 which is there on optical communication. So, now we will understand the ray theory, how the light is going to be transmitted inside the fiber optic here, ok. So, these are the learning outcomes. At end of the session, students will be able to describe the ray theory transmission and its total internal reflection that is known as a TIR. Before starting to the reflection and the refraction, what is that thing? So, just recall that which is the principle which light follows inside the core to remain inside the core only, ok. So, for that we will see in the upcoming lectures, what is that TIR that is total internal reflection here, ok. So, as we know that the light travels fastest in the vacuum. So, its velocity in the vacuum is given by 3 into 10 raised to 8 meters per second here, ok. So, but in whenever the light travels with the denser medium, like other substances of a velocity of light is always less than 3 into 10 raised to 8 meters per second here, ok. So, if you know how to calculate and refractive index here, so that is calculated by the ratio of velocity of light in a vacuum that is the velocity of light in a vacuum that is C to the velocity of light in another medium that is V here, then you will be able to calculate the refractive index here. As we know that the refractive index for air is 1 as water is more denser than this air. So, the refractive index for the water will be 1.33, ok. So, if it is more denser medium, so it will be having the higher refractive index here. So, the light behaves in different manner when in the vacuum as compared to the in the denser medium. So, when the light will get reflected and when light will be get refracted, so that is we are going to see because the light should remain inside the core only while transmitting the data in the optical fiber here, ok. So, we will understand what is a reflection and the reflection in this session. So, you can see this diagram here. So, this is an incident ray here, ok. So, when this incident ray is incident at the theta of I, so when the light passes from one medium, so this is an interface and this is the normal here and this is the medium N1 and N2 here. So, N1 is less than N2. So, N1 is less than N2 here, ok. So, this is the denser medium here, ok. So, when light passes from one medium to another that causes the change of speed here, ok. So, which results in the change of light traveling direction also. So, when light will transmit from the lower denser medium to the higher denser medium, so it causes the change in this angle as well as the speed of the light traveling, ok. So, that deflection of light is called as an refraction here, ok. So, this is a theta of I and this is theta of R that is whenever the incident ray is being reflected back with the same angle that is known as the reflected ray here, ok. But when the lower denser medium light is incident and it passes through the denser medium, so it has been deflection of light that is theta of T, so that is known as an refracted here because so it changes the speed as well as some angle, ok. So, that is why it is known as a refracted ray and a reflected ray here, but we are interested in the refracted ray here, ok. So, we will see how light will remain inside this optical fiber only. So, for that we should know the principle of total internal reflection here, ok. So, now we will see the relationship between the incident ray and the reflection ray that is theta i is equals to theta R here, ok. So, as shown in this figure what I have told, this is the angle will be same, so that is why it is theta i is equals to theta R, this is known as a law of reflection here, ok. Now, there should be some second relation that is the relation between the incident ray and the reflection ray here, ok. So, N1 is your refractive indexes and theta sin theta i is equals to N2 sin theta of T here that is known as a Snail's law here, ok. So, where N1 and N2 are the refractive indexes and theta i is angle of incident and theta T is the angle of a reflection here, ok. So, this is the angle of a reflection here, ok. So, that is the relation is given by the Snail's law here that is N1 sin theta i is equals to N2 sin theta of T here, ok. So, that is known as the Snail's law here, ok. So, now we will understand what is the meaning of total internal reflection that is T i R here, ok. So, we have seen this law of reflection that is theta i is equals to theta R and one more relation is what the Snail's law, so that we have to see now. So, from a Snail's law I shown in this figure A, B and C here, so this is known as the reflection, figure B is known as and the critical angle and figures is that the principle will explain about the total internal reflection here. So, from the Snail's law what we have seen that is N1 sin theta 1 is equals to N2 sin theta 2 here, ok. So, this is the according to the Snail's law here, ok. As the value of theta 1, ok, theta 1 goes on increasing as the value of n goes on increasing that theta 2 also increases. So, when theta 2 becomes 90 degree, ok, so when this theta 2 becomes the 90 degree, this is called as a critical angle here. So, this is called the critical angle. So, this ray becomes parallel to this normal here or perpendicular to the normal here, this ray what is the exit ray is showing in this figure. So, it becomes parallel to this line and it will be the perpendicular to this line here, ok. So, this is called as a critical angle which is theta C is given by that is sin theta C is equals to N2 upon N1. If you calculate this theta C equals to sin inverse of N2 upon N1. So, this critical angle also depends on the refractive indexes of N1 and N2 here. Ok, see now two rays should remain inside this core only. So, we should understand that the what is critical angle because as the theta 1 goes on increasing from the this side angle. So, theta 2 also goes on increasing but when it becomes perpendicular to this normal. So, this is known as a critical angle here. So, the critical angle when it is follow, so if the angle of incident is greater than your critical angle then the light is reflected back to that same medium here. So, that is important. So, whenever you are going to incident the light into the fiber optical cable here, your angle of incident should be greater than your critical angle here. Then the light is reflected back into the same medium here that is known as a total internal reflection as shown in this figure here. That is known as the total internal reflection here that is theta is equals to this theta here. Otherwise what will happen if this angle of incident is smaller than this. So, the light will get in this manner exited into the another medium that is the cladding here. So, we do not want this here. So, we have to incident the angle which is greater than the critical angle. So, that the light is reflected back into that same medium. So, that is known as the total internal reflection as shown in this figure here. So, now just we summarize this video. So, what we have seen the reflection and the refraction, how to calculate the refractive index because the refractive index is most important thing because whenever you are going to calculate the critical angle it depends on the refractive indexes of N1 and N2 here. So, for that reason the refractive index will be the velocity of light in the vacuum upon the velocity of light in the another medium here. So, we have seen the relationship between the reflection what is an refraction here. So, according to the snail's law we are going to implement the critical angle here. So, this angle whenever Theta 1 is going to be increased. So, the angle of incident should be always greater than your critical angle here. So, that the light will remain same inside the medium. So, this is known as a total internal reflection here. So, now these are my references. Thank you.