With the Approach of the Twentieth Century, The Classical Wave Theory of Radiation—A Widely Accepted Theory in Physics—Began to Encounter Obstacles

Reading passage question

With the approach of the twentieth century, the classical wave theory of radiation—a widely accepted theory in physics—began to encounter obstacles. This theory held that all electromagnetic radiation—the (5) entire spectrum from gamma and X rays to radio frequencies, including heat and light—exists in the form of waves. One fundamental assumption of wave theory was that as the length of a wave of radiation shortens, its energy increases smoothly—like a volume (10) dial on a radio that adjusts smoothly to any setting— and that any conceivable energy value could thus occur in nature.

The major challenge to wave theory was the behavior of thermal radiation, the radiation emitted by (15) an object due to the object’s temperature, commonly called “blackbody” radiation because experiments aimed at measuring it require objects, such as black velvet or soot, with little or no reflective capability. Physicists can monitor the radiation coming from a (20) blackbody object and be confident that they are observing its thermal radiation and not simply reflected radiation that has originated elsewhere. Employing the principles of wave theory, physicists originally predicted that blackbody objects radiated much more at (25) short wavelengths, such as ultraviolet, than at long wavelengths. However, physicists using advanced experimental techniques near the turn of the century did not find the predicted amount of radiation at short wavelengths—in fact, they found almost none, a result (30) that became known among wave theorists as the “ultraviolet catastrophe.”

Max Planck, a classical physicist who had made important contributions to wave theory, developed a hypothesis about atomic processes taking place in a (35) blackbody object that broke with wave theory and accounted for the observed patterns of blackbody radiation. Planck discarded the assumption of radiation’s smooth energy continuum and took the then bizarre position that these atomic processes could only (40) involve discrete energies that jump between certain units of value—like a volume dial that “clicks” between incremental settings—and he thereby obtained numbers that perfectly fit the earlier experimental result. This directly opposed wave theory’s picture of (45) atomic processes, and the physics community was at first quite critical of Planck’s hypothesis, in part because he presented it without physical explanation.

Soon thereafter, however, Albert Einstein and other physicists provided theoretical justification for (50) Planck’s hypothesis. They found that upon being hit with part of the radiation spectrum, metal surfaces give off energy at values that are discontinuous. Further, they noted a threshold along the spectrum beyond which no energy is emitted by the metal. Einstein (55) theorized, and later found evidence to confirm, that radiation is composed of particles, now called photons, which can be emitted only in discrete units and at certain wavelengths, in accordance with Planck’s speculations. So in just a few years, what was

(60) considered a catastrophe generated a new vision in physics that led to theories still in place today.

“With the approach of the twentieth century, the classical wave theory of radiation—a widely accepted theory in physics—began to encounter obstacles.”- is a GMAT reading comprehension passage with answers. Candidates need a strong knowledge of English GMAT reading comprehension.

This GMAT Reading Comprehension consists of 7 comprehension questions. The GMAT Reading Comprehension questions are designed for the purpose of testing candidates’ abilities in understanding, analysing, and applying information or concepts. Candidates can actively prepare with the help of GMAT Reading Comprehension Practice Questions.

Solution and Explanation

1. Which one of the following most accurately states the main point of the passage?

(A) If classical wave theorists had never focused on blackbody radiation, Planck’s insights would not have developed and the stage would not have been set for Einstein.
(B) Classical wave theory, an incorrect formulation of the nature of radiation, was corrected by Planck and other physicists after Planck performed experiments that demonstrated that radiation exists as particles.
(C) Planck’s new model of radiation, though numerically consistent with observed data, was slow to win the support of the scientific community, which was critical of his ideas.
(D) Prompted by new experimental findings, Planck discarded an assumption of classical wave theory and proposed a picture of radiation that matched experimental results and was further supported by theoretical justification.
(E) At the turn of the century, Planck and Einstein revolutionized studies in radiation by modifying classical wave theory in response to experimental results that suggested the energy of radiation is less at short wavelengths than at long ones.

2. Which one of the following does the author use to illustrate the difference between continuous energies and discrete energies?

(A) radio waves
(B) black velvet or soot
(C) microscopic particles
(D) metal surfaces
(E) radio volume dials

3. Which one of the following can most clearly be inferred from the description of blackbody objects in the second paragraph?

(A) Radiation reflected by and radiation emitted by an object are difficult to distinguish from one another.
(B) Any object in a dark room is a nearly ideal blackbody object.
(C) All blackbody objects of comparable size give off radiation at approximately the same wavelengths regardless of the objects’ temperatures.
(D) Any blackbody object whose temperature is difficult to manipulate would be of little use in an experiment.
(E) Thermal radiation cannot originate from a blackbody object.

4. The author’s attitude toward Planck’s development of a new hypothesis about atomic processes can most aptly be described as

(A) strong admiration for the intuitive leap that led to a restored confidence in wave theory’s picture of atomic processes
(B) mild surprise at the bizarre position Planck took regarding atomic processes
(C) reasoned skepticism of Planck’s lack of scientific justification for his hypothesis
(D) legitimate concern that the hypothesis would have been abandoned without the further studies of Einstein and others
(E) scholarly interest in a step that led to a more accurate picture of atomic processes

5. The passage provides information that answers each of the following questions EXCEPT:

(A) What did Planck’s hypothesis about atomic processes try to account for?
(B) What led to the scientific community’s acceptance of Planck’s ideas?
(C) Roughly when did the blackbody radiation experiments take place?
(D) What contributions did Planck make to classical wave theory?
(E) What type of experiment led Einstein to formulate a theory regarding the composition of radiation?

6. The primary function of the first two paragraphs of the passage is to

(A) describe the process by which one theory’s assumption was dismantled by a competing theory
(B) introduce a central assumption of a scientific theory and the experimental evidence that led to the overthrowing of that theory
(C) explain two competing theories that are based on the same experimental evidence
(D) describe the process of retesting a theory in light of ambiguous experimental results
(E) provide the basis for an argument intended to dismiss a new theory

7.  The passage is primarily concerned with

(A) discussing the value of speculation in a scientific discipline
(B) summarizing the reasons for the rejection of an established theory by the scientific community
(C) describing the role that experimental research plays in a scientific discipline
(D) examining a critical stage in the evolution of theories concerning the nature of a physical phenomenon
(E) comparing the various assumptions that lie at the foundation of a scientific discipline

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