Prisms, Refraction, and Dummies

Prism separating light colors

Many greedy self-appointed freelance writers flocked to eHow in the early days, driven by a desire for quick cash without much work. The worst of them gleefully reworded or otherwise mangled primary sources as they "answered" – we use that word loosely – a variety of questions harvested from the internet. Some claimed stronger qualifications than others, so it's most disheartening when we happen upon utter bullshit published on the site by someone with training in the field (it has happened, though). Today's DotD candidate is just that: J. T. Barett (aka John Papiewski?) says he has a B. S. in physics on his eHow.com profile, but there was precious little evidence of such training in the content he posted to Sciencing.com called "How do Prisms Work?"

Barett started off with an off-topic statement:

"When light passes from the air into glass, it slows down, and when it leaves the glass, it speeds up again. "

Besides the questionable punctuation,  what J. T. really wanted to say was that, "When light passes from air into glass at an angle, its path bends; and when it leaves the glass, it bends again" – but he didn't. Dumbass.

Barett never actually explained how prisms work. Oh, he gave a brief shout-out to Snell's Law, which he described in this rather dismissive manner:

"An optical principle called Snell's Law predicts exactly how this happens. Snell's Law deals with the angles that light enters and leaves a prism, and something called the index of refraction. The index of refraction shows how much light slows down when it goes into the glass." Now that is a good beginning for the correct answer, but he dropped it there, Instead of explaining what what an index of refraction is and how it makes prisms work, Barett then wandered off to discuss tangential topics. Oh, sure, he briefly discussed the rainbow effect of a prism in a paragraph he called "Color Changes": "The different colors of light, from red to violet, each get bent at slightly different angles. Red gets bent the least, violet the most. This causes the colors to fan out and become distinct"...

...but said nothing about WHY this happens (or, for that matter, why prisms and rainbows split up the colors the same way). And as far as we're concerned the real answer to "How Do Prisms Work?" is buried within that "optical principle called Snell's Law" Barett glossed over. You see, the index of refraction of any medium is the velocity of light in vacuum (that familiar ~186,000 miles per second) divided by the speed of light in the medium. The second thing J. T. neglected to mention is that different colors of light travel through a medium -- in this case, the glass of a prism -- at different speeds (red is incrementally faster than blue, for instance). Thus, a prism has slightly different indices of refraction depending on the color of the light falling on it. Snell's Law predicts that the different colors will separate as they cross the boundaries between air and the prism, then the prism and air -- just as we've all seen happen.

Barett didn't say that. No, instead he blathered about experiments and reflection; giving at best about a third of an answer. We figure that anyone who claims to have a degree in physics, no matter how old, should be able to put together the entire answer to that question. After all, it's often covered in middle-school science classes! But J. T. couldn't do it -- and that's why he's the Antisocial Network's Dumbass of the Day, scientific illiteracy division.

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