Ray Diagrams: 2nd Incidence Ray - Which Is Correct?

[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 isn't 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 I've 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!

 

[cesiumfrog]

the second ray must be drawn from the top of the object

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).

 

[charng]

Hello, thank you for bringing up this concern about ray diagrams. It is important to understand that ray diagrams are simply a visual representation of how light rays behave when they interact with mirrors or lenses. They are not always completely accurate and can vary depending on the specific situation or lens/mirror being used.

In terms of the second incidence ray, it is correct that it should be drawn from the top of the object through the principle axis and reflected parallel to the principle axis. This is because the second ray is used to determine the location and size of the image produced by the mirror or lens.

However, as you mentioned, the location of the object in relation to the principle focus can affect the behavior of the light rays and therefore the position of the image. When the object is placed closer to the principle focus, the image will be virtual and the second ray will appear to pass through the pole or follow the dotted lines through the focus. This is because the light rays are not actually converging at the location of the image, but appear to be coming from that point due to the refraction of the lens or reflection of the mirror.

So to answer your question, both ways of drawing the second ray can be correct depending on the situation. It is important to understand the principles behind ray diagrams and how they can vary in different scenarios. I hope this helps clarify any confusion.