Diffraction: Study with Video Lessons, Practice Problems & Examples

Diffraction is the phenomenon where light spreads apart when passing through a small slit, particularly when the slit width is smaller than the wavelength of light. Light typically travels in straight lines, but when it encounters a slit, it can become isotropic, emitting rays in all directions. This results in a diffraction pattern of alternating bright and dark spots on a screen, unique to the specific conditions of the light and slit. Understanding diffraction is essential in optics, as it illustrates the wave nature of light and its interactions with obstacles.

Diffraction

Hey, guys. In this video, we're gonna talk about a phenomenon called diffraction. Alright? Let's get to it. Remember that light travels in a straight line so long as it's not disturbed. Okay? We've seen one type of disruption of light before when we saw light encountering a boundary between 2 media and that light could reflect off of the boundary or it could transmit through and refract its angle as it passed into the second medium. Okay? This allows light. This fact that light travels, so long as it's not disturbed in a straight line, allows light to be described as rays. So just to refresh ourselves, we can draw any wave as successive wave fronts. Each of these wave fronts drawn in green is a point of maximum oscillation. In the case for an, an electromagnetic wave, in the case for light, it's a maximum electric field and we can draw rays such that they're perpendicular to all the wavefronts at all points. So I can draw Okay? And you can see clearly that it's perpendicular wherever you want to measure. Okay? And the distance between wave fronts, the distance between two peaks, as we know is just the wavelength. That's the definition of the wavelength. Okay? Now a common way to disturb light that we haven't talked about is for light to encounter a slit. Okay? And a slit is a small opening between 2 barriers of light. Alright? Let me minimize myself. We have here just light traveling. I drew 3 hypothetical light waves each of which is a different color. Okay? Here I've indicated 2 2 boundaries and we're gonna imagine that these boundaries, these barriers are completely reflective. Okay? Or not reflective at all but not transmissive. They completely block out any transmission of light. All the light that's allowed to transmit, then, the only light that's allowed to transmit is the one that passes through the slit. So the green light is the only light on the other side. Okay? Now depending on the size of the slit, depending on the width of the slit, right? This dimension, the rays may or may not be disturbed. They don't have to be disturbed as they pass through. They may or may not be disturbed. Alright? What diffraction is, is it's sort of a catchall term that refers to all phenomenon associated with light rays being spread apart when they encounter a slit. Okay? A slit between 2 barriers. Right? Diffraction isn't gonna occur for any slit. Okay? The slit's width I say the slit must be small, but what I mean is the width must be small compared to the wavelength of the light. Okay? So diffraction will only occur if this dimension right here is small compared to this dimension which is the wavelength. Alright? Now let's see what diffraction looks like. Right here I have 2 scenarios. Alright? I have light light of a particular wavelength encountering a slit of a particular width. And I've shown what happens when the wave fronts pass pass through that slit. Okay? So let's draw the rays and see if diffraction occurs here. In order to be perpendicular at all points to the wavefronts the rays before encountering the slit have to look like this. Okay? This by the way is referred to as collimated. This funky looking letter there is supposed to be an 'l'. Collimated light. Okay? Light that is all initially parallel to itself. All the rays are parallel. Okay? Now the wavefronts I have shown passing through the slit. What do the rays look like passing through the slit? Well they still need to be parallel to one another in order to be perpendicular at all points on the wavefront. So it's collimated before passing through the slit and collimated after. Those rays never spread apart. They're collimated entering. They're collimated exiting. That means that there was no diffraction here. Okay? But now choosing another hypothetical scenario. One where we have a larger wavelength and a significantly smaller hole. Now I wanna consider the scenario where the length is smaller than the width sorry, the width of the slit is smaller than the wavelength of the light. Okay? If I'm gonna draw the rays for this light, you can see that once again it has to be collimated. That's the only way to match rays to those wavefronts. Okay? But the wavefronts look different coming out of the slit. Now instead of them being parallel wavefronts, they're actually wavefronts that are moving spherically outwards. Okay? So in order to draw the rays, remember it has to be perpendicular to everything. This is perpendicular perpendicular perpendicular. But at a different angle. I need to draw the ray at a different angle. Right? So they point out equally in all directions. Okay? This is known as isotropic. And it's absolutely not collimated. Isotropic just means the same in all directions. Okay? Since the light intercollimated and exited isotropically, the light rays were disturbed. They did spread out and this is known as diffraction. Okay? Now something interesting happens when the light is allowed to diffract or when you allow for light diffracting. Okay? Light passing through a slit acts differently if you ignore diffraction. So in the left figure, we're gonna pretend like diffraction isn't a thing. Meaning that if we look at these two figures up here really quickly, no matter the relative size of the width of the slit to the wavelength, the light that enters collimated will always leave collimated. Okay. That's what we mean by no diffraction. So what that means is when light is entering the slit collimated, it's all coming out collimated and you're gonna get a single bright spot on some sort of screen behind the slit. That screen is just there to collect the light, to allow the light to, land on it so that you can see. But if you allow diffraction then so long as the width of this slit, so this dimension, that horizontal width, so long as that width is less than the wavelength of light what's going to happen is that initially collimated light is going to come out equal in all directions. And it turns out that you don't get a continuous band of bright light. You actually get alternating bits of bright light and dark light. Alright. So dark bright. And this alternating pattern of bright and dark spots of light is known as a diffraction pattern. Okay? And it's unique to the particular diffraction situation that the light is in. Okay? This wraps up our introduction on diffraction. Thanks for watching guys.

Here’s what students ask on this topic:

What is diffraction and how does it occur?

Diffraction is the phenomenon where light waves spread out after passing through a small slit or around an obstacle. It occurs when the width of the slit is comparable to or smaller than the wavelength of the light. When light encounters such a slit, it no longer travels in straight lines but instead spreads out in different directions, creating a pattern of alternating bright and dark spots known as a diffraction pattern. This spreading is due to the wave nature of light, where the wavefronts become spherical and isotropic after passing through the slit.

What is a diffraction pattern and how is it formed?

A diffraction pattern is a series of alternating bright and dark spots observed on a screen when light passes through a small slit. It is formed due to the interference of light waves that have spread out after passing through the slit. When the slit width is smaller than the wavelength of the light, the light waves spread out isotropically, and their overlapping creates regions of constructive interference (bright spots) and destructive interference (dark spots). The specific arrangement of these spots depends on the wavelength of the light and the dimensions of the slit.

How does the width of the slit affect diffraction?

The width of the slit significantly affects the diffraction of light. If the slit width is much larger than the wavelength of the light, diffraction is minimal, and the light continues to travel in straight lines. However, if the slit width is comparable to or smaller than the wavelength, significant diffraction occurs, causing the light to spread out in all directions. The narrower the slit relative to the wavelength, the more pronounced the diffraction and the more spread out the resulting diffraction pattern will be.

What is the difference between collimated and isotropic light in the context of diffraction?

Collimated light refers to light rays that are parallel to each other, maintaining a uniform direction. In the context of diffraction, collimated light entering a slit will remain collimated if the slit width is much larger than the wavelength, resulting in no diffraction. Isotropic light, on the other hand, spreads out equally in all directions. When collimated light passes through a slit with a width smaller than the wavelength, it becomes isotropic, leading to diffraction. This isotropic spreading creates the characteristic diffraction pattern of alternating bright and dark spots.

Why is understanding diffraction important in optics?

Understanding diffraction is crucial in optics because it illustrates the wave nature of light and its interactions with obstacles. Diffraction affects the resolution of optical instruments, such as microscopes and telescopes, by limiting their ability to distinguish between closely spaced objects. It also plays a role in various applications, including the design of optical systems, the analysis of wave behavior, and the development of technologies like diffraction gratings used in spectroscopy. By studying diffraction, we gain insights into the fundamental properties of light and improve the performance of optical devices.