 any other kind of surface. So, we have discussed how light get deviated because of reflection and because lights get deviated it not coincide with the object. You may get a feel that light is coming from somewhere else, not from the object because that is deviated now. So, that is why we talk about formation of image because ultimately what your eyes observe is the image where I feel that light is coming from. So, we have done entire reflection then after that we started reflection. Reflection is slightly bigger topic than reflection because we are not only dealing with lenses but interface between the two mediums. In reflection you have light that changes the medium, it goes from one medium and how you define the medium with what factor? Refractive. Now, how much light depends not only depends on the refractive index but on how the geometry is of the interface between the two mediums. So, we have only when the interface is horizontal straight or planar we have found out where the image will be how much shift in the object will be apparent shift. So, we have done all the reflection which is total internal fine. So, there will be lot of very nice question on reflection only this much of you know. So, and how is critical surface creates an image right. So, we also refraction because even which is lens from this particular equation ok. Because finding out lens formula if we already have interface formula while we need a lens formula. So, you guys notice image location you do not need to use interface formula two times because they are two different interface forms image ok. So, either you use interface formula and use two step approach to find the image or you use lens formula to directly get the location of image ok. So, that is what we have lens formula ok. And when we say lens formula both are same ok. And the difference of the glass could be anything glass or whatever substance it is made up of. But both sides should have same refractive index. Then only your lens formula which we are going to derive will be valid ok. Suppose because n3 then you cannot use lens formula. Then what you have to do? Use interface formula first between these two interface then. So, these are the few things and another assumption when we are studying lens is that these lenses are thin ok. So, write down few assumptions for lens. The assumptions are in our syllabus ok when we are talking about lens. So, we call them thin lenses ok. They are thin and also their aperture is less. Say that having in case of spherical lens ok. So, how about here also ok. So, before we shift we should first try to find out few terminologies which we will be using to explain about lens. Like for example in case of mirror what are things we have used to explain what is happening with mirror? There is a curvature center of curvature and then focal length aperture and all that right. So, similarly let us see what are the terminologies for lens ok. Now, before we will lens learn that dim lens ok. This is also point going forward is that we are not going to study all lens ok. And then we will be using sign convention because of that sign convention whatever formula you derive will be true for all cases. We are getting it?