6. principal axis
- line that passes straight through the middle of the lens
- point where all rays meet
- is distance from focus to lense
7. focal point
- point where all rays meet
- line that passes straight through the middle of the lens
- is distance from focus to lense
8. focal length
- line that passes straight through the middle of the lens
- is distance from focus to lense
- point where all rays meet
