[Music] in today's video we're going to take a look at how we can draw ray diagrams for convex and concave lenses let's start with concave or diverging lenses which are the ones that cause the light rays to diverge and spread out to represent a concave lens when you're drawing a vey diagram we use this symbol here and you can see that it has these outwards pointing v shapes at either end because they show that concave lenses get wider towards their ends next we need to add our axis which is a horizontal line through the middle of the lens and our focal points of which there will be one on either side and you might sometimes hear the focal point being called the principal focus instead but they both mean exactly the same thing we also normally add another dot on either side that are twice as far away from the lens as the focal point is and we label this one 2f these will become important later on when we do convex lenses in your exams though you'll normally be given all of this stuff so you probably won't have to draw yourself so let's imagine that this chicken is our object and that we need to draw a ray diagram to show where the image will form the first thing we do is pick a point near the top of our object and draw our incident rays coming out of the object at that point so one running parallel to the axis and another one going towards the center of the lens because this second ray went through the center it won't be refracted at all and i'll just continue on straight this first ray though will be refracted and to find the angle it'll be refracted at we need to draw a virtual ray between this point where it hits the lens and the principal focus or focal point which we labeled f earlier we normally draw this as a dashed line between the focal point and the lens but then a solid line as it continues past the lens now that we've drawn all of our light rays we need to find the point where they cross which would be this point here so this is the top of our image and we can now repeat this whole process to find where the bottom of our image should be but as a shortcut if the bottom of the object is on the axis like ours is then the bottom of the image will also be on the axis so there's no need for us to repeat everything we know that the image is going to be here in the exam once you've drawn your array diagram you might also have to comment on the image so in this case we would say that it's virtual because one of the rays used is a virtual array it's upright because it's standing the right way up and it's smaller than the object and if you're using a concave lens like this then the image will actually always be virtual the next thing we need to cover is exactly the same stuff but this time for convex or converging lenses which focus light onto a single point just like before the first thing we do is set everything up so with our lens our axis focal points the object and the two incident light rays the only difference so far is that the v shapes at the end of the lens are the other way around because this is how we represent convex lenses which get narrower towards the ends the ray passing through the middle of the lens won't be refracted at all and or just continue straight on the parallel ray though will be refracted so that it passes through the focal point on the other side of the lens and then carries on going and now that we've drawn all of the rays we just need to find the spot where they cross which is down here so this is the beak of our chicken and we draw it in again again you'll likely need to comment on the image in this case the image is real because these rays are both real it's inverted which just means upside down and it's smaller than the object bear in mind that in your exam you won't be given a complex object like our chicken instead you'll normally be given something simple like an arrow and just make sure that you always draw your rays from the very top of the arrow like we have here to make sure that you get all the marks now ray diagrams for convex lenses won't always look like this it's only because our object here is more than two times the focal distance away from the lens if our object was at two times the focal distance exactly then the image would still be real and inverted but it would be exactly the same size or if it was between f and 2f then it would be real inverted and larger than the object the weird one is that if the object is between the focal point and the lens itself then the refracted rays won't ever meet so to find our image we have to trace them backwards using virtual then we find the point where they meet and draw our image in these cases the images will be virtual upright and larger than the object anyways that's everything for today's video so if you found it useful then please do give us a like and subscribe and i'll see you again soon you