12 Motivating Context for Unit II
This unit will focus on how light (and in some cases electrons) travel through both empty space and matter as well as how those interactions can be used to manipulate the paths and make . The most familiar optical system to most of you is the one you are probably using to read these very words: your eyes! We will therefore be looking at the eye quite a bit throughout this unit, both human eyes and simpler eyes in the animal kingdom. Another common biological application of optics is the microscope. To make sure that everyone is on the same page, you will find below some information about the anatomy of the human eye as well as some basic information about microscopes. Please be familiar with this terminology as we will use it in class.
The Human Eye. Derived from 36.5 Vision by OpenStax Biology
is the ability to detect light patterns from the outside environment and interpret them into images. Animals are bombarded with sensory information, and the sheer volume of visual information can be problematic. Fortunately, the visual systems of species have evolved to attend to the most-important stimuli. The importance of vision to humans is further substantiated by the fact that about one-third of the human cerebral cortex is dedicated to analyzing and perceiving visual information.
Anatomy of the Eye
The is the region in the center back of the eye that is responsible for acute vision. The fovea has a high density of cones. When you bring your gaze to an object to examine it intently in bright light, the eyes orient so that the object’s image falls on the fovea. However, when looking at a star in the night sky or other object in dim light, the object can be better viewed by the peripheral vision because it is the rods at the edges of the retina, rather than the cones at the center, that operate better in low light. In humans, cones far outnumber rods in the fovea.
Problem 1: Identify the portion of the eye responsible for most of the focusing of light.
Problem 2: What is the purpose of the iris?
Transduction of Light
The rods and cones are the site of transduction of light to a neural signal. Both rods and cones contain photopigments. In vertebrates, the main photopigment, , has two main parts (Figure 4) an opsin, which is a membrane protein (in the form of a cluster of α-helices that span the membrane), and retinal—a molecule that absorbs light. When light hits a photoreceptor, it causes a shape change in the retinal, altering its structure from a bent (cis) form of the molecule to its linear (trans) isomer. This isomerization of retinal activates the rhodopsin, starting a cascade of events that ends with the closing of Na+ channels in the membrane of the photoreceptor. Thus, unlike most other sensory neurons (which become depolarized by exposure to a stimulus) visual receptors become hyperpolarized and thus driven away from threshold (Figure 5).
Trichromatic Coding
There are three types of cones (with different photo