 Okay, so the later part of the chapter you have some natural phenomena's like how rainbow gets formed okay and scattering why sun is red while it is about to sunset okay and the defects in the eyes so all that in your syllabus so we will do that and then towards the end the last part of the chapter is about optical instrument okay that is something which people leave I want you to pay a lot of attention towards the end of the chapter because what happens was the end students become slack they'll be like okay chapter is about to get over they don't pay attention to a later part of the chapter but this the person who don't make noise so the the person who makes the paper who sets the paper they also know this that you guys will be leaving away that portion of the chapter so they will set questions from that portion in fact last year you know current electricity is is a chapter in your syllabus fine so entire current electricity can be divided to three chunks okay and the third chunk is the smallest part which talks about some some instruments meter bridge we stone bridge which people ignore okay so two questions came and both of them were from that part okay so let us pay attention till the very end of the chapter fine so write down natural phenomena's write down rainbow the first one is rainbow it's a very very nice phenomena how rainbow gets formed now tell me how when it is when the rainbow is formed you might have seen the rainbow right so all of you fortunate enough to see the rainbow now tell me what are the conditions when rainbow gets on what I mean what happens during that was was it just rained and there's a sun also right it had just rained and there is a sun okay and rainbow gets formed in the direction of sun or in opposite direction of the sun in the direction of sun so you look at the sun and opposite direction it it happens in opposite direction you notice next time what happens is that there will be droplets in the air when the rainbow gets formed okay don't fight it's it's just about rainbow so there will be droplets here suppose this is a water droplet so we are going to see what happens because of the water droplet and then you will be able to visualize exactly how rainbow gets formed okay so sun is somewhere here okay sun is smaller than droplet but you know anyways so you are here you are looking at rainbow fine so what happens a sunlight will hit the water droplet let us say it hits like this okay then what will happen tell me this is a water droplet so reflective index is different than air so what will happen to this ray first you need to draw the normal normal passes through the center of curvature so let us say this is normal okay now there will be dispersion it will split into colors just like it had split into colors when it was prism similarly here also it will split into colors are you getting it okay then what will happen after this tell me after this what will happen so it it will come out like that so most of the time that will only happen but at times the angle of incidence this one becomes more than the critical angle then total internal reflection will happen fine and that total reflection may not happen for all the colors because it had different wavelengths and every wavelength had different refractive index so different refractive index means critical angles will be different for different colors fine so some color will get totally internally reflected so this angle will be equal to that angle so lots of refraction won't come in play law of reflection will come in play so this angle will become equal to that angle fine so after this see total and reflection can it happen here can it happen at point number one why total and reflection only happens when light travels from denser to rarer medium so here the light was about to travel from denser to rarer so it can only happen here okay and light will come out from this side like this okay it may so happen that total to the reflection happens only for red light okay so you will see a pattern over there so due to the geometry it may happen that the droplets here in this belt for these belt only red or violet will have total and reflection so this small belt will have a color of that and as you gradually move up this belt will be some other color okay this belt will have some other color okay so like that the rainbow gets formed okay so again this is also qualitatively in your syllabus the numerical as such they are not there see geometrically if that is happening okay if that is true it can be like you know whatever satisfied that that will be the shape of the rainbow yeah you will see an arch probably because the droplets over here satisfies that condition droplets are here also it's not that water droplets are not here they are there but that is not happening with them what happens for those droplets light comes out it doesn't come back and you are here you'll only see the light which is coming towards you so again this is just qualitatively in your syllabus there are no numericals as such on this formation just for your information alright so this is one of the natural phenomena the other one is scattering right down what is scattering have you learned about it in class 10 what is it dispersion is something else dispersion is splitting of this thing scattering is another phenomena of light just like see reflection refraction scattering polarization these are the four different kinds of phenomena from the light okay although in your syllabus scattering and polarization is not there in great detail but they are also you know as in they are also a way in which light behaves okay so scattering is some other phenomena other than reflection and refraction this phenomena happens like this the particles will absorb the light okay and then emit the light this is a phenomena now in this phenomena what happens is that when the light is coming towards a particle it will come suppose in a particular direction suppose this is a particle light comes from this direction this particle absorbs this light okay and now it emits which direction it will emit in all directions okay that is what you call scattering it scatters the entire energy in all the directions fine it absorbs the light coming from one direction and then scatters in all the directions okay our atmosphere has nitrogen and oxygen in it okay the scattering happens most for those wavelength which are comparable to the size of the particle okay so nitrogen and oxygen they are comparable to the size of the wavelength of the blue light so in our atmosphere these nitrogen and oxygen they absorb blue light and then they emit the blue light so whatever they absorb that that only they will emit isn't it so they absorb the blue light from the sun and then they scatter away entire blue light so that is why when you look at the sky even though there is no light source above sun is there you're not looking at sun you're looking at somewhere else still you feel that there's a blue light over there okay that is because of scattering the particles have absorbed the blue light and throwing away the blue light fine okay so one thing you should like keep it as a thumb rule that blue light or violet light gets scattered the most fine and red light get scattered the least in our atmosphere fine any doubt on it so when if this is true then you'll see that whenever there's a signboard suppose it is written in the blue fonts and if you look at from very far you'll not be able to make out what is written why because the blue light coming from the signboard the oxygen and nitrogen absorbs them and then throws away in all the direction so you'll not see light coming in some pattern towards you you'll see a blurred this thing but same thing if it is written in red color red light doesn't get scattered easily so you'll be able to see clearly what is written so that is probably another reason why red color is used more and it is also used in when you have to show the danger sign that is why red color is used for the danger sign so you so that you can easily identify from very far it doesn't get scattered okay now can you tell me why sun looks red when it is about to set just think over it exactly so if you draw a diagram like this suppose this is earth and this is the atmosphere okay and suppose this is what the sun the sun throws away light like this towards you so this sunlight has to travel very great distance inside the atmosphere this much from here till here when it is about to set but when it is about it has to travel only this much okay so scattering here is very effective so blue light gets scattered away only red light is remaining red light or its shades fine so you'll feel that the sun is having red color okay is this thing clear okay alright so this is what thing about natural phenomena