Transcription
In today's video, we're going to take a look at lenses and see how they can focus and disperse light using refraction. We'll also cover the difference between real and virtual images and see how a lens's power changes with its curvature.
Now, as we can see here, there are two main types of lenses: convex and concave. Convex lenses have an outwards bulge in the center, which causes them to refract parallel rays of light inwards to a single point, which we call the principal focus. Another name for a convex lens is a converging lens because they cause the light rays to converge, which just means to come together.
Concave lenses, on the other hand, cave in on either side, which causes them to refract parallel rays outwards, effectively dispersing the light. To figure out the direction that they'll be refracted in, we have to trace virtual lines from the lens's principal focus to where the rays hit the lens. Then, if we continue these lines, it shows us where the real rays will go.
Now, regardless of what type of lens it is, all lenses have a principal focus on both sides, and they'll be equal distances from the center. So our convex lens would also have a principal focus over here, and the concave lens would also have one here. You might sometimes hear the principal focus being called the focal point, but they're both exactly the same thing because they have a principal focus on both sides.
Lenses are basically symmetrical and can work both ways. So if we were to reverse the direction of the light so that it's traveling from right to left, everything basically flips, and the light focuses on the opposite side of the lens.
Another important thing about the principal focus is that it will always sit on the axis, which is this line passing through the middle of the lens. The distance between the principal focus and the very center of the lens is called the focal length, and the shorter the focal length is, the more powerful the lens will be, which just means that it will refract light more strongly.
In order to make a lens more powerful, we need to either make it more curved or use a different material that naturally refracts light more strongly.
The last thing we need to cover is the difference between real and virtual images, which, to be honest, is kind of a weird concept. So just bear with us, and hopefully it'll make sense. Now, regardless of whether it's real or virtual, images are formed at points where all the light rays from a particular point on an object appear to come together.
A real image, like we can see here, is formed when the light rays actually do come together to form the image. For example, these light rays from the chicken's head pass through the lens and converge in the bottom right, which is why we see the chicken's head down at the bottom. Meanwhile, the light rays from the feet converge in the top right, and so we see the feet at the top.
Because it's a real image, if there was a screen here, then the image could be captured on the screen. To give you some idea of where this happens in real life, whenever you look at a real-life object like a spanner, the light rays from that object pass through the lens in your eye and form a real image on your retina. It is actually inverted like this one here; we just don't notice it because our brain corrects it for us so that everything appears to be the right way up.
If we look at this diagram instead, though, we're seeing a virtual image. Virtual images are formed when the light rays don't actually come together where the image appears to be. For example, in this case, these dashed lines aren't actually light rays; they're just virtual rays that we trace back from these points towards the focal point. So because these are only virtual rays, what we're forming is only a virtual image.
The most common example of a virtual image is whenever you look in the mirror. The images you see aren't real images because, although the images appear to be behind the mirror, there can't really be any rays behind the mirror because it's a solid object that light can't pass through. So they're only virtual rays forming a virtual image.
Finally, when it comes to describing images in exams, as well as mentioning whether they are real or virtual, you also want to comment on whether the image is upright or inverted and whether it's bigger or smaller than the object. So in our example on the left, the image would be real, inverted, and smaller than the object, whereas in our example on the right, the image would be virtual, upright, and smaller.
Anyway, that's everything for this video. Hope you found it useful! If you did, then please do give us a like and subscribe, and I'll see you again soon.