one thing that waves do all kinds of waves do this light waves do this the same way we're going to see how waves reflect uh what I have here is a wave pulse that's moving to the right right over here what you can't see right there is that this is actually attached to a wall it's a spring attached to a wall and what happens is this this spring this wave is going to reflect from where it's attached to the wall and I want you to watch this and I want you to tell me what's different about the reflected pulse notice it reflects back and we'll see this live version as well but the wave when it hits the wall it is reflected but what's different about the reflected pulse in it's inverted the reflected pulse is inverted in other words it's upside down if you send a Crest and it's fixed in other words attached to the wall right there what you get back in the reflection is a trough okay uh so now what I'm going to show you is this here's the live version of that with a fixed end a Crest which is scent is reflected as a trough the wave is inverted when reflected from a fixed end so now what I'm going to show you is this we're going to have a free end now in this situation totally different situation what we have here is a ring and a post and this ring is free to just travel up and down the post in other words this end is free to move up and down now the first thing I want you to ask yourself is will this wave reflect if the end is not stuck on the wall go ahead and predict that and we'll just see what happens here we go does the wave reflect that's the reflected wave it does reflect okay notice that the wave is reflecting but what's different about this situation uh different than the uh reflecting from a uh fixed end reflection the reflection is not inverted if you send a Crest a Crest is what comes back here's the live version of that that demo a free end is created by just tying a rope to one end of the slinky so that end can move back and forth but you can see in a free-end reflection a Crest is sent and then a Crest is what reflects from a free end there is wave reflection but there is no inversion of the wave a Crest is sent and a Crest is reflected back we are going to to fill in the blanks here a little bit and then we're going to explain why this happens okay so when reflecting off a fixed end like a wall the wave pulse inverts upon reflection that means it Turns Upside Down a Crest turns into a trough now there's an important explanation for this when the wave hits the wall that's where I'm going to stop it what happens is this spring is pulling up on the wall the spring pulls up on the wall what does the wall do to the spring when the spring pulls up on the wall so because of one law if the spring pulls up the wall pulls down when the spring pulls up on the wall the wall pulls down on the spring that is due to Newton's hold up the number which law of Newton hold that up that is Newton's third law that makes good sense spring pulls up on the wall wall pulls down in the spring however when uh when you have a situation where the end is free to move like this notice that this end just keeps moving up it keeps moving up and that pulls the rest of the spring up with it so if you had to blame one of Newton's laws for that if this is moving up and it stays moving up which of Newton's Laws so the wave pulse uh it does not invert in when hitting a free end does not invert it does reflect but it does not invert as the free end goes up it maintains its velocity upward so which of Newton's Laws says well if something can maintain its velocity and there's no forces on it it will which law is that the first law now actually there is a a force on it will slow it down but the main point being if the spring is moving up like every point in the spring is it'll keep going up that way and pull the rest of the spring up with it and that's how that works so we do have reflection regardless of whether it's a fixed end or a free end like it is in this situation okay so this is going to allow us to make the next prediction which is going to be very important