 Hi, this is Dr. Lloyd Williams. I'm an ophthalmologist who practices in Salt Lake City. I'm also the editor of the Moran Corps International. The purpose of these lectures is to help you learn the things that you need to become a ophthalmologist who can practice well and take care of patients. One thing that is important as an ophthalmologist is to understand something about the quality and properties of light. Since the eye is the organ that detects light and everything that goes on in the eye is in some ways influenced by the properties of light, these lectures are going to cover the general basics and give you an overview of the properties and optics of light, but are not intended to go in-depth into any of the particular topics like you would if you were a physicist studying light. Potentially, you might be able to write an entire book about each of these slides, but we're going to cover them just briefly as in terms of what an ophthalmologist would need to know. Light is electromagnetic radiation. There are many forms of electromagnetic radiation, and in particular, the visible light is what we call the visible spectrum, but gamma rays, x-rays, ultraviolet light, infrared, microwave, and radio waves are all other forms of electromagnetic radiation that aren't detected by the eye. The shorter wavelengths of light, blue and violet, and the middle wavelengths of light, green, yellow, orange, and the long wavelengths of light, red, are all detected in the eye. Light is interesting in that from a physics standpoint, it has a dual nature. So when light was first studied, people looked at it as a wave, and light as a wave best describes the properties of light as it travels in a vacuum, air, or a transparent material. And so in general, for the purposes of ophthalmology, thinking of light as a wave will often make light make the most sense in the practical applications of your daily work. The wave-like properties of light were shown by the double slit experiment, and we'll talk about that in a little bit. And as light travels, it travels most like a wave. And refraction and optics and those things which you'll be doing every day in your clinic are times where thinking of light as a wave makes a lot of sense. Light also has a nature as a particle, and we call a particle of light a photon. And light as a particle best describes light that is either being emitted or absorbed. And particle properties of light are demonstrated in the photoelectric effect and in black body radiation. And we'll talk about the photoelectric effect also in the next few slides. As light emits or transfers energy, it acts most like a particle, and light as a particle therefore is useful for understanding processes like laser ablation, laser photo coagulation, laser-based refractive surgery, and some of those aspects of ophthalmology. The fundamental properties of a wave are its wavelength, which is represented by the symbol lambda, the Greek symbol, and its amplitude, which is the height of the wave, and its speed. And for all light, the speed of light is constant in a vacuum at three times 10 to the eight meters per second. So this is the double slit experiment, and this was the classic experiment that showed the properties of light acting as a wave. Here on the left of your screen, you see some example of water waves, and water in particular is a material where we're very easily able to see the waves and think of them. So if you have water that is interacting with other droplets or other waves of water, what you'll see is areas of peaks and areas of troughs. And so here in this image, you can see that there are peaks and troughs in the way that the particular surface of water is interacting with the waves that are in it. In the same way, if you pass light through a pair of slits, the light is diffracted by those slits and becomes two wave fronts that interact with each other, and consequently you get areas of bright light and areas of almost no light as you get constructive and destructive interference of the light, and thus you get peaks and troughs just like you did with the water. So when people originally did this experiment, they said, see, light acts like a wave. If you were to think of light as a particle and you did the double slit experiment, what you would see is that instead of interacting and creating a series of interference patterns, you would see all the particles land in two spots essentially, the two spots that were available for the particle to pass through and then everything else would be blocked on either side. So you'd get two areas where there would be a lot of particles striking the target and the rest of it would have none at all, whereas with the light, because of the interference pattern created, you get a series of bright and dark and bright and dark and bright and dark. And so when we do a double slit experiment, what we actually see is what occurs here on the right and not what occurs on the left here. Later, people did an experiment where they were shining light on a metallic surface and they noticed that light striking the surface resulted in emission of electrons. And so if you were to think of light as a wave, you would expect that as you increase the intensity of the light that you would get more emission of electrons and as you decrease the intensity of the light, you would get less emissions of electrons and that you would have a relatively smooth curve of greater intensity is greater emission, less intensity is less emission. But what was found in this experiment is that once you went, that the intensity didn't matter as much as the frequency of the light. And once you went below a certain frequency of light, then you had no emission at all. So if you think of light as a wave, the results of this didn't make sense. So people had to begin thinking of light also as a particle. And then the question was, well, which is it? And currently physics says, well, it's both. So I don't know if that's helpful to you in thinking about it, but like we talked about before, in certain applications, it's easier to think of it one way and in certain applications, it's easier to think of it another way. The equations for light that are most important are C equals lambda nu, meaning the speed of light equals wavelength times frequency and E equals h nu or energy equals Planck's constant times frequency. And I've given you the speed of light and Planck's constant here. Both of those are constant numbers. They don't change in a vacuum. Diffraction, as we saw in the double slit experiment, is where light is bent by passing through a small opening. And so in this little diagram on the right, you can see uniform wave front of light interacting with a slit creates a diffraction pattern. In order to create diffraction, you need a slit that is similar in size to the wavelength of the light passing through it and larger slits produce interference. Resolution of a telescope or a microscope is limited by diffraction. So larger apertures and smaller wavelengths will allow greater resolution. However, as you saw from the electromagnetic spectrum, that there's a limit to how small you can make visible light in terms of wavelength. As may have occurred to you, the eye itself has a small aperture or slit known as the pupil. And so light can diffract or interact with this structure as well. The amount of light entering the eye is proportional to the area of the pupil and the area of a circle is pi R squared where pi equals 3.14. The pupil radius ranges from about one millimeter to four millimeters or a diameter from about two millimeters to eight millimeters. So the amount of light entering the eye can vary 16 fold. In addition, the depth of field varies with pupil size. Inversely, the smaller pupils have a larger depth of field and that's what's responsible for the pinhole effect where if you look through a pinhole, you can see better even independent of your refraction or a inlay called the camera inlay where a presbyope can get greater depth of field by placing a pinhole inlay in their cornea. Refraction is another property of light and refraction is when light is bent as it travels from a media with one refractive index into a media with another refractive index. Understanding the principles of refraction is essential to understanding how light passes through the eye since there are many different media that light has to pass through in order to get from the air in front of your eye to your retina. Some of the primary interfaces are the air-tier film interface and the aqueous lens interface. In this case, we see that when light interacts with a surface, there is an incident ray, a reflected ray and a refracted ray. And the refracted ray is based the angle of refraction or the amount that a ray is refracted is based on Snell's law. Snell's law is sine theta one over sine theta two equals the velocity of light in the media one over the velocity of light in the media two or the index of refraction in the media two over the index of refraction in the media one. And so here in this graph, we see that the incident ray going from water to air has an angle of incidence of theta one and an angle of refraction of theta two which is greater than theta one. At a critical angle, the refracted ray will actually be refracted at the interface and not into the new medium or back into the medium from whence it came. And total internal reflection such as used in fiber optics is where the angle of refraction is actually so great that the refracted light actually goes right back into the medium from whence it came. One instance in ophthalmology where the total internal reflection makes a big difference is when we do gonioscopy. The reason the gonioscopy mirror has to sit on the eye is that light coming from the angle of the eye will strike the cornea at such an angle that all the light is actually experiencing total internal reflection. So you have to have the coupling of the glass on the surface so that you change the index of refraction from the index of the air to actually the index of the glass and that enables you to see into that area. Reflection occurs when an incident ray is bouncing off of a surface. It follows three laws. The incident ray and the reflected ray and the normal line are all in the same plane. The angle of incidence with the normal is equal to the angle of reflectance with the normal and the incident ray and the reflected ray are on opposite sides of the normal. And the normal to the surface is a line perpendicular to the surface. If the surface is not smooth, scattered reflection will occur and that's shown in this image on the right. Light is scattered when it strikes an irregular surface and it can also be scattered by particulates such as dust in the air or cell or red blood cells located in the anterior chamber or the vitreous. Scattering can also occur in other ocular tissues like the cornea due to things like granular dystrophy, spheroidal degeneration or other causes of scatter. Transmission of light in a media refers to how much light passes through the media. Absorption of light refers to how much light doesn't pass through or how much light remains in the media. And absorption is often expressed as an optical density. In general, black surfaces absorb most of the incident light and white surfaces reflect most of the light while transparent surfaces transmit most of the light. Interference is another important property of light. Constructive interference, two waves that are perfectly in phase and experiencing constructive interference will double the amplitude of the light. Two waves that are out of phase and experiencing destructive interference can actually result in the elimination of the wave of light. The principle of destructive interference is used in anti-reflective coatings. Coherence of light expresses the correlation between two waves. You may have heard me say something about light phase in the previous, talking about the previous slide. Light, which is coherent, is in phase with itself and light, which is incoherent, is not in phase. And so you can see in these two diagrams located at the bottom that the light at the top is in phase because the relationship between the two waves stays constant over time and the light at the bottom, the relationship with the two waves changes over time and so that light would be out of phase or incoherent, temporarily, meaning in time. On the right, you can see light which is on the top part, which is spatially in phase and on the bottom part, which is spatially out of phase or incoherent. Light can also be spectrally, meaning based on its wavelength in or out of phase. Light can also be polarized and we see this particularly with polarized glasses and with light being reflected off of surfaces like water. So polarized pair of glasses blocks a lot of the reflected light off the surface of the water, which has been polarized vertically, horizontally, or in any direction. And what that means is that the light in that wave has its electrical component, the red, and its magnetic component, the blue, which is abbreviated B, oriented the same way in the light in that beam. It can also be circularly polarized as shown to the right. And that becomes an important principle when we look at light reflected off of surfaces or reflected off of water and when we deal with polarized glasses. One practical application can be demonstrated here. So in a photo without a polarized camera lens, the window in this image acts very much like a mirror and you can see very little into the room. And that's because light bouncing off of the window is polarized by the window and comes off with a certain polarization. When we put a polarized lens 90 degrees to the polarization of the light bouncing off the window, we now see very little of that light and eliminate almost all of that glare and can see into the room. This also applies with water and that's why polarized glasses allow you to see much deeper into water than not using polarized glasses. Illumination is another property of light. Illumination is the amount of light striking a surface or in our field, generally the amount of light striking a retina. It can be measured in absolute terms, which is radiometry, or in terms relative to perception of the human eye, which is called photometry. Illumination is affected exponentially by distance. So for every meter further away, you lose the amount of light by the square of the distance that you're moving away. The human eye can detect vast variances in illumination. So for example, on a moonless night, there's .0001 lux of light. In a full moon, there's one lux of light, which is 10,000 fold more. Indoor lighting is 50 to 80 lux. And direct full sunlight can be up to 100,000 lux. So you can see that there is an enormous difference in the amount of light conditions that the eye can function in. The principle of photometry versus radiometry, or why would you even need two different measures of light, is that the human eye is not equally sensitive to all wavelengths of light. Bright light called photopic light and scotopic light, dim light, and mesopic light, which is in between, states exist for the eye and the eye has different ability to detect brightness depending on which state it's in. So if you leave an eye in the dark for say an hour, its ability to detect small amounts of light increases dramatically. And you can see this if you spend time outside on a moonless night, that over time your ability to see where you are and what's happening improves. Photometry uses the relative sensitivity of the human eye, but radiometry measures the absolute intensity of light without concern to the quality of the human eye. And this is important in that one watt of red light is equal to 2.7 lumens and one watt of green light is equal to 683 lumens, meaning that the eye is much, much more sensitive to green light than it is to red light. In addition, one watt of infrared light is zero lumens in the eye because infrared light isn't detectable by the eye at all. So here are some examples of measurements of photometry and measurements of radiometry. The last thing we'll talk about in terms of the property and physics of light that's essential part of ophthalmology is lasers. Laser is an acronym standing for light amplification by stimulated emission of radiation. Lasers are monochromatic, meaning they emit light at only one wavelength. So lasers have a specific color. They're directional, meaning that the beam is very narrow and spreads minimally over a distance. So the difference between a flashlight, which spreads quite a bit in a laser, is that a laser can put a very fine point, even a long distance away on a wall, whereas a flashlight is not capable of doing that. Laser light is coherent. It's all in the same phase. It's polarized and it's intense. So lasers can emit very high wattage beams of light. Some common practical uses of laser in ophthalmology are photo coagulation. We can do pan retinal photo coagulation, focal laser retinopaxi and destruction of tumors, photodisruption, such as a Yag capsuleotomy or argon laser trabeculoplasty. Photoablation, such as we use in refractive surgery like LASIK or PRK, or femtosecond laser cutting of cornea tissue. Or we can use lasers to analyze tissue, such as in optical biometry, measuring the length of the eye for predicting intraocular lenses or ocular coherence tomography, also known as OCT. I'd like to thank Wikipedia, Wikimedia Commons, where I got many of these images. These are the individuals who produced the images. Again, I'm Dr. Williams. I hope this has been a helpful introduction to light and its properties and some of the ways that those properties can affect your clinical practice in ophthalmology. Thank you very much.