Ray Nature Of Light: Study with Video Lessons, Practice Problems & Examples

Light can be described as rays, which are perpendicular to wave fronts, simplifying its behavior in different media. When light travels in a single medium, it moves in a straight line, but phenomena like refraction and diffraction occur at boundaries. Huygens' principle states that each point on a wavefront acts as a source of spherical wavelets, forming new wavefronts tangent to their apexes. This principle explains reflection and the propagation of light, emphasizing the importance of understanding wave behavior in optics.

Ray Nature of Light

Hey, guys. In this video, we're gonna talk about something referred to as the ray nature of light, describing light as a ray instead of a wave. Okay, let's get to it. Now light, as we know, is composed of electromagnetic waves. All waves have things called wave fronts, which is a point of maximum oscillation for that wave. In the case of light, it's a point of maximum electric field. Because of this, it's often convenient to describe these moving electromagnetic waves as just rays, which are lines that point perpendicular to those wave fronts. Consider the image above me. I've drawn a moving light wave, a propagating electromagnetic wave, as a series of green wave fronts. Those wave fronts, which are points of maximum electric field, their peaks have to be separated by the wavelength. The peak to peak distance is just the wavelength and the rays that are going to be drawn have to be drawn so that they're always perpendicular no matter where they are to those wave fronts. Now light is always going to travel in a straight line when in a single medium. Whether it's air or water, I wrote a vacuum, but a vacuum is technically the absence of a medium. Light being the only wave that can propagate in the absence of a medium. However, when light's disturbed, when it crosses a boundary between media, then interesting things happen. They may not be interesting to you but they're interesting to physicists and so you're forced to learn them. These are in particular refraction and diffraction which are going to be 2 things that we talk about while discussing light and optics. In order to understand these two phenomena, we have to understand something called Hagen's principle. Now Hagen's principle makes 2 points. The first point is that all points on a wavefront act as point sources for spherical wavelets. So if I were to draw a wavefront, then every single point on that wavefront is going to be producing these smaller waves, which are called wavelets. And the second point is that new wave fronts. So new in time. Where's the wave gonna be after some amount of time? New wave fronts are formed by the tangent line across the apex of the wavelets that were produced by the last wavefront. This is a lot of information, but Hagen's principle is fairly simple in application. Let's do, just a little bit of application so we can see what exactly it's saying. So on the left, I have a wavefront produced by light that is moving in a single direction. This light is what would be called collimated light. Collimated. Because if I were to draw rays for this light, all the rays have to be perpendicular to the front. Those rays are all parallel to one another. So collimated light is light whose rays are all parallel to one another. So if I were to choose a point on this wavefront, you'll see that these little wavelets, are being produced. And they travel some distance in some amount of time. The distance I chose is just the wavelength because that's where the next wave front is gonna be. So that's one point. I can choose another point on the wave front, and it will also produce these spherical wavelets? And a third point on the wave front and also these spherical wavelets are gonna be produced. Now what Hagen's principle says is that the next wavefront, located one wavelength away is gonna be produced perpendicular, sorry, tangent to the apex of each of these wavelets. So here's the apex for the red wavelet. Here's the apex for the green wavelet. Here's the apex for the black wavelet, and our new wave front is just going to be tangent to those apexes. This is the new front. And as you can see, the light doesn't change direction. Light that was moving to the right is still moving to the right. All of those rays are still pointing in the same direction as they should because in a single medium, light shouldn't change direction. Let me minimize myself for the next thing. According to the old wave front, we have light that's moving in all manner of different directions. It's actually moving in all possible directions. It's moving spherically and this kind of light we would call isotropic. The same in all directions. Now if I choose a point on this wavefront you can see that these spherical waves are emitted and they're emitted for a distance point, out, travel a distance of the wavelength, choose a third point, spherical wavelets come out, traveling a distance of one wavelength. Now I want to mark the apexes on all of these waves Because those apexes are gonna determine where the next wave front appears. And that wavefront has to be tangent. Sorry. My hand got a little squiggly there. Tangent to all of those apexes. So it's going to be another spherical wave front, and you'll see that once again the light continues traveling in a straight direction because it's still perpendicular at at all those points on the new wave front, and that is to be expected. Light should continue to travel in a straight direction unless it moves into a new medium. So let's do an example. We wanna explain light reflecting off of a mirror using Hagen's principle. For this, I'm gonna draw wave fronts. I have the light ray, I want to draw wave fronts. Each new wave front I draw is going to be at a new point in time. Right? This wave is traveling. So at one point in time, the wavefront is here. Then it travels a wavelength, and now it's here. Then it travels some more, and it's here, and then it's here, and then it's here, and then it's here, and then it's here, and then it's here. Where did the wave front or where did the electromagnetic wave first contact the mirror? It first contacted the mirror here. Where did it contact the mirror? It's alright first. Where did it contact the mirror second? It contacted the mirror second right here. And where did it contact the mirror last? It contacted the mirror last right here. That means that that third point of contact is gonna have the least amount of time for the wavelet to propagate. Right? Because it occurred last. So its wavelet is gonna be the smallest. The second point is gonna have an intermediate wavelength because it occurred second, so it has the second largest amount of time to propagate. So I'll draw the wavelet like this. Now the first point of contact occurred the earliest. It occurred the longest amount of time ago, so its wavelet is gonna be the largest because it had the most time to propagate. So its wavelet is gonna look something like this. And now I'm gonna redraw this scenario because that picture's already getting complicated, and I wanna see what the new wave fronts look like. So my blue point is here. My green point is here. My red point is here. My red wavelet is small. My green wavelet is intermediate. My blue wavelet is large. And now where are the apexes? Remember that the apexes tell us where that new wavefront is gonna be. There's an apex. There's an apex. There's an apex. And I'm just gonna draw a line like this. If this had been done properly with computers and everything, that line would be perfectly tangent to all of the wave fronts sorry, all the wavelets, but I'm a person so I can't draw things perfectly. But the line looks something like that which means that my new ray should look like this. And because it moves in a straight line, the new wave fronts propagate like this, and this is what my new light ray looks like. And that is how you explain reflection using Hagen's principle. Alright, guys. That wraps up our discussion on the ray nature of light. Thanks for watching.

Here’s what students ask on this topic:

What is the ray nature of light?

The ray nature of light describes light as rays, which are lines perpendicular to wave fronts. This simplification helps in understanding light's behavior in different media. When light travels in a single medium, it moves in a straight line. However, at boundaries between different media, phenomena like refraction and diffraction occur. This approach is particularly useful in optics, where it helps explain how light interacts with lenses, mirrors, and other optical devices.

How does Huygens' principle explain the propagation of light?

Huygens' principle states that every point on a wavefront acts as a source of spherical wavelets. These wavelets spread out in all directions, and the new wavefront is formed by the tangent line to the apexes of these wavelets. This principle explains how light propagates, including phenomena like reflection and refraction. For example, when light reflects off a mirror, the wavelets from different points on the wavefront create a new wavefront that continues in a straight line, maintaining the angle of incidence equal to the angle of reflection.

What is the difference between collimated light and isotropic light?

Collimated light consists of rays that are parallel to each other, meaning the light waves travel in the same direction. This type of light is often used in lasers and optical instruments. Isotropic light, on the other hand, spreads out uniformly in all directions from a point source. In isotropic light, the wavefronts are spherical, and the light intensity decreases with distance from the source. Understanding these differences is crucial in applications like imaging and illumination.

What happens to light when it crosses a boundary between two media?

When light crosses a boundary between two media, it undergoes refraction or diffraction. Refraction occurs when light changes direction due to a change in its speed as it enters a different medium. The angle of refraction depends on the indices of refraction of the two media, described by Snell's Law. Diffraction occurs when light bends around obstacles or passes through small openings, causing the light to spread out. Both phenomena are explained by Huygens' principle, which describes how wavelets from each point on a wavefront interact to form new wavefronts.

How does Huygens' principle explain reflection?

Huygens' principle explains reflection by considering each point on a wavefront as a source of spherical wavelets. When a wavefront hits a reflective surface, the wavelets from different points on the wavefront create a new wavefront that is tangent to the apexes of these wavelets. This new wavefront continues in a straight line, maintaining the angle of incidence equal to the angle of reflection. This principle helps visualize how light behaves when it encounters reflective surfaces, such as mirrors.