Final answer:
Unwanted reflections in optometers can be sourced to optical aberrations such as chromatic aberration, astigmatism, spherical aberration, and coma, which can be minimized with careful design and corrective measures.
Step-by-step explanation:
The sources of unwanted reflections in optometers, including those experienced with a contact lens floating on a film of tear fluid on the eye, are typically due to a combination of aberrations within the optical system.
These can include, but are not limited to, chromatic aberration, which causes wavelengths to focus at different points leading to a fuzzy image, and astigmatism, where the eye focuses light unevenly due to irregularities in the shape of the cornea or lens. Additionally, he inherent design of optical instruments can lead to other aberrations like spherical aberration and coma. Spherical aberration occurs when rays from the outer edges of a lens focus at different points as compared to rays closer to the center, leading to a blurrier image.
Coma is observed when the object is off-center resulting in parts of a lens or mirror not refracting or reflecting the image to the same point, producing a pear-shaped image. These aberrations can be minimized or corrected through various methods including improved component quality, adjusting the angle of incidence, and computational techniques.