However, the total internal reflection can partly break down if we apply something to the side of the glass tube, like touching it with a finger. The laser will strike through the glass cylinder and onto whatever is touching it, this is known as frustrated total internal reflection. What's supposedly happening in this product is the razor blade has a fiber with a laser being shone through it, when the fiber comes into the contact with a hair the laser exits the fiber and the thermal energy burns the hair.
The problems here are pretty severe.
• If such a fiber did exist it would give you severe burns on contact with your skin (a very close shave indeed). It would be like trying to shave with a red hot block of steel held up to your face.
• Ever burnt some hair? It smells ghastly. I burned my beard once and the nauseating smell didn't go away for hours. Trying to convince consumers to go on a date while smelling like burning keratin would near impossible.
• The amount of laser power going through the fiber would have the potential to remove your eyesight if you broke it. If you wanted to use this you would need to be wearing eye protection (sealed goggles), skin protection like welding gear, have signage and locks on your bathroom to prevent anybody unprotected from entering.
• The amount of energy required for this sort of effect would be colossal, lasers are inefficient and need a lot of cooling (high power ones are often water cooled). If this existed just the hand piece would be something like a petrol pump nozzle leading to a massive cooling system and power supply.
• Any slight impurity in the fiber would cause it to instantly melt when the laser was turned on. Unless something is perfectly optically clear there is some loss to heat as light passes through it. Glass fibers made for telecommunications are incredibly clear, but they still have enough loss that repeaters are needed on long runs due to losses.
[0]: https://en.wikipedia.org/wiki/Total_internal_reflection
