Overlooking the Diffraction Limit
The diffraction limit sets a hard boundary on how well an optical system, such as a microscope or telescope, can resolve fine details. This limit is determined by the inherent wave properties of light and the unavoidable phenomenon of diffraction when light passes through an aperture. To get the most out of any optical system, it’s crucial to understand what the diffraction limit is, how it works, and why ignoring it can lead to disappointing results.
The Basics of Diffraction Limit
In any optical system, from microscopes to telescopes, diffraction plays a critical role in setting a resolution ceiling. The diffraction limit is a fundamental restriction caused by the wave-like behavior of light. As light enters an optical system through an aperture, it bends and interferes with itself, producing a pattern known as an Airy disk.
Every optical aperture creates this Airy disk—a small, bright center surrounded by fainter rings of light. The radius of this disk, and thus the smallest point that light can be focused into, is given by the formula:
Ra = 1.22λ/2Na
Where:
- Ra is the Airy disk radius
- λ is the wavelength of the light
- Na is the numerical aperture of the system
This radius defines the minimum spot size, a key factor in how fine an optical system can resolve detail. As the numerical aperture increases, the Airy disk’s size decreases, allowing for finer resolution.