Section 27–3 The focal length of a lens

 (Spherical interface / Thick lens / Thin lens)

 

In this section, Feynman discusses the formulas of a spherical interface, thick lens and thin lens.

 

1. Spherical interface:

“Most of the lenses that we use have two surfaces, not just one. How does this affect matters? (Feynman et al., 1963, section 27–3 The focal length of a lens).”

 

Feynman says that we need a generalization of the formula (1/s) + (n/s′) = (n−1)/R (27.3) for a case where there are two different indices, n₁ and n₂, rather than only n. He adds that it is not difficult to prove that the general form of (27.3) is (n₁/s) + (n₂/s′) = (n₂−n₁)/R. In fact, the general formula can be proved using the formula (27.2) (h²/2s) + (nh²/2s′) = (n−1)h²/2R. One should realize that the formula (27.3) is simplified in the sense that the refractive index of the first term is 1 and the second term is n. By replacing 1 by n₁ and n by n₂, the formula (27.2) becomes (n₁h²/2s) + (n₂h²/2s′) = (n₂− n₁)h²/2R, and we have almost obtained the general formula.

Feynman elaborates that most of the lenses have two surfaces instead of only one. It is based on his broader definition of a lens and that is why the title of this section is “The focal length of a lens.” Specifically, the section is about the focal length of a spherical interface, thick lens, and thin lens. The title “the focal length of a lens” is potentially misleading because we tend to conceptualize a lens to have two surfaces. In general, a lens is defined as an optical device that comprises two refracting surfaces separated by a transparent medium (e.g., glass) and it can either converge or diverge light rays. Thus, the first and simplest situation could be more appropriately described as a spherical interface instead of a lens (that has one refracting surface).

 

2. Thick lens:

“…Therefore the thickness of the lens at the center must be given by the relationship (n₁h²/2s) + (n₁h²/2s′) = (n₂−n₁)T (27.8) (Feynman et al., 1963, section 27–3 The focal length of a lens).”

 

To derive the thickness of a lens, Feynman explains that the excess time in going from O to O′ is (n₁h²/2s) + (n₁h²/2s′) by considering the distances from O to P and P to O′ (diagonal distances) and initially ignoring the thickness T of a lens of index n₂. Then, he expresses the thickness of the lens T in terms of the radii R₁ and R₂ of the two surfaces as T = h²/2R₁ − h²/2R₂ (horizontal distances). The term h²/2R₁ is the horizontal distance from the vertex (or pole) of the first refracting surface to its center of curvature, whereas the term h²/2R₂ is the horizontal distance from the vertex of the second refracting surface to its center of curvature. However, one may define a thick lens as an optical device having two spherical refracting surfaces separated by a transparent medium that has a thickness T. One should realize that Feynman uses a special case (converging meniscus lens) to derive a general formula of a thick lens.

Feynman suggests that the thickness of the lens at the center must be given by the relationship (n₁h²/2s) + (n₁h²/2s′) = (n₂−n₁)T (27.8). Perhaps he could have clarified the formula n₁h²/2s) + (n₁h²/2s′) = (n₂−n₁)T is derived using the optical path, where the formula T = h²/2R₁ − h²/2R₂ is based on actual distance using only geometrical considerations. However, a better formula for thick lens can be expressed as n/f = n′/f₁′ + n′′/f₂′′ - dn′′/f₁′f₂′′ = n′′/f′′ (Gaussian formula) or P = P₁ + P₂ – dP₁P₂/n′ (Power formula) in which n, f, P, and d refer to refractive index, focal length, power, and the thickness of a thick lens respectively (Jenkins & White, 1981). These two formulas can be derived using similar triangle methods that are based on how light rays passes through the front focal point and rear focal point of the thick lens.

 

3. Thin lens:

“…if one of the points is at infinity, the other will be at a point which we will call the focal length f. The focal length f is given by 1/f = (n−1)(1/R₁−1/R₂)… (Feynman et al., 1963, section 27–3 The focal length of a lens).”

 

During Feynman’s derivation of the lensmaker’s equation, he mentions that if one of the points is at infinity, the other will be at a focal point given by the equation 1/f = (n−1)(1/R₁−1/R₂). Note that the equation relates the focal length of a thin lens to a ratio of refractive indices n = n₂/n₁ and the radii of curvature of the two lens surfaces. Perhaps Feynman could have emphasized the need of using the spherical refracting surface equation (n₁/s) + (n₂/s′) = (n₂−n₁)/R two times. Essentially, the image formed by the first refracting surface acts as an object (source of light) for the second refracting surface. Furthermore, the equation involves an idealization in which the lens is very thin in the sense that the separation between the two refracting surfaces is negligible compared with the object distance and image distance.

 

According to Feynman, it would be better to write the equation in terms of the focal length directly as (1/s) + (1/s′) = 1/f. In addition, the two focal lengths of an optical system are the same provided the initial and final refractive indices are the same. Curiously, one may observe that the lensmaker’s equation 1/f = (n−1)[(1/R₁−1/R₂)] does not include the size of the object. That is, one limitation of the equation is that the object size should be sufficiently small in comparison to R₁ and R₂. Furthermore, one may add that the lens is usually used where the refractive media on both sides of the lens are 1 (air), and thus, the symmetry of the foci F and F' would not be violated.

 

Review Questions:

1. Would you adopt Feynman’s definition of a lens that may have one or two surfaces?

2. Would you use Feynman’s method to derive the thickness of a thick lens?

3. How would you explain the idealization or limitations of the lensmaker’s equation?

 

The moral of the lesson: an important principle of a lens is to use the position of an image that is formed through the first refracting surface to find the new position of the image that is formed through the second refracting surface of the lens.

 

References:

1. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.

2. Jenkins, F. A., & White, H. E. (1981). Fundamentals of Optics (4th Ed.). Singapore: McGraw-Hill.