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Light is a form of electromagnetic radiation, and it can exhibit various colors based on the wavelength. Typically, visible light ranges from about 400 nm to 700 nm. Different colors correspond to different wavelengths:
- Blue light, which has a shorter wavelength, around 400 nm, appears at the start of the visible spectrum.
- Red light, with a longer wavelength, typically around 700 nm, appears at the end of the visible spectrum.
It's important to understand that the wavelength affects not just the color, but also the behavior of light in optical instruments like microscopes. Because of its shorter wavelength, blue light can provide better resolution in microscopy applications.

The Rayleigh criterion is a vital concept in optics, especially when discussing the resolution of microscopes. It gives the limit at which two closely spaced objects can be resolved, or seen as distinct points. The mathematical expression of the Rayleigh criterion is:
\[ R = \dfrac{1.22 \lambda}{2NA} \]
Here, \( R \) represents the resolution limit, \( \lambda \) is the wavelength of the light used, and \( NA \) is the numerical aperture of the microscope's objective lens.
This formula shows a direct relationship between resolution and wavelength: as \( \lambda \) increases, the resolution \( R \) becomes larger, meaning the microscope is less able to differentiate between two points that are close together. Hence, shorter wavelength light, like blue light, results in a finer, higher resolution according to the Rayleigh criterion.

The numerical aperture (NA) is a fundamental concept in microscopy that significantly influences a microscope's resolving power. NA measures the light-gathering ability of a microscope's objective lens and is crucial for extracting fine details in the sample.
Numerically, NA is defined as:
\[ NA = n \cdot \sin (\theta) \]
where \( n \) is the refractive index of the medium between the sample and the lens (typically air or immersion oil), and \( \theta \) is the half-angle of the maximum cone of light that can enter the lens.
A higher numerical aperture allows more light to enter the microscope, thereby enhancing resolution. This is why adjusting the medium – for example, using oil immersion objectives, can substantially improve the numerical aperture and consequently the resolution. High NA objectives capture more detail, providing sharper and clearer images of the samples under examination.