We can only see when light is present… without light, we are literally in the dark. When light is present, that light enters our eyes through our pupils and allows us to see. We think of light simply in terms of vision and color, but it actually is made up of tiny particles called photons. These photons travel from their source (the sun, a light bulb, a candle) in waves. Although they travel in waves, the waves run in one particular, straight direction.
Since the light travels in such a straight line from point A to point B, why did I bother to tell you that this line is wavy? Because the waviness tells us what color the light is. So, take a look at that picture above. I drew two light waves in it. The one on top is slower than the one on the bottom (only about 3 cycles are seen on the top one, while about 5 cycles are seen on the bottom one). The faster the wavelength (“higher frequency” in the diagram below), the more toward the purple end of the visible light spectrum, and the slower the wavelength, the more toward the red end of the visible light spectrum. But, whatever wavelength exists in the light we see, that light wave travels in a straight path from one point to another.
In order for us to see an object, light has to travel from that object to our eyes. That is easy to understand if you think about looking at a lightbulb– obviously, light waves are emanating from that lightbulb and coming into your eye. But how do we see a table or another person when they do not emit light? To understand that, you have to understand one more principle of light waves: light waves can be reflected or bent!
When light waves hit an object like a mirror or any other thick, solid object like a table or person, the light waves can bounce off that object. Just like when you hit a tennis ball against a wall, the light wave bounces off that solid object with an appropriate trajectory based on the angle of its approach; this is called light reflection. Note that some of the light is absorbed into the object (especially if it is a dark object). When light hits something that isn’t solid or thick, like water, rather than bouncing off, it bends. The bending of light waves is called refraction. Refraction also occurs when light hits your cornea.
Light hitting the cornea is refracted. The light continues on into your eye. Some of the light is blocked (absorbed) by the iris, but some makes it through the pupil. The light passing through the pupil gets refracted some more when it hits your lens. Because both the cornea and the lens are convex (rounded outward), all the light waves are bent in toward one another. This brings them very close so that they even cross. After they have crossed, they hit the retina. The refraction of the light waves brings them into focus on your retina. If your lens is not perfect, you’ll need glasses to help refract the light properly and bring the visual image into focus on your retina.
Note that, as shown in Figure 12.39, this crossing of the light waves causes the visual image you saw to get flipped upside down and backward on your retina. Therefore, the retina views our visual world upside down and backward. You’ll see that our brain maps it out that way, too. But we, obviously, interpret this visual image properly.