Archive for June, 2010

Why is the sea blue?

June 28, 2010
Garibaldi Lake: Notice the blue colour

Garibaldi Lake: Notice the blue colour

There are some things which everyone ought to know, but which seem to evade understanding until later life. I know I ought to be grateful about discovering areas of profound ignorance, but somehow I found the latest example of ‘You learn something new every day‘ rather hard to take. And I am still not sure I grasp it fully. The gist of it is this:

the sea is blue because water itself is blue.

Now when I first came across this, I was immediately dismissive. I ‘know’ that water is not blue: I have looked at water quite a lot lately and I would say it was broadly colourless and substantially transparent. However a colleague assured me that he had seen a vial normal water and a vial of heavy water (- deuterium oxide – in which the hydrogen atoms were replaced by their heavier isotope deuterium) and that viewed side by side, normal water was visibly bluer. I was skeptical, and I am still not sure this is the correct explanation. But my alternatives lack any quantitative support.

While I provisionally accept what I have read, my scepticism arises from the fact that I never been aware of the blueness of water in a context which does not involve a source of blue light – namely the sky. I will make some efforts to take a look at some heavy water in a  lab and report back.

As an aside I would just like to note how shocked I am about this! How could I have lived so long and not known about this! Ahhhh….

Painting Mountains White

June 17, 2010
Painting a mountain white

Painting a mountain white

A strange story on the BBC today about a team of people in peru who are painting mountains white in order to encourage the return of glaciers to their mountains. Truly the story is sad.

First the physics. Yes, it is true that painting the mountain white will cause more sunlight to be reflected and will cool the average surface temperature of the mountain. If the average surface temperature is just above zero degrees Celsius then the paint may cause the average temperature to fall below zero and allow ice to form and snow to settle. And could this cause a glacier to re-form? Well, No.

Why not? Well one only needs to reflect on the fact that a glacier did exists there previously and that it was on top off the rocks. At some point in history – probably only 100 years ago –  it would have presented an extraordinarily white reflective surface to the Sun. The surface would have been far smoother and more reflective than any surface finish that could be achieved by throwing buckets of lime on the rocks. And yet the slow action of climate change and possible industrial soot caused the destruction of the glacier in just a hundred years or so.

So let us imagine that the white paint caused ice to reform and survive over the summer at this location. The paint would then be irrelevant – and exactly the same climate forces which destroyed the previous glacier will destroy the the next one.

And so to me this story is just sad, sad, sad. The glacier is gone; the spring water it supplied is gone; and it is not coming back anytime soon. The residents would be better employed building a dam to create an artificial lake that would trap some of the winter melt water.

Exam Standards: Good News and a suggestion

June 17, 2010

I have written here previously about my anger at the decline in exam standards, particularly at GCSE level and particularly in Science. And most particularly in Physics. My concern is that children from state schools which ‘teach to the exam’ will be denied the possibility of careers in Science. The Qualifications and Curriculum Development Authority oversaw a perncious system which systematically drove down exam standards year after year and penalised anyone – teachers, publishers or schools – who tried to object.

So it is great to report that the QCDA is moribund, and I am delighted to find that Ofqual has finally found the teeth to object to this decline. After rejecting the next syllabus revision as too low in standards today I read that they have sent back the exam boards revised syllabi as still not challenging enough. Hurray! Ideally, exam standards could rise year upon year as OfQual drove standards higher and higher. I am not hopeful, because the same political pressure which drove the previous government to devalue educational qualifications still exist. However I do have a suggestion.

My Suggestion

When I took my O level exams, the grade awarded was determined not by any absolute standard, but by where one came in the ranked exam order. Thus an A indicated that one had achieved a mark in the top 10% (say, I don’t know the exact fraction used) of the exam cohort. Nowadays an A indicates a mark exceeding 70% (I think) and can be achieved by any fraction of the exam cohort. It thus no longer serves to discriminate among exam candidates. And discrimination amongst candidates is – like it or not – a key purpose of exams.

If the old procedure was reintroduced it would automatically condemn a certain fraction of the cohort to ‘failure’, no matter what they achieved. However there is no need to use the system universally. A mixed system could be used: An A grade could still indicate the top 10%, B would indicate the next 10%, and C would indicate a norm-based pass above some nominal pass mark. There would be no limit to the number of people who could get C. This allows for improved teaching to result in improved pass rates without a guaranteed fraction ‘of failures’ while allowing employers or universities to look for academic high achievers.

  • Is there something wrong or unfair with this?

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