Fundamentals of geometrical optics
Focus, focal distance, focal plane
Figure 26
[zoom...]
![[zoom...]](javascript:window.open(%22../res/fig24_1.jpg%22,%22_blank%22,%22width=%22+Math.min(800,screen.availWidth)+%22,height=%22+Math.min(600,screen.availWidth)+%22,left=%22+(screen.availWidth-800)/2+%22,top=%22+(screen.availHeight-600)/2+%22,scrollbars=yes,resizable=yes%22)?void(0):void(0)){kind=link}
If the object is to infinity (1/z = 0), according to (17), the image, in F', is such that z' = f ' = R/2.
Similarly for the object focus F, we have 1/z' = 0 and z = f = R/2.
F and F' are confounded and are at the centre of segment SC.
Figure 27
[zoom...]
![[zoom...]](javascript:window.open(%22../res/fig25_1.jpg%22,%22_blank%22,%22width=%22+Math.min(800,screen.availWidth)+%22,height=%22+Math.min(600,screen.availWidth)+%22,left=%22+(screen.availWidth-800)/2+%22,top=%22+(screen.availHeight-600)/2+%22,scrollbars=yes,resizable=yes%22)?void(0):void(0)){kind=link}
The focal plane contains the images of the points to infinity.
An object to infinity is caracterised by a beam of parallel rays forming an angle θ with the optical axis. Following figure 27, the ray l going through C reflects to itself, ray 2 going through F reflects itself in parallel to the axis. Ray 3 going through S reflects itself symetrically in relation to the axis.
The dimension y' of the image is : y' = -θ.R/2