Fiber Optic Effect in Convex Lens?

Hey all. Was fiddling around with my biconvex magnifying glass and my lasers and noticed something... odd.

Here are some pictures....



Now it may look like the reflection of the beam is just re-entering the lens. But here is proof that (at least some of) it never exits....



From what I know about fiber optics, it is a material through which the light passes through that has a "threshold" angle, below which light will be reflected internally.

Now, this was done with my 405nm PHR, but when I tried it with my 200mW 532nm greenie, it didn't work as well, and when I tried it with my LOC, it didn't work at all.

Any explanations?

Thats pretty cool I did the same thing a while back with this lens


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Total internal reflection, TIR, same reason a fiber optic cable works. But it works with any light in any medium where you have a boundary with a medium with a lower refractive index.

Critical angle for TIR = arcsin(n2/n1). n2 is the refractive index of the lower index, outside medium (air in this and many other cases, n2 = 1). n1 is the refractive index of the high index material, the lens in this case.

The critical angle is measured from the normal to the surface. So, for total internal reflection to occur, the incident wave must hit at an angle greater than the critical angle. If the critical angle is 40degrees from normal, then a wave hitting at an angle greater than 40degrees from normal would undergo TIR, and a wave at less than 40degrees from normal would not.

Index of refraction varies with wavelength, this is called dispersion. The lower the wavelength, the higher the index of refraction. So, the index of refraction in the lens for 405nm light is higher than for any other visible laser, which results in a critical angle smaller than any other visible laser. Higher index of refraction in the glass decreases the critical angle (measured from surface normal) and causes a larger acceptance angle for TIR. The critical angle is greater than the angle of incidence for all your other wavelengths (so no TIR), but less than the angle of incidence for the 405nm laser, so only the 405nm laser sees TIR.

Hey, thanks pull! Makes a lot of sense

Lol, just went through this topic in depth in class. Awesome pictures for this guys!

Haha. What a coincidence, eh?

Sorry Had to post these.
After reading this I had to try it. WOW
at some angles, my 12x looks like is going everywhere...lol

God that's so cool!