Friday, June 15, 2007

Why is it dark at night?

Many years ago someone made a short film, asking Harvard graduates at commencement to explain why we have seasons. It's Harvard, so each answer is eloquent and delivered with perfect confidence -- and every one is badly wrong. I wish I could find a copy of that film on YouTube!

Motivated by the film, I've been asking the following question on exams given in courses on "conceptual physics" or physics for non-scientists. I've found that at both Berkeley and U Oregon, about 25% of students answer the question incorrectly (note this is at the end of the term, not the beginning). Remember, in our political system the votes of the people who get this question wrong count as much as yours or mine!

Question: It is dark at night in Eugene/Berkeley because

a) the night sky absorbs UV light
b) the moon blocks out the sun
c) the earth blocks out the sun
d) none of the above

Hint: draw a picture!

Further comments: Surprisingly the success rate for Berkeley students was about the same as for Oregon students, despite the difference in admissions selectivity. Nearly all of the students in these classes are humanities or social science majors. Many are upperclassmen, even seniors, trying to fulfill a minimal science requirement in order to graduate. These days the main claim of humanities programs is that they "teach critical thinking" :-)

See here for related discussion.


Anonymous said...

None of the answers are exactly correct although (c.) is close.

It is bright in daytime because the closest star, our sun, has flooded space around us with light.

There are stars at night as well. Billions and billions of them and the farther out you look the more there are. By that measure, night should be as bright as day but, it isn't. Why?

Light from our local star reaches us in eight minutes or so. There's the clue; Because of the vastly greater distances to other stars there hasn't been enough time since those stars were created for their light to fill in the night sky. This is indirect evidence that the universe had a begining; there really was a Big Bang.

DB said...

It is dark at night because, among the landscape of string theory vacua, those whose parameters lead to dark nights are most favorable to intelligent life. :-)

steve said...

Both answers listed above are eligible for extra credit, but do not qualify as "basic knowledge" compared to knowing the relative positions of Eugene, the earth and the sun at night :-)

An economist questions one-man-one-vote here and here. I've heard his book is pretty good, but I wonder why he does not find markets as irrational as voters.

Anonymous said...

The video of the Harvard students is available here.

Anonymous said...

Yes; I suppose your playing, but there is an off-putting smugness here which is probably not necessary.


Anonymous said...

You're, really.

De Selby said...

Darkness is simply an accretion of 'black air', i.e., a staining of the atmosphere due to volcanic eruptions too fine to be seen with the naked eye and also to certain 'regrettable' industrial activities involving coal-tar.

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