 video, we have this 1.5, we need to find focal length of this lens, so when you hear focal length then you are okay, so 1 by f should be equal to 1, mu 2 minus mu 1 divided by mu 1, 1 by r1 minus 1 by r2, now object is this side, observer is that side, so the instant ray comes from here, is it not, so mu 2 is not 1.5, mu 1 is 1, this what is r1, plus 10, because I am measuring in this direction, radius of curvature and instant rays also coming like that, so 1 by 10 minus r2 is not, plus 20 minus 15, plus 20 only, so this is 20, so 1 by f you get as 1 by 2 into 1 by 20, so f comes also to be 40 centimeter, so now we have almost every formula, whatever we had to learn from this chapter, we can mix and match, so we can have lens and mirrors together, we can have multiple lenses together, we can have multiple mirrors and question, a point source S, 15 centimeter from converging lens of focal length 10 centimeter on it's principal axis, so a point source S on it's principal axis, focal length 15 centimeter, this should a diverging mirror, where should you place a diverging mirror, so that real image form on the source itself, where you will not be able to solve the question, if you do not further ray area, so are you able to visualize what is happening right now, exactly, but able to see what is happening, you may not be able to do it, just right now, but if you draw ray area, it will give you a bridge for understanding more the situation, and once you understand the situation properly, then only you can use the formula more effectively, so try to first draw the ray diagram and see what is happening, here we have a converging lens and a diverging mirror, what is the converging lens, what kind of lens is this, so we have a convex lens and a, and convex mirror also, fine convex mirror diverges, convex lens converges, so the situation is something like this, you have a convex lens and you have a mirror, okay, and then you have a source here, fine can you draw a ray diagram, it goes like this, what will happen to this ray, it will come back, it will come back, so image will be on this line somewhere, okay, now this is another ray, what should happen to this ray, so it goes something like this, then what happens to this ray, what, if it goes up again like this, so the image gets found here, are you getting the point, image should be found here, so what should happen to this ray, it will be somewhere here, no, it goes down and goes through the lens again, what, it goes through the lens again, okay, correct, so if it goes like this, then what will happen, it will converge, so it may converge like this, so yeah exactly, so what I am saying is, what is that specific position, focal length is 10 centimeter, so focal length for the lens is 10 centimeter and focal length for the diverging mirror should be given here, 12 centimeter, focal length of this is 12 centimeter, you need to find this distance, okay, this distance is given as 15 centimeter, what is this x, okay, but what if it strikes the mirror along the normal, suppose this is center of curvature, what will happen to this, then what will happen to this ray, it can reflected back along the same line, so it replaces its path and then image gets found here or not, okay, so the situation here should be that this ray of light hits the diverging mirror along the normal, okay, so the image due to this lens should be found where, should be found at the radius of curvature of this mirror, are you getting it, now try to solve this question, okay, what is the radius of curvature focal length is 12, 24, so this should be 24, right, so if this is 24, what is the distance of image for the lens, it will be x plus 24, is it not, so we have the situation where u is not minus 15 centimeter, v is not 24 plus x centimeter and f is plus 10 centimeter, right, so I will use the lens formula here, 1 by v minus 1 by u equals to 1 by x, so 1 by 24 plus x, minus 1 by minus 15 is equal to 1 by 10, fine, you are getting x plus 5, so it has 2 focal points here and there, so why you taking plus 10, why not minus 10, what is focal length, when you have object here, focal points should be considered this side, because it parallel to that side, okay, hand into the diverging mirror.