Introduction
Although we commonly speak of seeing objects, we actually see light. Read about the various stages in light’s journey from the sun to the eye.
The image in this activity depicts light coming from the sun, moving through the blue sky, and striking a red flower. Some of the light bounces off of the flower and enters a woman’s eye. The light entering the woman’s eye is focused by the lens onto the retina in the back of the eye.
The Sun
The majority of light rays that eventually strike humans’ eyes originate from the sun. The sun generates vast amounts of electromagnetic energy, including light rays with wavelengths between 400 and 700 nanometers, which constitute the visible spectrum for humans. (Other types of electromagnetic energy include ultraviolet waves that can burn your skin and infrared waves that we feel as heat.) When all these visible light rays are added together, they produce “white” light. Isaac Newton discovered that when sunlight is passed through a prism (the image depicts a white ray of light striking a triangular prism and then emerging as a rainbow of colors), rays with different wavelengths (and therefore different colors) become separated.
The Atmosphere
Some light rays strike dust, water molecules, and other particles in the atmosphere. These particles absorb some of the light rays and scatter others in random directions. The inset image shows light rays and dust molecules. The light rays either pass by the dust molecules without hitting them, hit the molecules and are absorbed, or deflect off the molecules and appear bluer. The daytime sky looks blue because short (blue) light wavelengths are scattered more than long (red) wavelengths and you mostly see the scattered light when you look at the sky. Sunsets and sunrises are more orange and red because at those times of day, the sun’s rays must travel through more atmosphere than during other times of day and therefore the blue wavelengths have more chances to scatter, leaving more red wavelengths (see textbook Figure 2.1B).
Solid Objects
Light rays that penetrate the atmosphere eventually strike the surfaces of objects. When this occurs, these surfaces absorb some of the light rays and reflect the rest. (The inset image shows a beam of light composed of the colors blue, cyan, green, yellow, and red striking a red flower. The blue, cyan, green, and yellow light rays are absorbed and the red light ray bounces off the flower.) Some wavelengths are absorbed or reflected more than others. The light rays that are absorbed by a surface never reach an observer’s eye (this would be the blue, cyan, green, and yellow light rays in the image); it is the light rays that are reflected off the object that the viewer sees (this is the red ray of light in the image).
For example, the petals of the flower appear red because most of the light rays with short and medium wavelengths (e.g., the blue, cyan, green, and yellow rays in the image) are absorbed by the surface of the petals. Long wavelengths, which fall at the red end of the spectrum, are reflected by the petals.
You will learn much more about color vision in Chapter 5.
Focusing Structures in the Eye
Some of the light rays reflected off the flower reach the cornea, the clear outer covering of the eyeball. A few of these rays reflect off the cornea, but because it is clear, most rays are transmitted through it into the eye. The cornea, along with the aqueous humor, lens, and vitreous humor, refract the light rays, bending them to form a clear image of the world on the retina. (The inset image shows a light ray moving past the cornea and into the pupil of the eye where it is bent by the lens before it strikes the retina at the back of the eye.)
For some young people, and almost all older people, these eye structures are not perfect image-formers. Glasses and contact lenses add refractory power and can compensate for any focusing errors of the eyes (see textbook Figure 2.5).
(The cornea, aqueous humor, lens, and vitreous humor are described in more detail in the activity on Eye Structure.)
The Retina
Once a light ray makes it all the way through the eyeball (from the front of the eye where the cornea is located to the back of the eye where the retina is located), it strikes a photoreceptor cell in the retina (the inset image shows the two main types, rods and cones). Photoreceptor cells contain special molecules (e.g., rhodopsin, a variant of vitamin A) that change shape when they absorb a light ray. This absorption causes a chemical chain reaction that eventually leads neurons in the retina to fire action potentials that are transmitted to the brain. Thus, the retina transduces light energy into neural energy.
Read more about this process in the activity on Retinal Structure.
Grass
Grass, like other objects, absorbs some light wavelengths and reflects others.
The inset image shows five rays of light striking the grass. The rays of light are colored blue, cyan, green, yellow, and red. The green light ray bounces off the grass but the blue, cyan, yellow, and red light rays are all absorbed by the grass.