they change direction as they enter a new medium. This occurs because different transparent materials transmit light at varying speeds, meaning light changes speed when passing from one material to another. The degree of refraction depends on the angle of the incoming light.

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