# 2: Lens & Mirror Calculations

- Page ID
- 7079

- 2.2: Limitations
- In this chapter, I am going to ignore lens and mirror aberrations. I may possibly prepare a separate chapter on lens and mirror aberrations sometime, but that is not the topic of the present chapter. Thus, in this chapter, I am going to assume that all angles are small.

- 2.3: Real and Virtual
- Whereas you can project a real image on to a piece of card or a photographic film, you cannot do this with a virtual image. The reason that you can see a real image with your eye is that the additional optics of your eye bends the diverging light from the virtual image and makes it converge on to a real image on your retina.

- 2.4: Convergence
- Converging light has positive convergence; Diverging light has negative convergence.

- 2.5: Power
- The function of a lens is to change the convergence of a beam of light. Indeed the difference between the initial and final convergence is called the power P of the lens, or of a refracting interface, or of a reflecting mirror.

- 2.6: Magnification
- If the magnification is positive, the image is erect; If the magnification is negative, the image is inverted.

- 2.8: Derivation of the Powers
- Up to this point I have defined what is meant by convergence, and I have defined power as the difference between the final and initial convergences. I asserted without proof formulas for the powers of a lens, a refracting interface, and a mirror. It is now time to derive them.

- 2.10: Designing an Achromatic Doublet
- A combination of two lenses in contact, a converging lens made of crown glass and a weaker diverging lens made of flint glass, can be designed so that the combination is a converging lens that is almost achromatic.

- 2.13: The Lazy Way
- The convergence and power method has great advantages when you have a complex systems of many lenses, mirrors and interfaces in succession. You just add the powers one after the other. But I expect there are some readers who don’t want to be bothered with all of that, and just want to do simple single-lens calculations with a simple formula that they are accustomed to, which is appropriate for the “real is positive” sign convention.

*Thumbnail: Parallel rays coming into a parabolic mirror are focused at a point F. The vertex is V, and the axis of symmetry passes through V and F. For off-axis reflectors (with just the part of the paraboloid between the points P1 and P3), the receiver is still placed at the focus of the paraboloid, but it does not cast a shadow onto the reflector. Image used with permission (Public Domain).*