What makes a light source safe?

Working in a science comes with an occupational hazard of  head-slapping. It doesn't matter how tolerant, how understanding you are of the fact that not everyone is a scientist and therefore lacks some of the insight that we take for granted. On a semi-regular basis, you find yourself face-palming, banging your head against a wall and generally weeping for the scientific literacy of humanity. Whether it's  a friend from college who has gone all homeopathic, a movie with impossible physics or a local news item that gets you to scream at the television, it's part of the territory.

Which is why I couldn't be all that surprised when, upon complimenting a coworker's manicure, I learned that her mother had gotten a free UV nail lamp because the cosmetology board had decided to replace all their old bulb UV lamps with LED light sources because they were 'safer'. My coworker laughed as she told me this, because we both know that it doesn't matter if your UV light is naturally emitted from unicorn horns fed only the most organic of herbage, it's still UV light and it can still give you cancer.

So, what makes a light source 'safe'? It depends, in part, on what you are using the light source for.

For example, in your house, you want a light bulb that isn't going to set your house on fire, explode, or release toxic gases. You aren't really worried about whether they can give you cancer, because the light they output is in the visible range, and sometimes into the infrared, all of which is non-ionizing. There are three options widely available to average person these days. There is old school incandescent bulbs, halogen bulbs and LED bulbs.

 Incandescent are familiar, and what most people alive today grew up with. They have a filament that glows white hot when you pass a current through them. They do not have any toxic gases and most people seem to think of them as the non-toxic bulb (though the tungsten in the filament is highly toxic, it's sitting there and who is going to lick it?) But it's incredibly energy inefficient. Most of the energy it uses goes to heat (infrared), not visible light. You can burn yourself by touching one that's been on a little while, and they can explode from thermal shock if you accidentally sneeze on one that has been on long enough to get hot (yes, I have done this).

 Halogens are becoming more familiar. They work just like any good old fluorescent bulb, with less flickering, by exciting electrons in a diffuse gas until they give off light, which then excites a coating on the bulb into giving white light. They are more energy efficient since heat is a byproduct and not the means of producing light, but they aren't hugely more efficient and they contain small amounts of mercury. They have to be disposed of properly at hardware stores or recycling centers, and it's not clear what you should do if one breaks.

 LEDs are the newest contenders. They use light emitting diodes to create light, which means they are semiconductor based. Semiconductors aren't the nicest things in the world to make, but they are not toxic if they break. They are very efficient (some of the better ones barely get warm) and very pricey. They have by far the longest lifespan, and are probably the nicest looking.

So for safety, in my house, I am switching over to LED bulbs as each of the other style bulbs give up the ghost. Lower fire risk and no risk of mercury poisoning. This is what a safe bulb in my house means.

But the safety question when it comes to things like tanning beds and nail polish curing is very different. The customer is unlikely to have to deal with broken bulbs and they aren't immediately concerned with the energy efficiency or fire risk. The questionable safety of such devices arises from the specific wavelengths of light used, namely ultraviolet or UV light.

UV light exposure is concerning over long periods because the wavelength of UV is small enough to interact with DNA molecules and energetic enough to damage them. When DNA gets damaged, it leads to mutations, some of which are harmless and others that can be very harmful  indeed.

 Now, we can put this to good use in sterilizers using UV-C, because it doesn't involve chemicals that might be dangerous to us or that bacteria might grow resistant to. The light destroys the  bacteria from the inside, like someone smashing your hard drive and motherboard would effectively destroy your computer. This is a good use of UV light.

UV light can be produced by any number of bulb types, including fluorescent bulbs, lasers of various types and LEDs. Older models of nail lamps used fluorescent bulbs, which are relatively cheap and give even light coverage.

So does switching over to a different type of bulb make it any safer for people who want to use these lamps? Nope. So long as they are using the same polymers that require the same wavelength of UV light to cure, the LED bulbs will be giving off the same  UV radiation as the old bulbs.

How safe are any of these nail lamps? Depends on who you ask. It is difficult to predict cancer risks in any population. This letter to the Journal of the American Academy of Dermatology suggests that, at least for the two national brand models they tested, in three minutes your hands are getting the equivalent of 4-6 hours of allowable UV exposure for construction workers. Each lamp puts out over 4 times the amount of UV energy than the sun. So while using them on occasion won't bring any more risk than staying outside in the sun all day, you may not want to use them on a regular basis.

Me, I'd rather get my skin cancer risk from taking a walk on a nice day.

Final Exam Grading Marathons

The past five days have been consumed in end of the semester final exam madness. I am kinda past the stage of having final exams of my own to take (thank heaven), and I'm not yet to the stage where I am writing the exam and interesting posts like this on the thoughts of the exam writer/proctor. No, I'm stuck in the stage where I get to help proctor, and grade, and keep the other TAs on track.

Every semester, we grade a couple thousand exams. There are 4 common finals for 4 physics classes (intro to mechanics with and without calculus, intro to electromagnetism with and without calculus) and  enrollment ranges from a couple hundred to a thousand plus. The latter number is usually full of people in online sections, which poses a special problem in that there are no TAs assigned to those sections.

In the past, since the exams are all taken on a Saturday, we graded them all on Sunday (the next day). When I first started grad school two and a half years ago, this schedule was brutal, but doable. There were closer to 2000 exams, and 10 TAs could grade properly and enter the grades in 12-14 hours.

Problem is, the further the professors get from grading their finals themselves, the longer they make them. We once had to grade an exam with 18 problems, the lowest six of which got dropped, but that still meant they all had to be graded. Combine this with the (kinda understandable) push to include more online sections and you have a perfect storm of grading problems. The number of exams have doubled in number for some of the class but the number of TAs has remained the same or reduced. Grading them in one day was  no longer an option. When you have about 5 seconds to grade each problem, you cannot do more than a pass/fail analysis, which isn't fair to anyone, so that had to go.

So this year we spread it out over 4 days, and gave ourselves over 24 hours to get it done (it took 30 hours all told).

For consistency and an ability to give partial credit appropriately, the multi-day method wins hands down. But it also takes more time away from the TAs own exams (I'm an oddity. Most TAs are newer grad students and so still have exams), and it drags out the stress of grading from one intense day to 4 intense days.

Most of us have a love/hate relationship to final exam grading. It's a time suck, it's exhausting and it can be kinda depressing.  On the other hand, it generates a kinda of camaraderie among the TAs, that we have done this mountain of work together, that we have passed through all the emotional stages of grading together and when its done we celebrate.

 You start out vaguely hopeful--yes, there are a lot of exams, but come on guys, we can do this!

Then you hit a grinding stage where you start expostulating over mistakes--this person can't multiply 2 numbers, they can't do a basic line integral, they said it equals zero and then say it equal pi with no explanation--and celebrating correct answers--this person got everything right! They got everything but the unit! Our lives were not a total waste this semester.

Then you hit the pessimist stage--this will never be done, no one is getting this problem right, why can't you add two numbers together, did I teach you nothing this semester RARRR!

 Eventually, somewhere are around the 10 hour mark you hit the giddy stage. You start giggling at everything. Someone starts making random noises. You start laughing at the absurd mistakes people made, not maliciously but as a way to keep from crying. You hold up the particularly egregious ones and ask someone, anyone, to provide an explanation for what this person was thinking so you can award some partial points. The ones that are just full of 'brain barf' provide at least 3 minutes of laughter and commentary and searching for some relevance. The problem asks them to solve for the time to discharge a resistor-capacitor circuit half way, but they have labeled capacitors as resistors and the resistors as inductors, they seem to have thrown every equation they ever learned ever on the page, and ended up trying to solve it using some mismash of Gauss's law and rotational motion and give you an answer of 7 million Newton Joules per Amp radians. It makes no sense at all, not a single thing on the page is right, but there is just so much effort given.  A quarter point out of 10 because somewhere among the mess there is a vaguely-relevant-if-you-squint-hard-enough equation or unit.

Somewhere around hour 15 or the 1000th exam, whatever comes first, you move into exhausted stage. This needs to be done. There is still another box of exams, but they have to be graded by midnight. People who have completed their grading pitch in to tally and sort the exams as they are finished, while someone else enters the grades into the spreadsheet  just so everyone can leave sooner. Your eyes start having trouble focusing at any distance other than 2 feet, and you aren't sure if you stand up your legs will work anymore, because you haven't moved significantly since you grabbed some food some vague number of hours ago.   You are chugging energy drinks, coffee, spicy candy, anything to kick your brain into gear for another hour.

And then its done. They are all graded, even the ones that got stuck in the bottom of the box. They have been sorted according to the professor's wishes, alphabetized and entered. They are back in their appropriate boxes and safely stored for dispersal at an hour when normal human beings conduct their business. High, low and average scores are announced and congratulated and fretted over. You walk outside to breath fresh air for the first time in more hours than you would like to admit, and then you go home and sleep, and wait for the flood of student emails in 2 days.

Ah, the life of a TA.

The Versatility of the Pound Cake

Desserts are a bit of an odd thing in our house. Dear Husband and I love them, but sporadically. We have gone weeks without eating a proper dessert. A chocolate square here and there, but nothing that could be called dessert. Partly it's because there's only two people who "don't eat very much"* so if I make a whole cake, we either get sick of it or have to throw half of it away. So I've started trying three different strategies: 1) ice cream 2) things that can be easily doled out, such as cookies 3) 1-2 person desserts.

In the last category, pound cake of all things is a shocking easy and useful base. A pound cake is defined not by a particular recipe, but particular ratio of ingredients. An equal amount by weight of eggs, butter, sugar, flour and a dash of flavoring. Which practically means any multiple of 2 oz (the weight of an egg) can be made into a cake. A two-ounce cake turns out to be the perfect amount of batter to make 2 small cakes  or a largish 1 serving cake.

And it is versatile. Besides the basic cake, which is itself delicious, you can use the batter to make caramel cakes, peach caramel cakes, upside down cakes, marble cakes, short cakes, mini trifles, pretty much any cake-based dessert. So long as you have softened butter, you can have cake in about 30 minutes.

You need more? Double it, triple it. More than a pound per ingredient, you might want to just make two batches.

Delicious versatility.



*Heard from every relative and friend who has seen us eat not at Thanksgiving