Other mistake: Take a look at the size of the rock that the boys push onto Piggy. A rock of that size, falling from that height would've exploded Piggy's head like a ripe melon. It also would've crushed his body into a mangled heap.
Other mistake: After there has been the vote for chief, behind one boy you can see a piece of wood reading "Anthony M," a marker for showing where the actor (Anthony McCall-Judson) should sit. (00:13:25)
Answer: The key factor there is the focus of the light over distance. The light coming through the glass is refracted and focused on a single point. But it's bent like a ribbon. There is a "sweet spot" so to say where you have to hold the magnifying glass or lens at just the right distance and angle from the object to focus the center point of the light on it. Typically, this means holding the glass out a good several inches or even a foot or so away from what you wish to ignite to get the focal point of the light on it. Someone wearing glasses has them pretty much right up to their face. And so the light can't reach a focal point. Also keep in mind that for focusing the light through a lens, it needs to be angled just right for the light to go through it at the optimal angle and focus. Usually this means facing the sun directly. Typically people don't look up directly at the sun, at least not for more than a second. Especially with glasses on.
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