 Okay, let's do this problem. Are we ready? Yes. Okay, let's do it. This one says the retina of human eye can detect light when radiant energy incident on it is at least 4.00 x 10 to the negative 17th joules. For light of 600 nanometer wavelength, how many photons does this correspond to? Okay, so I've written up there the relevant information that you're going to need or that the problem gives you. But how do we relate these things? Well, one thing you may remember is that there's an energy equation. So, do you remember what that one is? No. Okay, so it's E equals NH frequency, NH speed. You remember that one now? Yeah. Okay, wonderful. Okay, and then do you remember the speed of light? Constant. Well, the constant, yeah, but the equation for converting wavelengths to frequency, because you're going to need that too. So it's wavelength times frequency equals the speed of light. Okay, so you need to know that. So in order to do these things, you're going to have to know the speed of light constant, which you alluded to. But you don't have to memorize that. Okay, so I'll give that to you. But also H here, that's Planck's constant. Okay, you have to be given that too. So 6.636 times 10 to the negative 34th joule seconds. Okay? So if you don't remember those, don't worry, because they'll be given to you. Okay, you just need to know where to plug them in. So frequency is not here, okay? But we have a way to equate it to something that has wavelength in it, which we have over here. Right? Yes. Okay, so let's rearrange this equation. So frequency equals speed of light divided by wavelength, like that. Is that all right? So now we can plug that in for this. So we're going to have NHC over lambda, like that. Is that okay? Yes. Okay. So now let's just write this equation out. So E equals NHC over lambda, like that. But one thing we want to realize is that this is in meters, right? And this is in nanometers. Okay, so we're going to have to convert one to the other. I'll convert this to meters. Okay? So when we do that, and I multiply this, well, down here it's going to be 1 times 10 to the ninth, of course, nanometers per one meter. That's going to cancel and cancel, like that. So when we do that, we should get 6.00 times 10 to the negative 7 meters. So now let's rearrange this equation, because what are we looking for? We're looking for N here. So we've got to rearrange this equation to be solving for N. So let's do that. N equals E lambda divided by HC, like that. Okay? So N is the number of photons. So E, where do we get that from? Well, that's given to us in the promo, 4.00 times 10 to the negative 17 joules. Lambda, that's given to us. But we've converted it to meters here, right? 6.00 times 10 to the negative 7 meters. All divided by points constant, 6.636 times 10 to the negative 34 joules seconds, like that. And then multiply that by the speed of light constant, 3.00 times 10 to the eighth meters divided by one second. Okay? So are we cool with that? Okay, so joules, cancels with joules, seconds with seconds, and meters with meters. So we're just left with the number of photons, okay? So whatever number we get, that's going to be the number of photons, okay? Do you have a question? I was going to ask you to have the units, but you screwed up. It's just the number of photons, okay? So it's just number. That's what it's looking for. So 4.00 seconds. And I get, for my answer, 121 photons. So that's the three significant figures, because this and that are both the three significant figures. Any questions on that one? No. That's a pretty good problem. It kind of puts a couple of different equations together. Can we kill it? Yes.