Up to top page(Amateur Sci, Weird Sci, etc.) LENS MISCONCEPTION William J. Beaty 1994 Email discussion Are people aware of the presence of a screwy diagram that appears widely in popular books and children's textbooks? I noticed it in a major grade-school science text, and then started noticing it in many other texts, in popular science books, newspapers, etc. Here is a crude version: ______________ /\__ __ - | /^\ / \ --__ __-- | | | | --__ __-- | | | | --__ | | | | __-- --__ | | ______________| |__-- --__ | \ / \|/ \/ object lens screen This diagram purports to explain how cameras and eyes form images. Yet it teaches quite a number of incorrect concepts: - That there is a special "point" between lens and image where all the light comes to a focus, - That lens/object distance doesn't affect the sharpness of the image, - That an object must be smaller than the lens in order to "fit", - That as the screen is moved closer to the lens, the inverted image on the screen will contract to a point, then become erect, - That if half the lens is covered, half the image will vanish, - That cameras based on lenses use an image-forming mechanism which is fundamentally different than that used by pinhole cameras This diagram has direct bearing on the whole "science misconception" area which has attracted so much attention recently. Misconceptions are formed in early years and then present serious barriers to learning during later schooling. Early concepts are a stumbling block for many of us, and are probably a major force for turning people away from science classes. But note that the above misconceptions do not occur naturally. They are specifically taught by faulty textbooks. If believing in this faulty diagram will forever make optics difficult to understand, then any books which contain it are doing actual harm. And the number of books is not small (see below.) I've seen this diagram in about twenty five books now, and in quite a few newspaper and magazine artices. In an anti-evolution article, it was offered as proof that eyes could not function in intermediate states of evolution. In an AJP paper by Goldberg and McDermott on student misconceptions, the college freshmen participating in dialog were obviously basing many of their naiive optics notions on just this diagram. A number of gradeschool science texts use the diagram to explain near and far sightedness. It even appears on the cover of a college-level photography text! I fear that it has become firmly embedded in the popular imagination, and also seems to be spreading from textbook to textbook like some sort of mind-virus. One point of this message is to find out if instructors and authors are aware of these "communicable misconceptions" which have spread from book to book, and even now are being used to misinform the young. There are a number of other examples out there as well, both in the form of diagrams and written explanations. By the way, here are a couple of corrected versions of the diagram... This one is appropriate for explaining image formation: ___--- | /\ ___--- | /|\ ---___ / \ ___--- | | ---___ | | ___--- | | --| |- | | __| |_ | | ___--- | | ---___ | | __--- | | ---___ | | -- \ / ---___ | \/ ---__ \|/ object lens screen It shows that a simple-lens camera is fundamentally similar to a pinhole camera, that the image projected on a screen will contract in size as the screen approaches the lens, and it shows the process by which the image is "turned upside down." This one shows how light from the object is focussed to create a sharp image: | __ /\ ___ | /|\ __-- / \ ----___ | | __-- | | ---____ | | __-- | | ---___ | |-==__ | | ___=== | | --__ | | ____--- | | --__ | | ___--- | --__ \ /___---- | \/ \|/ object lens screen It shows why the image becomes sharp at one special distance, why a large lens gathers more light than a small lens, and why a large image can get through even a tiny lens. Keep in mind that these two diagrams are intended for children. Any optics textbook will obviously use much more exact and complex diagrams. Some books containing the diagram: CHILDREN'S SCIENCE BOOKS "How It Works, Vol II" 1974 p191-194 "How It Works... the Telescope and Microscope" 1971 p11 "Light and Radiation" 1968 p30 "Eye and Brain" 1973 p23 "Light and Vision" 1966 p35 "First book of Light" 1962 p48 "Understanding Light" 1960, p102,106 "The Story of Light" 1952, p23,33-37 "Light and Sight" 1963, p49 "Optics: Light for a New Age" 1987 TEXTBOOKS AND EDUCATIONAL MATERIAL "Physical Science" Prentice-Hall, 1988 p569 "Holt Science" fifth grade text, 1984 p130 "The New Exploring Science" 1982 p172 Silver-Burdett "Science" instructor's workbook p69 "Elementary School Science and How to Teach It" 1984 p561 "Teaching Children Science",1982 p352 "Teaching Science Through Discovery" 3rd ed., 1985 p505 "Physics for Scientists and Engineers" 2nd ed, 1986 "Perception" 1984 p2 "Photography" 1984 (cover, and several diagrams) "A Sourcebook of Elementary Science" 1971 p244 Carolina Biological Supply, transparency 50-6950 OTHER "Academic American Encyclopedia" 1986, entry under "eye" "Encyclopedia Americana" 1986, Vol E, p812 "Science and Technology Illustrated" Ency. Brit. 1983, under "Lens" "Physics in Everyday Life" 1979 p239 "LIFE Science Library, Light and Vision" 1966 p35, p54 "Life Library of Photography, The Camera" 1970 p64, p77 "Time-Life Inc "Light and Film" 1970 (frontspiece) "Eye and Brain" 1973 p23 --------------------------------------------------------------------------- Some Misconception Resources: F.M. Goldberg and L. C. McDermott, AN INVESTIGATION OF STUDENT UNDERSTANDING OF THE REAL IMAGE FORMED BY A CONVERGING LENS OR CONCAVE MIRROR, Am. J. of Phys. 55 (2) Feb. 1987 pp108-119. PROCEEDINGS OF THE 2nd INTL SEMINAR ON MISCONCEPTIONS AND EDUCATIONAL STRATEGIES IN SCIENCE AND MATHEMATICS, VOL 1-3 1987 Cornell U., Ithaca NY Mario Iona, WHY JOHNNY CAN'T LEARN PHYSICS FROM TEXTBOOKS I HAVE KNOWN Millikan Award Lecture, Am J. Phys. 55 (4) Apr 1987 pp299-307 Mario Iona, HOW SHOULD WE SAY IT? Series of columns in The Science Teacher, 1970-1972 Mario Iona, WOULD YOUR BELIEVE? Series of columns in The Physics Teacher Teacher, 1970-1972 Email discussion If you comments to make or great URLs to add here, mail me at billbeskimo.com __________________________________________________________________________ This page is on the Eskimo North service, in Seattle, WA. Try the Main Eskimo home page.