Reading Comprehension

PT115 · S3 · P3 · Q19 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 )

19.

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

  1. Correct

    Radiation reflected by and radiation

    Why this is right

    This is inferable from the second paragraph. It's a weird answer to the question stem, since this answer choice is saying something true about all objects, not just blackbody objects, but the question stem is saying, "what can we infer from the description of blackbody objects", which I guess we can take to mean "from the discussion of blackbody objects in the 2nd paragraph". The reason physicists use blackbody objects to measure thermal radiation (the energy emitted by an object, not reflected by the object) because with blackbody objects you can be confident that the radiation you're measuring is emitted, not reflected. The logic here is that "if you have to find an object that doesn't reflect much/any radiation in order to measure how much radiation an object emits, then apparently a normal object that reflects and emits radiation would be too hard to measure: you wouldn't know how much of the measured radiation was from reflection, how much was from emission, since radiation reflected by and radiation emitted by an object are difficult to distinguish from one another.

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

    67% picked this

  2. Any object in a dark

    Too Strong: any / nearly ideal

    The gist of this answer makes some sense. The less light there is in a room, the less we'd have to worry about measuring reflected light. But dark room is not the same as "pitch-black room". There are still photons bouncing around in a dark room. Also, we just don't know enough about measuring thermal radiation, from this passage, to know whether any object would be nearly ideal, as long as you dimmed the lights.

    4% picked this

  3. All blackbody objects of comparable

    Too Strong

    Too Strong: all / same Contradicts Common Sense This seems counterintuitive to what we might know about temperature. Wouldn't a hotter object be giving off more thermal radiation than a cooler object, even if they were the same size? By definition, it seems like thermal radiation is very related to the temperature of an object.

    8% picked this

  4. Any blackbody object whose temperature

    Out of Scope: difficult to manipulate

    Nothing in the second paragraph (or passage) is talking about manipulating the temperature of objects. We're only talking about measuring the emitted radiation of objects.

    12% picked this

  5. Thermal radiation cannot originate from

    Contradicted

    Blackbody objects are useful to physicists who want to measure only thermal radiation. All objects are emitting thermal radiation; most objects are also reflecting radiation. Blackbody objects (basically) only do the former.

    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