Transcription
Hello, we are going to talk about the physics of the eye, how images of objects are formed inside our eye so that the brain can interpret them. Let's start with a super simplified diagram of an eye. Light coming from objects enters through the cornea and reaches the crystalline lens, which is a converging lens. The wonderful thing about this lens is that thanks to this muscle, called the ciliary muscle, it can vary its curvature. The ciliary muscle contracts or relaxes, and thus can bulge or stretch the crystalline lens, with which the power of this lens can vary to focus both objects that are very far away and objects that are very close. Well, once it passes, we will talk about this in more detail later. Once the light passes through the lens, it refracts and an image is formed here on the retina. The information related to that image travels through the optic nerve to the brain. The eye in its normal state has the ciliary muscle relaxed, meaning it is prepared to see objects that are far away. Let's draw the ray tracing for an object that is far away. Here is the object. The ray parallel to the optical axis is deviated, passing through the focus, and the ray that passes through the optical center is not deviated. And this is where the image would be formed, a sharp image on the retina. This happens if the eye has no defect, of course. And what happens if we bring the object closer? Let's do the ray tracing. We bring the object closer, but we would not change the lens, meaning the focus remains the same. We will have a new ray, the ray parallel to the optical axis that passes through the focus, the ray that passes through the optical center that is not deviated, and the image will be formed behind the retina. What are the consequences of this? Another person owning the eye would see this object quite blurry. For the image to be formed on the retina, not behind it, the focal length of the lens must decrease, meaning its power must increase. This is achieved by making the crystalline lens more spherical. This is where the ciliary muscle comes into play, which contracts, bulging the crystalline lens, making it have more power. There is a bulged crystalline lens, so that if we trace the rays again, we see that by having moved the focus a little to the left, the image now falls right on the retina. This ability to adjust the curvature of the crystalline lens to focus at different distances is called accommodation. Finally, let's define the far point, which is the farthest point at which an eye can focus to see distant objects sharply. The far point of a normal eye is infinity. The near point is the closest distance at which an eye can focus thanks to the accommodation mechanism we have seen previously. The normal eye has a near point of approximately 25 centimeters, but this will vary quite a bit from person to person and also with age. In the next videos, we will see the refractive defects of the eye: myopia, hyperopia, and presbyopia. So, see you then. Goodbye.