2016 Mar

Physics behind Accommodation of the Human Eye

March 10, 2016

Eye Accommodation

Eye accommodation is the act of physiologically making adjustment to the crystalline lens elements to vary the power of refraction and bring into sharp focus close and far objects. The eye accommodates by changing the lens’ curvature which is performed by the ciliary muscles. They relax the suspensory ligament fibers, narrow the ciliary body diameter, and allow relaxation of the lens, forming a more convex shape. The more convex the lens are, the stronger their light refractive power and focuses diverging light rays onto the retina, allowing good focus on close objects.

Eye focus is facilitated by a couple of elements including the lens, cornea, iris, and muscle tissue, which alters the lens’ shape so the eye can suitably focus on objects at different distances from the observer. This is shown in the figure above. At the point when ciliary muscles contract, they relax the ciliary strands joined to the envelope of the crystalline lens. Since the lens is flexible, it unwinds into a more bended shape, expanding its refractive energy to suit for closer survey. The iris capacities as the gap stop for the eye, shutting to around 2 mm in breadth in brilliant light and opening to a limit of around 8 mm in faint light.

The convenience of the eye can be effectively displayed as shown in the below diagram; the scale model eye is utilized with the cornea through the front surface of the lens held steady at the model qualities. The network for that part of the eye is computed, and afterward the thickness d and back surface force P are changed.

Eye Modeling

Light detection at the molecular level

Vision is founded on the light absorption by photosensitive cells in the eye. The photosensitive cells are light sensitive in a relatively narrow electromagnetic spectrum region, with wavelengths ranging from 300 nm to 850 nm as learnt during our A Level Physics tuition class. The human eye possesses two kinds of photosensitive cells, named cones and rods based on their typical shapes. Rods function well in dim light but they don’t distinguish color, whereas cones work well in bright light and distinguish color vision. A person’s retina contains approximately a hundred million rods and three million cones. Notably, rod cells are able to respond to a distinct photon, and the brain needs fewer than ten of these responses to detect the sensation of the light.

Rods are slender but elongated structures. Their outer section is specialized for photoreception. It comprises of about 1000 discs in a stack, which are sacs enclosed by membrane; densely packed with photosensitive molecules. The light sensitive molecule is usually referred to as a visual pigment since it is highly colored due to its light absorption ability. Rhodopsin is the photosensitive molecule found in rods, which consists of the opsin protein connected to 11-cis-retinal, a prosthetic group.

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