# Ray diagrams query

1. Jan 25, 2010

### luysion

Hello.
I have a concern in relation to ray diagrams. So I read that the first incidence ray I draw must be parallel to the principle axis and be reflected such that it passes through the principle axis.
Ok so this part isnt too much of a worry.
However it states that the second ray must be drawn from the top of the object through the principle axis and reflected parallel to the principle axis.
I know that this is only possible when the object is placed infront of the principle focus (i.e. further away from the pole)
I know that when the object is closer to the pole then the principle focus that the image is virtual. In this case Ive seen the 2nd incidence ray drawn to be either going through the pole OR being drawn such that it follows the dotted (virtual lines) that are drawn through the focus.

which is correct? or are they situational.
Cheers!

2. Jan 26, 2010

### cesiumfrog

Obviously there are no laws about the order of rays. Ray diagrams are frequently poorly understood though.

The ray along the principle axis is trivial, that's a good place to start.

The lens (by definition) will transform between (parallel) paraxial rays and rays radiating from the (appropriate) focal point, in both directions. It is convenient to use this fact to draw different second and third rays, both originating from the same off-axis (top) point on the object.

For a given object and lens/mirror (by the thin lens approximation) every ray from any point in the object (plane) will be redirected to (at least virtually) pass through the corresponding (i.e., merely scaled or inverted but not otherwise repositioned) point in the image plane. This fact allows you to identify the location and properties of the image (from the previously mentioned three lines), and makes it easy to draw in any further rays (which is useful if your diagram will contain multiple optics).