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.

Everyday Optics: Cosmetic Mirror

Last week, while helping a friend study for the qualifying exam, I posed him this question--explain how a cosmetic mirror works.

For those of you who have never used one, a cosmetic mirror is a mirror that creates a magnified image. Usually they are small and hand held so you can use them to apply things like eye liner and see what you are doing.

I am embarrassed to say, while we had the right instincts in this matter, it took us a day to figure out how to do the ray tracing to prove we were right, so I figured I'd make a blog post out of it.

To start with, let's examine the three types of basic mirrors. There is the flat mirror, which is the kind that hangs over your bathroom sink and is the kind of mirror pretty much everyone is familiar with. It can not magnify, either positively (make it bigger) or negatively (make it smaller). So that one's out.

Don't you love my white board illustrations?

There is the convex mirror, which is bowed outward and is the kind you see in gas stations as a security measure. They create smaller, distorted images of whatever is in front of it. So that's out.

No? Too bad.

Lastly, there is the concave mirror, which bows inward. This is the  most complicated mirror, because what it does depends on what region you are in, as shown below.

I do need new markers though....

So this is the kind of mirror we need, and we know we need to be inside the focal point for this to work. That's fine, because you are usually holding this close to your face anyway. However, the image that it creates is imaginary, and that's the part that was tripping us up while we were drawing the ray diagram.

As you can see, to demonstrate the effect we know occurs, we need to trace partially real rays, and partially imaginary rays. The imaginary rays are what we perceive happens, the virtual image that is created 'in' the mirror.

Diagram a la Hecht

So there you have it. How a cosmetic mirror works. Incidentally, this also applies to the image created  in bowl of spoon. See if you can find the focal point!

~AMPH

Everyday Optics: Rearview Mirrors

I have been helping some friends study for the qualifying exam lately, and part of that has been coming up with problems for them to ponder and answer. As I've done this, I've realized just how much optical phenomena surround us everyday, and just how much of it can be treated in terms of simple geometrical optics. (Why I didn't discover this when I myself was studying is anyone's guess).

Take, for example, your rearview mirror. If you drive, you know this mirror is a good friend. And if you do a lot of night driving, you know that moving that little lever on the bottom forward means you don't have to be blinded by the headlights of the guy behind you. But you probably have not thought about why that works. You are just thankful it does when the idiot behind you has his brights on. 

The way that this works is simple, cool and demonstrates the usefulness of basic optics.

First, let's look at the case of the normal, daytime mirror. In this situation, it works like any other mirror. You have a piece of silvered glass (glass with a highly reflective material on one side) that is angled so that it directed light from objects directly behind the driver's right shoulder into the driver's eyes. 

Yes, I do illustrations on my whiteboard.

From an optical standpoint, there are two reflective surfaces or interfaces. Reflections occur wherever there is an index mismatch, and the stronger the mismatch, the stronger the reflection. How much reflection occurs can be found using the Fresnel Equations.  In the case of a rearview mirror, we have a air/glass and a glass/reflective coating interface. One other thing to note is that rearview mirrors are not like your bathroom mirror, which is made of planar glass. Rather, rearview mirrors are prismatic, which is to say if you cut one in half from top to bottom, you would notice that the glass is ever so slightly trapezoidal, like this:

This allows you to choose which reflection you want to use--the silvered surface reflection for daytime driving, where everything is the same brightness, thanks to sunlight.

Or the first glass surface at night, where you just want enough light to know someone is behind you, because you aren't going to get any kind of detail from the reflected image anyway. Notice that the light still is reflecting off the silvered surface, but now it is being reflected at the ceiling. In fact, if you accidentally leave it in the night position during the day, you'll notice a very faint reflection of what's behind you, and a much stronger reflection of your car ceiling.

Behold! PhysicsGal in her minivan, parked safely in her garage.

Alright, I admit it. Geometrical optics is kinda cool and useful. Only took me...6 years to figure that out? I think I'm ashamed of myself.