Reading Comprehension

PT115 · S3 · P3 · Q17 Planck and Wave Theory

A free, expert breakdown of this official LSAT Reading Comprehension question.

  • Save & drill this skill — build targeted practice sets from questions like this one

  • Video walkthroughs — watch every question solved step by step

  • 81 official LSATs — as questions, timed sections & full-length tests

Start practicing free

With the approach of the twentieth century, the classical wave theory of radiation—a widely accepted theory in physics—began to encounter obstacles.

Topic

Wave theory of radiation

Framework

old / new

Paragraph 1 The very first sentence warns us that we're probably reading an Old/New type passage (along with the fact that Old/New is the most common framework for Science) passages.

An accepted theory is encountering obstacles?

When you suspect an Old/New framework, you try to proactively predict the New, by defining it as the logical opposite of the Old (in the hopes of priming your brain so that it's easier to process the payoff sentences when they arise).

Old New all EM radiation exists some EM radiation in the form of waves isn't in wave form?

as length of wave as length of wave shortens, its energy shortens, its energy increases smoothly doesn't go up like (volume dial) a smooth volume dial?

Any conceivable some energy values energy value could can't occur in occur in nature nature?

Paragraph 2 The transition sentence warns us that we're about to hear the story of the "obstacles" that wave theory started to encounter. Many science passages with an Old/New structure have a section where they're saying: Scientists thought / assumed X, which meant that Y should be true. But then data showed that Y wasn't true!

This happens when we hear that physicists originally predicted they would find more energy at short wavelengths, however they found almost none.

This tension is what sets us up for the New.

Paragraph 3 The second sentence gives us Planck's answer to this conundrum -- "get rid of the Old volume dial / continuous view of energy, start using a New incremental-click / discontinuous view".

We even get a mini Old/New at the end of the paragraph as we learn that "at first" the physics community was deeply reluctant to accept this, since Planck had no physical explanation to back this up.

Paragraph 4 This paragraph opens with a continuator that says Einstein and others came to the rescue by coming up with a different theory of energy that could explain why Planck's incremental-click / discontinuous view could be legit.

The big breakthrough here was the idea of positing particles (photons) that are discrete units and certain wavelengths. (a particle is a quantum of energy, a specific quantity of energy ... which is where the name quantum physics comes from )

17.

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

  1. If classical wave theorists had

    Wrong Emphasis

    The main clause of this answer is "Planck's insights would not have developed and the stage would not have been set for Einstein". Why would we say the main point was talking about some counterfactual world? Wasn't the passage trying to tell a story about the actual world? When we say, "If hadn't been for X, Y would have never happened", we're making X seem crucial and important. So we could hear this answer choice saying, "Classical wave theorists were super crucial and important to focus on blackbody radiation!" That's definitely the wrong emphasis. The main character of this passage is Planck or the its the new conception of radiation as clicky, not smooth.

    2% picked this

  2. Classical wave theory, an incorrect

    Unsupported: Planck's experiments

    Planck had an insight, a hypothesis. He never actually performed any experiments that demonstrated radiation exists as particles. He just proposed a new assumption about radiation: "what if it's made up of discrete energies that jump from one unit to another?"

    9% picked this

  3. Planck’s new model of radiation,

    Wrong Emphasis

    The main clause of this answer is that "Planck's new model was slow to win support". That was not the main point. The main point was more like, "Planck had a new model that thought of radiation as operating in discrete units, rather than in a smooth continuum."

    5% picked this

  4. Correct

    Prompted by new experimental findings,

    Why this is right

    The main clause here is, "Planck discarded an (Old) assumption and proposed a (New) picture of radiation". The old assumption was "smooth and continuous". The new assumption was "discrete and discontinuous". The other details check out: Planck's work was prompted by experimental findings that showed that blackbodies do not emit more short wavelengths than long wavelengths. And his work was further supported by theoretical justification: the first sentence of the final paragraph says that Einstein and others provided theoretical justification.

    Skill tested: Main Point · how this choice captures the passage's function is the move to repeat next time.

    75% picked this

  5. At the turn of the

    Wrong Catalyst

    Planck didn't modify classical wave theory; he kind of blew it up. It generated "a new vision in physics". It led to Einstein's discovery that radiation is composed of particles. And it wasn't in response to results suggesting that "the energy of radiation is less at short wavelengths than at long ones". It was in response to experiments showing that blackbody objects weren't emitting energy at the amounts predicted for short vs. long wavelengths. The fact that experimental observations didn't match wave theory's predictions alerted physicists to the possibility that something was wrong with wave theory.

    9% picked this

Continue the review in LSAT Lab

Save this question, watch the video walkthrough, and drill similar questions in your LSAT Lab account.

Save and practice this skill
LSAT Lab

Turn this review into a targeted study plan.

Save this question, drill more like it, watch the video walkthrough, and track your progress in your LSAT Lab account.

Start practicing free