What is Optical Path Length?

In this video, I describe what optical path length is, in terms of how far light "thinks" it has traveled. I give an expression for the optical path length in terms of the refractive index, and a more general expression if the refractive index varies over space.
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This is part of my series on introductory electromagnetism, where we explore one of the fundamental forces of nature - how your phone charges and communicates with the rest of the world, why you should be afraid of the sun, and the fundamentals of electric and magnetic forces and fields, voltages,

[@jayanthnv2577]

Optical path length is the equivalent distance travelled in air..the video makes us think it's the distance travelled in the medium which is not true...if the wave can cover a distance d in a medium refractive index of n, then the equivalent distance covered is n x d in air. The distance travelled increases as the velocity is increased, but the time taken is the same in both media. Keeping in mind the frequency is constant, the wavelength changes, whereas the number of wavelengths in the time t is the same. Ie; here in time t, the no. of waves is 4 and will be the same in the medium, but the wavelength will be halved in the medium..

[@educationtanishq5076]

Thanks

[@fishwishpishpish9165]

Great video

[@anqizhao5769]

hi, just wanna know does it make sense to ask the optical path for a electron wave?

[@y.xiaochun4918]

Very good videos. I watched a lot videos from Dr. Jordan. He is really good at teaching.

[@Farhan-sv1eh]

i love you

[@richardlevy2625]

Wow thank u for this mr youtuber this is soo helpful for my physics 8b final coming up :)

[@pablomarco5118]

thank you very much. I learned a lot from you

[@HimanshiYadav-ds5yr]

Thank you😊

[@sahilpande7943]

Love your way of approaching

[@Rajkumar-v3n5q]

Thank you so much

[@golakbiharisahoo9697]

Thank you

[@shahzcoc3498]

Thanku for these videos

[@tinakwok7600]

thank u😄

[@houmi7798]

Does light in free space travels 4λ uses the same time as travels 8λ in the n=2 material?

[@binayakpanigrahi4082]

No. But light will have same frequency in both air medium and the medium of index 2

[@mohamedirfan2816]

Frequency of light is constant, so in the medium of refractive index 2, it will have half velocity and wavelength

[@simt-vs9sx]

Yes, same time period.
As frequency= reciprocal of time period, and frequency is constant, time period is also constant

[@TheAllimania]

Isn't the way the information is presented here highly misleading? from your sketches one would assume that the wavelength changes depending on the index of refraction, which can clearly not be the case due to conservation of energy and real world experience with windows.

[@blzKrg]

Wavelength does change with refractive index.

[@mohamedirfan2816]

Wavelengths do change but frequency is constant

[@ianj.2651]

Wavelength does change while the light is propagating through the medium. The reason we don't see its color change is bc it goes back to its original wavelength after exiting the medium and before entering our eyes.