left and right focal points.

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In Optics.js, we defined

    // left focal point (F)
    this.leftFocalPointProperty = new DerivedProperty(
      [ this.positionProperty, this.focalLengthProperty ],
      ( position, focalLength ) => position.plusXY( -Math.abs( focalLength ), 0 ), {
        units: 'cm',
        tandem: options.tandem.createTandem( 'leftFocalPointProperty' ),
        phetioValueType: Vector2.Vector2IO,
        phetioDocumentation: 'focal point F, at a distance f to the left of the optic'
      } );

    // right focal point (F)
    this.rightFocalPointProperty = new DerivedProperty(
      [ this.positionProperty, this.focalLengthProperty ],
      ( position, focalLength ) => position.plusXY( Math.abs( focalLength ), 0 ), {
        units: 'cm',
        tandem: options.tandem.createTandem( 'rightFocalPointProperty' ),
        phetioValueType: Vector2.Vector2IO,
        phetioDocumentation: 'focal point F, at a distance f to the right of the optic'
      } );

In phet brand, one does not know about the implementation details and we can use the above information to determine the position of the two focal points.

However, from the point of view of an instructional designer, there is no sense in tracking the left or right focal point. Instead, one would like to track the principal (a.k.a. primary) focal point or the secondary focal point.

The principal focal point is the point on the axis of a lens or mirror to which parallel rays of light converge or from which they appear to diverge after refraction or reflection.

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The same comment can be applied, although to a lesser extent, to the left 2F and right 2F points. The 2F points are really only relevant for converging lenses and converging mirrors, so it is not of paramount importance.

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Discussed with @arouinfar on 4/18.

In summary, we will not change the current behavior for the following reasons. The leftFocalPointProperty and rightFocalPointProperty are instrumented for convenience but are unlikely to be leveraged. In fact it may be useful to not feature the position of such a point. Since the visibility of the two points are tied by a single Visibility property, there is not much use in making semantic distinction between the two focal points. The focal length (magnitude and sign) is instrumented, which is really the important quantity, rather than the actual positions of the primary and secondary focal points.

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Ok, closing