I had the same question as in Molecular chirality and optical rotation

In the answer, it says that there can't be such a mirror position where the effect of one molecule be cancelled by other. But I'm so confused. But don't the first and the third image in the original answer show molecules such that the both in combination could cancel each other's effects?

enter image description here enter image description here

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    $\begingroup$ These two are difficult to compare because in one, we are looking at the discs from the right, and in three, we are looking at the discs from the left. Go back to the answer of the other question and compare the first with the fourth image. $\endgroup$
    – Karsten
    Commented Jan 3 at 16:25
  • $\begingroup$ It's a bit like turning a screw so that the head is on the other side. $\endgroup$
    – Karsten
    Commented Jan 3 at 16:31
  • $\begingroup$ Thank you so much @Karsten. I understand now, the screw analogy really helped. $\endgroup$
    – Leon Raj
    Commented Jan 3 at 17:19

1 Answer 1


Let's just look at the first situation. What you're asking is, in effect, what happens when I run the situation in reverse because for every orientation that looks like the first scenario, there is one that matches the reverse.

But look at the effect down the axis from both sides. In both situations, the plane of polarization has shifted about 45° counterclockwise. So no matter the direction of light in this situation, the sense of rotation is the same, so there is no canceling of the net rotation due to symmetry; the symmetry reinforces the sense of rotation.

This makes perfect sense if you were to look at screw. Regardless of which end you look at it from, for a regular right-handed screw, you always turn to the right (clockwise) to drive it in (away from you).


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