Self-study
P1
With the approach of the twentieth century, the classical wave theory of radiation—a widely accepted theory in physics—began to encounter obstacles. ████ ██████ ████ ████ ███ ███████████████ ███████████████ ██████ ████████ ████ █████ ███ █ ████ ██ █████ ████████████ █████████ ████ ███ ██████████████ ██ ███ ████ ██ ██████ ███
Challenge to Existing Theory ·
Classical wave theory claims that all radiation exists as waves
If this theory is being challenged, that must mean that some (or all?) radiation doesn't take the form of waves?
Theory's Assumption ·
Wavelength and energy are continuous
Continuous means that any conceivable energy value could occur in nature. But if that's the assumption, then it probably will turn out to be false. So... what's the opposing concept to "continuous?"
P2
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Phenomenon (Unexplained by Wave Theory) ·
"Blackbody" radiation
"Blackbody" radiation is the heat(?) emitted by an object. The object is black so that it doesn't reflect other forms of radiation (e.g., light in the visible wavelengths). That way, physicists can be sure that they're measuring radiation emitted by the object, as opposed to radiation that merely bounced off that object.
Wave Theory's Prediction ·
Blackbody objects would radiate more at short (compared to long) wavelengths
Prediction False ·
Physicists found almost no short wavelength radiation
The theory predicted that they would find a lot of short wavelength radiation, e.g., ultraviolet radiation. But experimental results contradicted that prediction.
P3
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New Hypothesis ·
Planck theorized that energy was discrete
Planck discarded wave theory's energy continuum (like a smooth turning dial). He didn't think that energy could take on any value. He hypothesized that energy could only take on discrete values. So to move from one energy level to the next involves a jump (like a dial that clicked into place).
Reception of Hypothesis ·
Physics community initially skeptical
Because Planck didn't provide a mechanism to explain how discrete energy jumps occurred.
P4
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Support for Hypothesis ·
Einstein (and others) provided theoretical justification
Found that energy values were discontinuous and found threshold beyond which no energy is emitted.
New Theory ·
Einstein theorized that radiation is composed of particles called photons
Photons can be emitted only in discrete units and at certain wavelengths, corroborating Planck's hypothesis.
Passage Style
Phenomenon-hypothesis (RC)
Single position
18.
Which one of the following ████ ███ ██████ ███ ██ ██████████ ███ ██████████ ███████ ██████████ ████████ ███ ████████ █████████
a
radio waves
The answer is volume dials. (See the text linked in the explanation below the question stem.)
b
black velvet or ████
The answer is volume dials. (See the text linked in the explanation below the question stem.)
c
microscopic particles
The answer is volume dials. (See the text linked in the explanation below the question stem.)
d
metal surfaces
The answer is volume dials. (See the text linked in the explanation below the question stem.) The author mentions that metal surfaces give off energy at values that are discontinuous, but this doesn’t show that metal surfaces are used to illustrate the difference between discontinuous and continuous energies.
e
radio volume dials
Correct. (See the text linked in the explanation below the question stem.)