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## Physics library

### Course: Physics library>Unit 15

Lesson 1: Reflection and refraction

# Total internal reflection

Critical incident angle and total internal reflection. Created by Sal Khan.

## Want to join the conversation?

• By Fast and Slower medium he means Rarer And Denser Medium , Right?
• Yes, that's right. It's going from denser to rarer.
(1 vote)
• In this video total internal refraction is shown through light going from slower medium to faster medium. Does same phenomenon occurs when light travels from faster medium to slower medium ?
• The critical angle is defined as the inverse sine of N2/N1, where N1 and N2 are the index of refraction (which is essentially a ratio of how fast light will travel through that substance). Therefore, in your example, the ratio of N2 to N1 will always be greater than 1, and the sine function is only defined between -1 and 1, so that would be an undefined value of sine, which means that no, it is not possible to have total internal reflection when going from a faster medium to a slower medium.
• So what are the conditions necessary for total internal reflection?
• First The ray should enter from high refractive index to low refractive medium
Second The angle of incident ray should make an angle grater than Critical angle
• How can fiber optic cables be bent when placed in the ground without light escaping them through refraction? Is there a limit to the degree at which they can be bent in order for total internal reflection to occur, or is there some other special property that prevents the escape of light from fiber optic cables?
• Fiber optic cable manufacturers specify a minimum bend radius that should be adhered to during installation. It's typically about 10 times the outer diameter--so something like 30-40mm for a typical 3mm fiber, which isn't too difficult to maintain in a proper installation. In less-than-proper installations you'll get attenuation, though in practice things often still work because there's enough power budget between the transmitter and receiver that the attenuated signal is still usable.

I'm not aware of fiber using a secondary reflective coating. Sometimes there's a metallic layer, but it's there purely to add strength in certain applications.
• What is a critical angle?
• the critical angle is defined as the angle of incidence that provides an angle of refraction of 90-degrees.
• I did not quite get the definition. What exactly is total internal reflection?
• Let's consider a light ray travelling from air to glass. Now we know that a light ray bends towards the normal when passing into an optically denser medium so the light ray will bends you can see in this photo. Now imagine an angle at which the light ray on getting refracted is parallel to the normal. This is called the critical angle. Now even an angle of 0.0001 degrees more will cause the light ray to refract back into its own medium. This can be called reflection. And we call it total internal reflection.
• Its pretty interesting to think that such a simple principle is responsible for long range transmission of information through fibre optic cables..... However my question is that is it possible for the material constituting the cladding fibre to lower the efficiency of transmission?
• No, if total internal reflection really occurs at every part i.e. if the angle of incidence is large enough, it should have nothing to do with refractive index or the nature of the cladding material. However, irregularities in the boundary between the core and the cladding fibre results in loss of intensity (attenuation).
• sal said that refraction angle is bigger then incidence angle, is it only in the case of slow to fast medium or always?
• I am super late answering this but for others who might be wondering the same thing, when light goes from a denser (slower) medium to a less dense (faster) one, light bends away from from the normal, thereby making the angle of refraction larger. In case light goes form a less dense to a denser medium, light would bend towards the normal, making the angle of refraction smaller.