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I have just watched this episode. MacGyver used a rosary to focus light into a tight beam and to burn through a rope (made of roots). This wouldn't work with such a rosary, that would be hard to achieve with a magnifying glass but not with a rosary made of many small polygonal glass beads.
Also, there is just one focused beam coming out of so many glass beads?
And the rope burned through just right when the enemies came...

I always had a problem with the glass bead laser.

yes it is VERY far fetched, he did a similar kind of thing in Ugly Duckling with the jeep headlights, I had equally as much trouble believing that as well.

It doesn't help that you can actually see the beam either.

Despite the nit-picks, I loved this episode. It was heartbreaking for me at the end when Deborah didn't choose Mac, though obviously she had to not, for among other things, the story to go on.

Very convenient that only one bead reflected light, extremely far fetched and not likely to happen if you tried it in real life.