PT7.S1.Q16 - Comets do not give off their own light...

udayanti.mukherjee

PrepTest 7 - Section 1 - Question 16

Can someone explain?
Admin Note: Edited title. Please use the format: "PT#.S#.Q# - brief description of question”

maco4538

We are looking for an answer that must be true, and anything that CBT, CBF or MBF is the wrong answer.
The stimulus tells us:
- scientists estimate the mass of the comets by their brightness
- the material of which Halley's comet reflects 60 times less light per unit of mass than had been previously thought.

The scientists assume that the mass of a comet will be directly proportional to its brightness (1:1). So, if it was double the mass then we would see double the brightness. So, if we were to measure the brightness of Halley's comet, we would be assuming that the brightness we measure will be proportional to the mass of the comet (1:1). But if the material that Halley's comet is made of reflects 60 times less light per unit of mass than previously thought then the mass of Halley's comet is 60 times greater than previously thought.

or as B puts it: previous estimates of the mass of Haley's comet were too low.

cjcjcjcj

The stimulus is intended to confuse you with fancy wording but just focus on this: More mass, More light.
The stimulus ends with only mentioning "less light". Then, the answer must mention "less mass".
B says it: "mass...were too low".

Daniel L

(A) The stimulus states that the Halley's comet material actually reflects 60x less light per unit of mass than was previously thought for Halley's comet. The 60x less light that Halley's comet actually reflects is compared to what scientists had previously estimated for Halley's comet, not compared to other comets. We have no idea whether any other comets exist whose material reflects 60 times more light per unit of mass than the material of Halley's comet.

(B) The stimulus says that the greater a comet's mass, the more light reflected. If scientists have previously estimated that a given unit of Halley mass reflects 60x more light than it actually does based on the amount of light that has been reflected, it would have to be 60x greater in mass to have reflected that amount of light. Thus, previous estimates of the mass of Halley's comet were too low. This is the right answer.

Example:
This comet has reflected 1200 photons of light, we estimate that the comet reflected 60 photons/kg of mass and the comet is 20 kg.

We've found out that it only reflects 1 photon/kg of mass (60x less than expected). Thus, since it reflects 1200 photons of light, the comet's mass is 1200 photons * 1 kg/photon of mass = 1200 kg. Previous estimates of the mass (20 kg) were too low.

(C) The stimulus only implies that the total light reflected per unit of mass is less than previously thought, not the that the total light reflected by the whole comet is less than previously thought.

(D) We have no idea whether this is true from the stimulus. All we know is that the actual reflective properties of the material of a comet can be overestimated by scientists, but the stimulus provides no indication that reflectivity varies from comet to comet (even though in real life this is true).

(E) We have no idea whether the satellite probe results mean scientists have enough or not enough information to estimate the mass of Halley's comet, so this isn't the right answer choice.

Mary - Student Service

Hi @"Daniel L". I have edited your comment. Please do not post the entire question and answer choices for the LSAC question, this is copyrighted content and is against the Forum Rules. Thank you.