Essays for Midterm 2


On the second midterm exam, you were asked to write an essay on one of the following two topics:


1. The eye is a complicated feature of the human body. How does it work?  Discuss the relevant physics.  In your essay, include answers to the following questions: what does it mean to be nearsighted or farsighted? How does aging play into this process? How does the eye see colors? What does it mean to be colorblind?


            For an essay that earned the full 20 points, see the one by Derek Chen.




2. Greenhouse effect.  Explain how it works, and how it affects climate. How is it affected by human emissions?  Why is the greenhouse effect "controversial" in public debate?  How does it relate to the "ozone problem"? 


            For an essay that earned the full 20 points, see the one by Stephanie Ho.



The eye by Derek Chen


The eye works by having light focus on your retina through the help of two lenses, the cornea and the lens.  The brain then interprets the signal and makes an image.  To be nearsighted means to have the light focus too quickly because the cornea is too curved.  (This can be solved with a concave lens such as contacts or glasses).  To be farsighted means to have light focus beyond the retina.  This problem is solved with a convex lens.  Aging plays into the process of sight because as you grow older your lens loses its flexibility and there is a loss of accommodation.  You cannot see things close up because you're farsighted.


The eye senses brightness with rods and color with cones.  Cones can only sense the colors red, blue, and green so it can be fooled into thinking a computer screen is white when the screen is actually made up of tiny red, blue, and green lights.  Colors other than RBG are sensed by measuring the intensity of each color received and having the brain that message.  To be colorblind means to not be able to distinguish from red and green because one of your cone sensors doesn't work.  It does not mean you see in black and white.  In a sense we are all colorblind because we see only three colors.


Greenhouse effect by Stephanie Ho


Contrary to popular belief, the greenhouse effect is not a completely negative aspect.  In fact, it is this effect that keeps the Earth at a temperature hospitable to human life.  How the greenhouse effect works is that sunlight enters the Earth's atmosphere, where it is absorbed by the Earth's surface (which is heated up).  The Earth's surface then emits light in the form of infrared waves.  However, not all of the IR waves are able to escape the Earth's atmosphere, as roughly half of it is trapped by the molecules in the air.


The infrared waves can be absorbed by carbon dioxide, O3, and water vapor, among other molecules.  As the average temperature of the Earth's surface has increased 1C over the past few years, environmentalists and the media have portrayed this problem to be a direct consequence of human activity.  Increased burning of fossil fuels have released an increasing amount of carbon dioxide, a molecule known to further trap IR.  Evidence that human activity can substantially affect the environment has been found in "the ozone problem."  Human usage of Freon (a chloroflorocarbon) has already been proven to break up the molecules of the ozone into O and O2.  An "ozone hole" has already been found over Antarctica, where nitric acid crystals that form in spring enhance the problem.


However, the greenhouse effect is controversial because studies have shown that even with the noted increase of CO2, the temperature of the Earth should have increased by only 0.1-0.2C, rather than the observed 1C.  Some like to say that the Earth has experienced temperature changes in the past and that the increase is natural while others still argue that even this excess can be caused by second-hand effects of CO2 resulting in water vapor.  In the end, correlations cannot definitely prove cause and effect and there is no way to tell for sure if humans are directly responsible for the increased greenhouse effect.  It comes down to whether the risk that it is is worth gambling.