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Anderson Video - Diverging Lens

Professor Anderson
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 Negative lenses or diverging lenses -- we drew a few pictures of those -- but how do those form images? So, here's our optic axis. A negative lens looks like this: Okay? It is a biconcave lens. And now, let's see if we can figure out where this object will form an image. And now, the reason that you call it a negative lens is because the focal length is in fact negative. It means you still have two focal points but they're flipped. All right? So, rule number one still applies. Light coming in parallel is going to go through the focus. But, it doesn't go through this focus. It goes through that focus. Okay. But, it doesn't bounce off this thing. It's not a mirror. It refracts, and it refracts out at an angle that looks like it was coming from the focus. Okay? This is why you call it a diverging lens because it diverges those rays away from the optic axis. That is ray number one. I'm not going to draw ray number two, but ray number two would go through this focus. It gets a little complicated to see it all. We're just gonna go straight to ray number three because ray number three always applies. Rays through the center do not bend. All right. Here comes my ray through the center. That is ray number three. Where those two rays meet is where the image is located. And, it looks like they meet right here Okay? This is our image. Now, it's made up of one real ray but one virtual ray, which means it's a virtual image. It is clearly pointing upwards, so it is upright. And, it is clearly smaller than the object, so it is de-magnified. Here's the special rule about negative lenses, diverging lenses they only form virtual images. You can never form a real image with a negative lens. And so, if you're stuck on a desert island and you have a companion that has glasses, you had better hope that those glasses are positive lenses because you can use those to start a fire, not negative lenses. If they're negative lenses on their eyeglasses, they're not going to help you start a fire at all.