First, assume a spherical cow of uniform composition...

kw: book reviews, nonfiction, collections, science journalism

OK, I assume you know the joke about asking a physicist to help improve the yield of dairy cows. That's how most folks look at physics, isn't it? Physics is math on steroids, right? Here's another one:

Are you an engineer or a mathematician? You enter a room. A sign says, "Boil the Water." You see a mug of water on a table just to the left of a microwave oven. OK, no prob; just put the mug in the microwave, and heat for a couple minutes.

Another sign says, "Go through the next door." When you do, you see a sign, "Boil the Water." You see a mug of water on a table just to the right of a microwave oven.

What do you do? Would you put the mug in the microwave, and so forth? If so, you're an engineer. A mathematician would move the mug to the left of the microwave, and state, "I already solved the rest."

What is Physics, really? Jennifer Ouellette's book will give you a good beginning, of seeing how physics has progressed during its 500-year history. Of course, "physics" wasn't its name in the 1500s. Instead, it was the major element of "Natural Philosophy". The 38 re-edited columns in Black Bodies and Quantum Cats: Tales from the Annals of Physics present highlights of the past half-millenium from a very human view.

The author is editor of American Physical Society News (APS News), and her monthly columns have been used by physicists worldwide to help their "physics-phobic" friends understand a little of what they do. Physics is not just math and esoteric experiments. It is a most human endeavor, and the stories Ms Ouellette tells are very human. Her love of history and fascination with the way science really happens come through loud and strong.

I am going to take a long sidetrack here. Nobody can be an expert in everything, and the author's blind spot seems to be optics. In particular, when discussing the compound microscope, and then telescope optics, she gets something very crucial completely backwards. I'll explain.

Simple, single-lens microscopes worked better than compound microscopes until the invention of the achromatic lens, which uses two lenses of different types of glass to cancel out most of the color aberration that is present in any single lens.

A single lens is used as an adjunct to your eye's lens to let you look at something really close. Most younger people's eyes work best at distances of ten inches or greater. We use a ten-inch distance as a benchmark. A lens with a 1-inch focus allows you to look at something one inch from your eye, so it looks ten times larger. The smallest lenses made by Leeuwenhoek in the 1680s had a focus of 1/30th inch, so they "magnified" by 300x. But you had to hold the lens so it nearly touched your eye. That is why his microscopes are built the way they are.

The compound microscope uses a kind of image relay to achieve large magnifications without sticking something right into your eye. If you have a large magnifier, try this. Instead of holding it close to your eye, hold it farther away, at arm's length, but a few inches from something. If you hold it close to the object, you see the object a little magnified. As you move the lens farther, the image you see gets bigger, then starts to be really distorted. Farther yet, and you see an enlarged image again, but upside-down. It may be hard to focus on also, because it is closer to you than the lens is. That is called a "real image", because if you put a piece of paper there, the image will be visible on it.

A compound microscope works by making an enlarged real image, then magnifying that with an eyepiece lens. For example, an ordinary school microscope might have a 10x objective, and a 10x eyepiece. The objective is made so that when it is 16mm (0.63 inch) from an object (the "slide" on the microscope stage), it produces a real image 160mm (6.3 inches) farther up the tube. Another 25mm (1 inch) farther up, the eyepiece, with a focal length of 25mm, works with the lens of your eye to focus the image on your retina, and it is magnified 100 times, compared to the slide.

While you can make a rudimentary compound microscope with two simple lenses, there will be strong rainbow effects, because the color aberrations of the second lens will multiply those of the first. So, special glasses are used to make lenses that don't have much color aberration, so you can see a clear image.

Now, in Chapter 3, Ms Ouellette states that the lenses in a compound microscope are "complementary", "one convex, the other concave." She states elsewhere that the concave lens (which spreads light, doesn't focus it) is the objective, while the convex lens (which does focus light) is the eyepiece. I have seen one of these historical microscopes, and I can assure you, both lenses are convex. What is going on here? Where did she get this idea?

It may be from an imperfect understanding of achromatic lenses. These do have a complementary pair of convex and concave, but the convex is the stronger. We want these lenses to focus, after all. But I think it might have been from partial knowledge of Galieo's telescope, and of common nautical "spyglasses" of the 19th century and earlier. These do use a convex and concave lens, one at each end of the tube. However, the convex lens is the objective, and the concave lens is the eyepiece. How this works is subtle.

The long-focus convex lens at the "business end" of a telescope has the job of producing an real image near the observer's eye. The eyepiece then has the job of relaying this image into the eye. Galileo and others of his time used a concave lens, which is easy to use. Concave lenses don't focus light by themselves. However, when light is coming to a focus because of a nearby convex lens, a concave lens can shift the focus, or even reverse it, so to speak.

A concave lens has a "negative focal length". If you produce a real image using a convex lens, then put a concave lens at just the right distance from that image, but intercepting the light before it gets there, it will just counteract the focusing effect, so the light seems to be coming from an object much farther away, but of a different size than the original object being imaged. They, when you place your eye near the concave lens, this nearly-parallel light will be focused by your eye's lens onto the retina and you'll see a magnified image. If you move the concave lens a little closer to the convex one, the size of the image will decrease a little, and you'll need to focus your eye as though the object were closer, but it is like a zoom lens.

This makes spyglasses easy to use. There is one setting where you get maximum magnification, and your eye's lens is relaxed. There are other settings with a little less magnification, as long as you can accommodate to viewing an image that seems to be closer. Of course, if you push the eyepiece in too far, you can't focus at all.

If this is so easy to use, what is the drawback? The geometry of the arrangement means you have a very narrow field of view. Looking through a spyglass, you see a small circle with a magnified image in it. But when you look through most modern telescopes, you see a much larger circle. Sometimes, you even have to look around to see the edges of the view. Why?

This is because they use convex lenses as eyepieces. A "positive" eyepiece stands farther from the real image produced by the objective, and relays it to your eye. Because a convex lens is a focuser, it can gather light over a larger angle and put it into your eye, so the view is much wider. There is a further, really big difference. A spyglass has its view right-side up, while a modern telescope has its view upside-down. There are simple ways to turn the image back over if you need to (binoculars use internal mirrors or reflecting prisms). Finally, though, the focusing range is more narrow, compared to a spyglass eyepiece. That is why most spyglasses just have a slide tube holding the eyepiece, but most telescopes use a mechanical focuser.

So, whether it is a microscope or telescope, an objective lens produces a real image of an object, and an eyepiece accommodates that image to your eye, further magnifying it in the process.

OK, back to the book. I really like the author's approach. She uses everyday analogies and popular culture (Addams Family Values, or Back to the Future...) to associate physics concepts to more familiar things. So, let's think about this. What do you do when you get curious about something?

Did you ever see the rainbow in the spray of a sprinkler on a sunny day? If you look closely, you can see that as a droplet of water moves, the color it sends to you changes. Perhaps close to the sprinkler head, it is silvery, but not very bright. Then as the drop moves up, it'll brighten to blue, green, yellow, red, then back to a dimmer, silvery color. Perhaps you'll see it fall back through the colors in reverse order before it hits the grass. If you want to, you can hold a drop of water in a tiny wire loop indoors, and shine a flashlight on it, to see what angles give you what colors. You're doing physics!

The big, theoretical formulations by Einstein, Dirac, and others, are one thing. Yes, there can be some really heavy math involved. But the foundations of physics rest in simple things like rolling a ball on a tilted table with a stopwatch in your hand. Ms Ouellette is to be thanked for bringing physics a bit closer to the everyday world.