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Name: Melissa
Status: educator
Grade: 6-12
Location: Mi
Date: December 2007

Question:
This is nearly word for word an objective out of the Michigan high school curriculum for physics in the "nature of Light" section. The objective reads "Identify the principle involved when you see a transparent object (e.g., straw, a piece of glass) in a clear liquid." I am not sure what I am suppose to be teaching. Any thoughts?



Replies:
Hi Melissa,

I think that this refers to the fact that when you do this and look into the glass sideways, the top of the straw appears to be at a different angle/position than the top of the straw due to the differences in the refractive indices of air and water. Have a look at

http://en.wikipedia.org/wiki/Refraction

for more information.

Best,

Dr. Topper


Melissa,

This is probably a lesson in refractive index and angles of refraction. Your students are required to understand that the angle of incidence will be different than the angle of transmittance if light enters a medium with a different density than from where it came from. This will mean that the piece of straw will appear bent. Another interesting experience is that an object with the same refractive index as the liquid it is immersed in will not be visible. This is an issue of having the same refraction effect.

Greg (Roberto Gregorius)


I would check out refraction, reflection and magnification. I would also call the state education board and ask them just what they mean by this benchmark.

Patricia Rowe


Hum. Good question about a questionable objective. At first I thought it might be after refraction (pencil in glass of water looks bent due to differing speeds of light) but the "transparent object" part makes no sense and it says "in a ...liquid" not part in and part out. I suspect the person/committee that wrote the objective does not understand what you are supposed to be teaching. Change State (ha, ha).

Mike Loop


Hi Melissa They might be referring to the index of refraction of the liquid and the transparent medium. If they are the same, it looks like the object disappears when submerged in the liquid. You should be able to demonstrate this with a glass test tube and a beaker of Wesson oil. If you slowly lower the test tube into the oil, the test tube will seem to disappear leaving only the writing on the tube visible in the oil. Give it a try! Hope this helps.

Bob Froehlich


I went ahead and looked up the section (Michigan has it posted on-line) just so we are clear (ha!), so here it is:

P4.9 Nature of Light
Light interacts with matter by reflection, absorption, or transmission.

P4.9A Identify the principle involved when you see a transparent object (e.g., straw, piece of glass) in a clear liquid.

I agree it is not exactly clear (again!) what they mean. Perhaps this refers to refraction, and how different materials bend light differently. The light bends as it passes from one material to the other, allowing you to distinguish two transparent objects. I am a bit confused since 'straws' are not necessarily transparent, but it makes me think of a straw appearing 'broken' when it sticks out of a clear liquid, also a consequence of refraction.

Burr Zimmerman



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