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Diffraction refers to the way waves, such as light, spread out when they pass through an opening or around an obstacle. This spreading causes overlapping and limits the ability to see fine details.
In the human eye, diffraction occurs at the pupil. The smaller the pupil, the more significant the diffraction, which limits the eye's maximum resolution capability.
Light waves entering the eye through the pupil bend, creating a blur that limits the detail you can see. Diffraction effectively sets a fundamental boundary on how small details can be resolved.
The Rayleigh criterion provides a formula to determine when two points of light can be considered separate based on angular resolution. This criterion, defined as \( \theta = 1.22 \frac{\lambda}{D} \), gives the minimum angular separation between two sources that the eye can distinguish. Here, \( \theta \) is the angular resolution, \( \lambda \) is the wavelength of light (550 nm), and \( D \) is the pupil diameter (2.0 mm).
By calculating \( \theta \), you can determine how diffraction limits the resolution of objects viewed by the eye.

Retinal cells are minute structures in the eye responsible for detecting light and forming images.
Each retinal cell collects light, and their size sets physical limits on how small details can be recorded and converted into visual information.
In the exercise problem, each retinal cell is about 5.0 µm in diameter, restricting the eye’s capability to discern small objects or fine details without blurring.
For instance, at a standard close viewing distance of 25 cm, this retinal cell size limits objects to a minimum height of about 50 µm to be resolved clearly.
If an object is smaller than this threshold, it will generally blur into surrounding details. Therefore, both retinal cell size and diffraction work together to set the ultimate boundaries on visual acuity.
This interplay between the physical structure of the eye and light behavior emphasizes the complexity and limitation of human vision.

Angular resolution is a critical concept that helps describe how well the eye can distinguish two separate points. This separation is expressed in terms of angles. The smaller this angle, the finer the details that can be resolved.
The Rayleigh criterion mentioned earlier helps quantify this by providing the minimal resolvable angle based on light wavelength and pupil diameter.
When the angular resolution is calculated, it can be further converted into a linear object size. This allows us to determine the smallest visible object size at a given distance (here, 25 cm), using \( h = \theta \times d \), where \( h \) is the height and \( d \) is the distance.
For this problem, the angular resolution can be converted into an angle expressed in minutes, revealing how closely it aligns with experimental observations of about 1 arcminute.
Angular resolution illustrates the intrinsic interaction of optical physics with biological structures, dictating how sharply and clearly we perceive the world.