Transcript for:
Understanding Ray Diagrams for Lenses

let's see how to draw Ray diagrams for convex lenses imagine we have a convex lens of focal length 10 cm and there's an object kept at 30 cm I want to know where the image is going to be what its nature is going to be how do we figure this out the first step is to draw a diagram we'll draw the lenses we'll draw the focal length so this focus is at 10 cm from the optic Center and then we'll also Mark 2f which is twice the distance 20 cm from here this means that object which is at 30 cm must be Beyond 2f in this particular situation so we'll draw it over here now there are three rays that we can draw the first Ray from the top going par to the principal axis this after refraction will go through the principal Focus because that's what principal focus is all the parel Rays of light after refraction goes through it the second ray of light we can draw through the optic Center that's the speciality of the optic Center it goes undeviated so it will not refract it'll go straight now these two rays are enough but if you want you can also draw a third Ray which is passing through the principal Focus now after refraction it will go parall to the principal axis this is exactly opposite of the first one if the incident Ray is parallel the refracted ray goes through the focus if the incident Ray is going through the focus the refracted d goes parall to the principal axis and now wherever these three meet that's where your image is going to be and therefore you can see you'll have the tip of the object going down giving you an inverted image and so our image is between F and 2f which means it's going to be between 10 cm and 20 cm we've also seen that it's inverted which means it's real image and you can see the size is smaller so it's diminished the point is you don't have to remember any of this you can always draw these Ray diagrams and you can figure it out so let's practice the remaining cases so here are some other cases and all the cases I've kept object at different locations why don't you pause the video and see if you can draw a couple of Ray diagrams and identify where the image is going to be why don't you pause and try all right I'm going to start with this one and for all the kids I'm only going to draw the yellow aray and the blue aray and we will see we'll be able to get all the information of the image all right so here we go look parallel goes through the focus and optic Center goes undeviated notice where you're getting the image in this case oh you're getting it exactly at 2f so when the object is at 2f image is at 2f and if you look carefully the sizes are the same inverted meaning real image okay let's go over here again same story I'm going to draw the same two rays parallel goes through the focus and the one going through the optic Center goes undeviated see where they meet they meet Beyond 2f which means when the object is between F and 2f notice I'm getting an object which is beyond 2f it's inverted and it's bigger than the object so this time I'm getting a real magnified image okay what if I'm having the object at F again same story two rays of light parallel goes to the focus and the other one goes straight unded to the optic Center notice these two rays are exactly parallel to each other which means they're not going to meet ever so you'll get no image in this particular case when the object is at F you get no image at all and in the final case you have the object in between F and O well again same thing you have one R parel going through the focus another one goes unded to the optic Center this time also they don't meet but they're not parallel you can see they're diverging the two rays are going away from each other and therefore if I extend them backward there they meet this means the Rays appear to be coming from here so we're going to get an enlarged image but this time it's erect this means it's a virtual image you can't capture this on a screen you get a virtual erect enlarged image okay finally let's go to a concave lens and again even here you can try to draw these two rays of light and see where the image is going to be why don't you pause and try it yourself first okay this time the parall ray of light will get refracted not through this Focus remember these are diverging lenses so it'll go diverge away from the principal axis but it'll appear to be diverging from this Focus that's the key that's the key difference between these two and of course the second Ray I can direct it at the optic Center just like before and it'll go UND deviated this will be the same whether you have convex or concave because there's no refraction happening and now just like this case you can see these two rays are going away from each other they're not meeting but they appear to come from here see they're not really meeting this Ray is not really here it appears to come from here so they appear to meet and therefore we get a virtual erect image over here and you will find that regardless of where you keep the object wherever location you keep the object You'll Always Find a diminished virtual image between fno and that's something that you can try it out yourself so isn't this amazing whichever lens you're dealing with wherever the object is you can figure out everything about the image where it will be its size whether it's real or virtual all by drawing two rays of light amazing isn't it