Learning Objectives
By the end of this section, you should be able to
- 6.1.1 Describe the region of the electromagnetic spectrum that is perceived by our visual system, and the relative energy of photons at long and short wavelengths.
- 6.1.2 Describe the major parts of the eye and their role in focusing light to create a clear image.
In this section, we will meet the range of the electromagnetic energy spectrum that we call “light,” and see how the structure of the eye provides an optical system that creates a sharply focused image of the visible world on the sensory structure at the back of the eye, the retina.
We Capture Photons of Light Reflected from Objects Around Us
Light is a form of electromagnetic radiation, energy packaged in particles called photons. Physicists understand that light is both a particle and an electromagnetic wave, and although light can be described by the energy in each photon, light is more typically described by its wavelength on the electromagnetic spectrum. Electromagnetic wavelengths stretch from long wavelength, low energy radio waves to short wavelength, high energy gamma rays. (Figure 6.2)
Sunlight includes wavelengths from infrared to ultraviolet (UV) that penetrate the atmosphere, but only the middle of that range penetrates water, which is where vertebrate vision first evolved. This is the visible spectrum that our eyes can detect. Different components (wavelengths) of the visible spectrum are absorbed or reflected by the surfaces of objects and our visual system interprets the reflected wavelengths as an object’s color and shape. Shorter wavelengths within the visible spectrum we call blue while longer ones look red. White light combines wavelengths from the entire visible spectrum. Bright lights produce many photons per second, while dim lights emit few photons per second.
Anatomy of the Eye
The eye is the primary sensory structure that intercepts electromagnetic waves in the visible spectrum, eventually allowing us to perceive light. Figure 6.3 shows the major parts of the eye.
At the front of the eye, the eye’s optics are like the lens of a camera. The rays of light reflected from objects spread out in space. The cornea and crystalline lens intercept the arriving rays of light and bend them to form a focused image on the retina at the rear of the eye. Interestingly, because lenses invert images, the projection of the world on the retina is upside-down and left-right reversed. Figure 6.4 shows how light reflected from objects diverges in space but is refocused and flipped around for its projection on the retina.
The ciliary muscle and zonule fibers pull on the lens to flatten it and shift the focus to distant objects; when relaxed, the lens rounds up to focus on nearby objects. A clear liquid, the aqueous humor, fills the space between the cornea and the lens, and a clear, jelly-like vitreous humor fills the globe of the eye. Blood vessels spread over the inner surface of the eye next to the vitreous humor.
At the back of the eye, photoreceptors (rods and cones) embedded in the retina respond to the light hitting them, and several layers of nerve cells process the image, responding to borders between light and dark. The final retinal neurons, called retinal ganglion cells, send their axons out of the eye, bundling together to form the optic nerve, which transmits visual information to the brain. The optic disk is the region where the ganglion cell axons leave the eye and blood vessels enter. There are no photoreceptors in the optic disk, which creates a “blind spot” in our vision, although we are normally unaware of it. The fovea at the center of the retina is a region of tightly packed photoreceptors that provide our highest visual acuity. When we look at an object, we turn our head and eyes to project the object’s image onto the fovea, where we perceive details most clearly.
The remaining structural features include the choroid, a layer behind the retina with blood vessels that nourish the retina and a screening pigment to absorb stray light and prevent internal reflections. Behind that is the tough white of the eye, the sclera.
Correcting Optical Flaws: Myopia, Hyperopia and Presbyopia
Sharp vision requires the image projected on the retina to be in good focus, but for many people, the focus is in front of the retina (myopia) or behind the retina (hyperopia) (Figure 6.5). Corrective lenses restore sharp focus on the retina. Although in some cases people are born with an eye that is too short or too long for good focus, and thus need to wear corrective lenses from an early age, there is also some indication that constant close-up work can affect the continued development of the eye’s shape and lead to myopia.
Even people who do not need corrective lenses when they are young eventually require reading glasses as they age. This condition is called “presbyopia,” literally “old eyes,” and it results from the lens of the eye becoming less flexible as people age. The lens normally becomes rounder to focus on nearby objects, but as people age, the lens becomes more rigid and is unable to change its shape to focus on nearby objects such as pages of text. At that point, corrective lenses are needed to supplement the lens’s ability to focus.