Performing reflections: line (old)
About this video
Sal uses the reflection tool to reflect a line across the horizontal line y = -4. He then explains why the image of a line under a reflection is still a line: the original has zero endpoints, so the reflected image also has zero endpoints.
You'll be able to
- identify the line y = -4 as a horizontal line.
- explain why reflecting a line gives a line, not a segment or ray.
- tell the difference between points on a line and endpoints.
Key words
- reflection
- A flip of a figure over a line, called the line of reflection.
- line
- A straight path that keeps going forever in both directions, with no endpoints.
- line segment
- Part of a line between two endpoints.
- endpoint
- A point where a segment or ray stops.
Transcript
We're asked to reflect this line across the line y is equal to negative 4. So you have this line right over here, and they actually give us all of these transformation tools right over here where we can Translate, Rotate, Reflect, Dilate, or Undo what we were about to do.
So they want us to reflect this line across y is equal to negative 4. So let's get the Reflection tool out. So I clicked reflect and let's make this line be y equals negative 4. So let's see. That would look something y equals negative 4. So we're going to reflect this line across y equals negative 4.
So let's reflect it, and there you have it. We reflected it across y equals negative 4. And then they ask us to answer some questions. When a line is reflected, the image created is a line. We know this is true because a reflected image, well, the original image had zero endpoints.
These are just two points on the line, they're not endpoints, they're not defining the end of the line. If they were both endpoints it would be a line segment, but this line keeps going in both directions. So we know this is true because a reflected image has zero endpoints, and we got it right.
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