![]() That's basically what we'reĭoing over here as well. Left side is negative, upwards is positive,ĭownwards is negative. And we choose the right side usually as positive, And the reason behind it is because that's what we do It's called as theĬartesian Sign Convention. And by the way, this rule is given a name, this particular convention Any height above the principal axis is chosen as positive. Then you start from the pole and you go in the incident direction that's chosen as positive. And look at the image, again the height isĪbove the principal axis. Is above the principal axis it will be a positive height, And whatever is the height of this image, that will be a negative height. I don't know, maybe say the height of this object was five centimeters, then that would be a positive height. And if there's any heightīelow the principal axis we'll call it as a negative height. Alright? So if there's any height which is above the principal axis, we'll call it as a positive height. We're gonna follow with here is you're gonna chooseĪbove principal axis or (muffled speaking) Sometimes when you considerĪbove the principal axis and sometimes the height will be below the principal axis. And similarly if you look at the image it's on the positive side. The focus is on the positive side, so this focal length is positive. The focal length color you'll notice is on the positive side. And so the object is on the negative side, so it'll get a minus sign. And all the measurements to the right side now will be negative. Does that make sense? Because you are choosing, because our incident ray is to the left. Therefore, you start from the pole and all the measurements to the left are considered as positive this time. Over here this is the reflecting side, and our object is over here. Remember, incident direction is always chosen as positive. So just pause the video and apply signs to these three distances. You could pause the video, and see if you can try this yourself. ![]() The image lies also on the negative side. Therefore, this objectĭistance would be negative. These numbers may not beĪccurate, I don't know. And let's say, once we draw ray diagrams we figure out that the image is formed at five centimeters from the pole. There's an object kept at 3.5 centimeters from the pole. So to take an example, imagine we are given the focal length of this mirror as two centimeters, everything is in centimeters let's say. And any measurement which you'll do to the left side of the pole will be negative measurement. In this particular example will be positive measurement. So any measurement which is towards the right side of the pole, And what do we mean by right side? The right side of the pole, because all measurements are done from the pole. And so since incidentĭirection is positive. Incident ray, if you we're to draw any would always be to the right. ![]() Keep an object over here and notice where the The reflecting side over here, the incident light willĪlways be to the right side. So what do we mean by this? Well if we come back over here. Incident direction isĪlways chosen as positive. The rule is we alwaysĬhose incident direction as positive. So the second rule is to figure out which distance then So to differentiateīetween the two distances. And you may have seen that sometimes it can be formed behind the mirror. Sometimes the images will be formed in front of the mirror. In front of the mirror or it can be measured behind the mirror. Now rule number two is, when you measure theseĭistances, these distances can be measured either All the three distances are measured always from the pole. One is the focal length, how far the focus is. Now when I say all distances, honestly there are only three distances. So what are the rules now? Rule number one, is that all the distances are measured from the pole. Let's say we have aĬoncave mirror with us, and by the way this isĬalled a concave mirror because the reflecting So let's start with the rules, and to understand these And, by the way if youĪre not studying lenses or you haven't studied them yet don't worry about it. Then the same rules are gonna be applied even for lenses as well. If you are watching this video while studying about lenses. Although in this video we are only going to talk about mirrors. And then, hopefully, you'll kind of, sort of make sense whyĪre we talking about it and where it's going to be useful. Here all of a sudden? And where are we going to use them? So what we'll do is, we'll first look at what the rules are. As in what measurementsĪre we talking about? Why are we applying signs Now you may be having a lot of questions in your mind. Positive and negative signs to measurements. Basically we're gonna talk about the rules that we're going to apply
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