7 2 En - Parallel rays through lenses

Hi, welcome to this next lesson. In this lesson we will talk about a few terms and symbols as well as common definitions associated with lenses. So from the previous lesson, we had learned about two types of lens. One of them is convex lens and the other one is concave lens. Convex lens is also known as converging lens. So what does this means in simple English? It basically means focus. The word converge means it focuses. So convex lens basically means it will focus the light that passes through. Whereas concave lens is known as diverging lens. This means it spreads the light. So when the light passes through the concave lens, the light will spread. Before we talk about ray diagram, which is by the way one of the common asked questions in SPM, we need to familiarise with the symbols and definitions of common optical terms. Now, every lens has a centre. So for convex lens the centre is somewhere here, right? And for concave lens, the centre would be here. So this represents the centre of the lens. So rays of light that passes through it will not be reflected. And it is represented by the letter O. So the letter O refers to the optical centre which also means the centre of the lens. And the line that passes through the optical centre is known as an axis. This is what we call principal axis. It is a straight line that passes through the optical centre and it is perpendicular to the lens. This means it will form a 90 degree with the lens. So the line is called principal axis. There are no alphabets that represents it, so we'll just draw a line and label it as principal axis. The next thing that we want to look at is the focal point. Focal point is the point on the principal axis where the rays of the light parallel to the axis will either be converge or diverge, depending on the type of lens that we use. So let's talk about convex lens. It is a converging lens, so if I have light rays coming into the lens at a point, all these light rays will be converge which means they will be focused on a certain point. And always remember to draw mini arrows for light rays. So let's say I have another ray of light coming in, at some point the light will be converging at the principal axis. And this point, where all the lights converge, this point is known as the focal point. And we can represent if with the letter F. Focal point. And for concave lens, since it is a diverging lens, so lights that come into the lens will be diverged instead of being converged. So the light will be spread up instead of being focused. And all spreading seems to originate from somewhere - in front of the lens, right? So if we join the lights together, it seems to originate from one point. And this point is the focal point. So the focal point is the point on the axis where the rays of light would either converge or diverge. And let's take a look at the small letter f, which stands for focal length. It basically means the distance between the focal point and the optical centre. So for our first picture, let me use a different colour. This is the centre, which is the focal point. So the distance between these is known as focal length. So this is the focal point. This is the optical centre. And the distance between them is known as the focal length. Another thing that we want to look at is the letter u which represents the object distance. So let's say I place an object in front of the lens here. So from this object position until the centre of the lens, this is known as the object distance. So let's say I place an object here for the concave lens. So from here until all the way to the centre, it is known as the object distance. Image distance is represented by the letter v, which is the distance between the image and the optical centre. So, the letter v stands for the image distance. So let's say after the light from this object passes through the lens, it forms an image on the other side of the lens. Let's say this is the image. The distance between the image and the optical centre is known as the image distance. And for our SPM, we need to know the images that are formed by convex as well as concave lens. For convex lens, the images changes according to the distance of the object. So the image will either be real, virtual, upright, inverted or the size of the image could be larger, smaller or even the same size as the object. But for concave lens, the object position does not matter whereby the images that are formed is always virtual. Virtual means it cannot be captured on screen. You cannot capture the image on screen. And the image will always be erect or what we call upright, which means it face the same direction as the object. And the size of the image is always diminished, which means it is always smaller than the object. So let us move on to the next lesson to learn more on this. Subtitles by the Amara.org community