The Audience
Understanding Physics is written primarily for undergraduate college students not intending (at least initially) to enter careers in science or engineering. These may include liberalarts students, business majors, prelegal, and prospective architecture students. We have found that when the course is taken with laboratory work, it has been deemed suitable by medical schools for premedical students.
An important group that this course is intended to serve are persons who plan to teach, or are already teaching, in K-12 classrooms. As has been widely discussed, there is a special need for improvement in the science education of current and future teachers as an important step toward achiev-ing greater scientific literacy in general. Many states have recently incor-porated the contextual approach used in Understanding Physics into state science education criteria. It is in part to meet the challenges of teacher education tl1at this course was developed as a resource, along with the usual pedagogical training.

Since college students in introductory science courses usually represent a wide spectrum of expertise in science and mathematics, this book asswnes no prerequisites in science or mathematics beyond high-school algebra, geometry, and general science.

In this text we have taken great care to de­rive all necessary equations very patiently, but whenever possible we have used narrative text instead of equations to convey由e meanings of laws and concepts. Even if srudents have taken physics in high school, they often still lack proficiency in even the most basic concepts and techniques. One of山e aims of this course is to enable all students to gain experience and confidence with physical-science concepts and quantitative me出ods, and wiili an understanding of the nature of science itself. Of course, for classes in which the students are sufficiently prepared, instnictors may decide to place more emphasis on quantitative or other aspects of physics as appro­priate. The course is designed with such flexibility in mind.

The Approach
A unique feature of this text, like its predecessor, is that it places the fun­damental concepts of physics within the broader humanistic and historical contex'ts in which they arose, but without handicapping students in tests that compare their performance with students who have taken a less broadly conceived, conventional physics course. Research has shown that students exposed to our approach gain a much deeper 皿derstanding of both the content and the processes of scientific research, as well as an appreciation not only of what we know, but also of how and why we think we know it. This approach has been endorsed by several national organizations, in­eluding the National Science Fo皿dation, the Research Council of the National Academy of Sciences, and Project 2061 of the American Associ­ation for the Advancement of Science. The National Research Cow1cil stated in its National Science Education Standards:*

In learning science, students need to understand that science re­flects its history and is an ongoing, changing enterprise.

The stan­dards for the history and nature of science recommend the use of history in school science programs to clarify different aspects of sci­entific inquiry, the human aspects of science, and the role that sci­ence has played in the development of various cultures.

13.7 EINSTEIN'S THEORY OF THE PHOTOELECTRIC EFFECT
The explanation ofthe photoelectric effect was the major work cited in the award to Albert Einstein of the Nobel Prize in Physics in 1921.
 Einstein's explanation, proposed in 1905, played a major role in the development of atomic physics. He based his theory on a daring hypothesis, for few of the experimental details were known in 1905. Moreover, the keypoint of Einstein's explanation contradicted the classical ideas of the time. Einstein assumed that the energy of light is not distributed evenly over the whole expanding wave front (as the classical theory assumed). Instead, the light energy is concentrated in separate "lumps." In addition, the amount of energy- in each of these regions is not just any amount, but a definite amount of energy that is proportional to the frequency/of the lightwave. The proportionality factor is a constant (symbol h); it is called Plancks constant for reasons which will be discussed later. Thus, in this model, the light energy in a beam of frequency comes in pieces, each of amount E = hf where h = 6.6X 10~34J/s.
The amount of radiant energy in each piece is called a light quantum {quantum is Latin for quantity), or a quantum of light energy. The quantum of light energy was later called a photon.