Total internal reflection | Geometric optics | Physics | Khan Academy

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Total internal reflection | Geometric optics | Physics | Khan Academy

433 447 views | 9 Dec. 2010
433 447 views | 9 Dec. 2010

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

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Physics on Khan Academy: Physics is the study of the basic principles that govern the physical world around us. We'll start by looking at motion itself. Then, we'll learn about forces, momentum, energy, and other concepts in lots of different physical situations. To get the most out of physics, you'll need a solid understanding of algebra and a basic understanding of trigonometry.

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austin webb

How do you calculate the things you do??

Marina Li

waaay better than my teacher. I love khanacademy <3 <3 

q q

Thanks a lot. My teacher made the explanation complicated. This sums up what I need perfectly.

EM mobile

you can use free app EMwaveRT to change for 3D visualization of total internal reflection

Dwight Schrute

Is this what causes all reflection? Do mirrors, for example, have such high refractive indexes that they reflect all light?

Hardik Vyas

And what if the incident travels along the normal i.e. i=0 degrees and when i=90 degrees

Mohammed Al-Bashiri

once again, I don't know what would I do without your videos. so much love to you Mr. khan

575 Gaurav Raj

Too good

Shahbaz Mughal

That fiber optic cord example at the end... whoa now I’ll never forget


I need my teacher teach like you,interesting and easy-understanding

Casey Leung

thank you so much,you are the best Physics teacher in the entire world!! by a student from HONG KONG

Senara Fernando

Thank you so much, you really helped me for my test!

ramsey eleryan

what is this app that ur using?

Sarah Holland

Sal, you are a lifesaver! Summer classes have us learning this whole playlist in 3 days and it's going to make up around 25% of the final. I was so stressed but 3 hours of sitting down and watching this playlist actually makes it manageable. Thank you, thank you, thank you!!!

Igna Cio

How do you display the calculator?


This is awesome thanks! Anyone else came here from graphics/game programming?

T. Z

Thank you ,that was really helpful


Tips: when you have got degree from water you can find out how deep is the lamp is (deep = length) ;)

mehh a tell u sym

I have a text Monday in this and I knows nothing expect what u teach me in 5 vid


How am I suppose to do arc sine in my head for the MCAT? Can't use calculators

Hana Ahmeti

You're a life saver Mr. Khan. Now I know I won't fail my physic exam.Lots of love

Lucinda Haasbroek

Thank you Khan Academy... You guys are always life savers !


Where have you been all my life sigh

Daniel Coleman

Sooooooooo helpful, my exam is tomorrow and this is really going to help

Abhi Murali

The best explanation ❤️

Freya Souter

Thank you SO much! This has been immensely useful and makes far more sense than it did before I watched the video! Many thanks

Christabel Oghinan

you lost me at the graphing part, I haven't learnt tht yet lol

Avi Wadhwa

0:49 Whoa! that really makes a lot of sense! thank you <3

Twiggy Gordon

Learned more in 6 minutes than an hour with my college professor... thanks so much.

Noah El Rhandour

0:38 i love that you always have such good examples to explain such phenomenons. this has actually something to do with the wave character of light. the example with the car is perfectly fitting. the light reenters the medium it just left due to its diagonal micro movements and looses speed. reentering the fast medium again, it made a curve.


Thanks for this video, helped explained this better rather in class. Fully understood the concept behind critical and total internal reflection. - grade 10 student

Ade9 372615

1:26 to 2:45 is what i found most useful, thanks mate



Muhammad Muzamil

thank you so much, man. my teacher has such a strong German or Austrian accent and its hard to understand anything thanks so much

Arturo G.

So is it safe to assume that regardless of the media the ray is initially in and the final media it is in, we can use the media with the lower index of refraction in the numerator, and the higher index in the denominator? Because there's been a couple situations where I get undefined errors in my calculator. For example when a ray goes from glass to air and I use the glass in the numerator and the air in the denominator because I'm thinking n1 = glass since it was in glass first.

Taylor Deasley

Thank you so much!! It all makes sense now!

Evangelina Musique

Thank you, Mr. Khan! You are the one who helps me to escape from my boring physics lesson!

Mihir Shah

Thank you Mr.Khan ,your videos are very helpful .

Tim Clouter

Reasoning it out, your explanation from 0:28 onwards was fantastic - it just immediately clicked. Thanks for your help!


ooooooooooooooooooohhhhhhhhhhhhhhhhhhhhhhhhhhhh 1:40 shoot I GET IT now!

Shajeeah Ramjaun

Thxs a million!! Khan Academy videos have been helping me for 2years now! Sal's the best at making complex things simple!!

mayur kalsekar

The car example was brilliant

shubhank jain

can you upload a video on some problems+solutions on TIR or a give me link or something? please i need it 


This was 8 years ago what in the world I was learning the alphabet at that time

Guilty Pleasure

I swear no one teaches like Khan Academy!
This is amazing, I had been trying to understand this whole concept from other videos, but for some reason, all those teachers or speakers assumed that everyone is a genius and automatically knows what they are talking about so instead of explaining, they just carry on saying a bunch of words that I have never had of.

Orkids Panchkula

Images not clear

Zara v

Thx!!! This helps so much I hope I wont fail my physics exam

John Davis

Cloak of invisibility?

Yash patel

What happens when light rays coming from 90°?


Thank you so much

dhai badr

These videos are the only reason that I'm able to pass my physics exam.. THANK YOU SO MUCH!


Man, why cant i have teachers like this? Mine just sits there and talks like i can imagine what the .....*.... he is talking about. Thanks.


Thanks a lot

Chris Guillen

You said that at the critical angle the light won't even travel along the surface, but then you said that it will and bounce back. Idk, I believe the second one

Tirso Acuña

Thanks a lot for this video! It's nice to see you're constantly uploading lots of videos. Many of them are so helpful! Thank you so much for keeping this channel up and going.


bless you sal

Reiys Ho

I found it reasonably interesting! Thanks! :3

Jessica Kaur


Why does total internal reflection only occur when light travels more slowly through the first medium than in the second and not the other way around?

Nahill Sharyf

Really helpful 

Classified level 10

The way u explain is top man

Alina Karkee

Is it ok if we write 1 refractive index of air instead of 1.00?
Thank you

Nikiyta Naude

Thank you SO MUCH!!!! This helped to explain the concept and reasoning of the critical angle.

fred yufeng

I was wondering if there is a way to pay for it coz i want my quality video

zeshan imtiyaz


Tobias Pattison

You have carried me through my GCSE's

Mathilde Amz

thank you so much ! I litteraly study in french and english is not my first language and i’ve learned more from this video than from class

Csr Racer

yes, it's. :)

Laurens H

This was the one part of my optics subject in school i never understood. Thanks for explaining clearly!

Yash vardhan

Man ,you are awesome

Keanu Breen

Are you a wizard?


This really helped a lot, I'm 14, i need to learn this in foreign German though :c this is going to be so hard xD

Venk tesh

can one get sunlight inside a dark room via a window using this TIR ...?

Mohamed Hassan

Who is watching in 2017 ? Like!!

Tate M

You sound like Othello. Thanks for the video.

Cuenta Luismiguel

Thanks! <3



Suhas Bangari

I want to keep it simple. You do great!!

bilal farooqui

what is your name?

Viswanath Hemanth

thank you very much .it helped me a lot for my physics exam .actually i did not understand about the critical angle when my teacher was teaching it but i clearly understood about the critical angle after watching this video.


i love you thankyou so much dear jesus you are amazing

zak khan

what is the middle dotted line. (normal)

Aayam Sharma

i think so


This makes so much more sense!


Thanks a lot..!! It was very helpful


why don’t I study from home no need for teachers lol

Ritika Gera

This is a really great vid, I just have one quick question...
Snell' slaw states that Nr=sini/sinr, but with TIR the critical angle is sini so it becomes Nr=sinr/sini. Do you have any idea why it flips? Thank you!!

Milton Weinhold

i love your voice and teaching skills Sal, thank you so much for taking the time to make these wonderful videos you are awesome <3

The Indian Offense

Thanks you very much for the video I was facing trouble understanding total internal reflection...... Thanks a lot


4:50 u can just calculate it with ur calculator and put the unknown given as an X

sumit kumar

My new PHYSICS SOLVING APP.More then 150+ formulas,Solves for any variable you want,Covers up all now.

Mohamed Ahmed

Thank you very much i have a test tomorrow and i didn't know how to do it


I love this guy's calculator


Cleared things up for me, thanks!

Fiona K

thank you! It only takes me 8 mins to understand what total internal refraction is. I can't even figure out what my teacher is teaching before watching this video.

Neha Sharma

This is awesome!

Praveen kumar

Thank you sir!!!

amitha venepally

Amazing , just amazing

Sakina Hassan

Nice explanation

Oken Keithellakpam

I sub

kLd. madi

Thaaanks , that was really helpful !