 Okay, so the wavelength of a xenon-flash tube is 4.1 times 10 to the negative 7 meters. What is the frequency of this light source? So, well it gives you the wavelength. The wavelength is 4.1 meters. And well, normally you'll be given the value of the speed of light. So that's the equals 3.0 times 10 to the 8 meters per second. Probably you'll have to remember the equation for this, right? So the frequency wavelength equation. Does anybody know that? What is it? Yeah, so c equals frequency times wavelength. Okay, so what are we looking for? We're looking for frequency, right? So we just want to isolate that variable. So, like that. So the way I like to do things is just to cancel units out. Okay, so I would do this like c times 1 divided by a wavelength, okay? So if you do that, you get 3.0 times 10 to the 8 meters per one second, okay? And then you multiply that by 1 over your wavelength, 4.1 times 10 to the negative 7 meters. Okay, so hopefully you can see that cancels out meters like that, right? And that gives you per seconds for your units, okay? So everybody hopefully is cool with the value of per seconds as being hertz, right? So it's kind of hertz is kind of a derived unit. So if you want to think about it this way, 1 per second equals 1 hertz, like that. So all you've got to do now is plug and chug. So hopefully I brought my calculator with me. Now, again, we're doing this in hertz. We've already got it there. So how many sig figs is this going to go to? Just 2, right? Okay, you don't use sig figs on constant, okay? You want to usually just use it for the values that you've measured in the experiment, okay? So in this case there's only 2, okay? So we're going to put this one to 2. So correct me if I'm wrong, but I got 7.3 times 10 to the 14. And this, if you want to think about it, is per one second, right? Put this in the same thing as second to the minus 1 or 7.3 times 10 to the 14 hertz. And hertz is the frequency unit, so we're cool. Does anybody have any questions about that one? Okay, so pretty straightforward on that one. You guys did a good job.