Sat 5/2/09

A Daily Walk With DietPower

Walking is the best exercise for weight loss. And the things you see!

I found this gasoline rainbow on Judith Drive. I'm sure you've seen this phenomenon yourself, and if you're a curious person (I pray you are), you've probably wondered what causes it.

It's difficult to explain and usually involves diagrams, but I think I can make it clear in words. ( if I'm wrong.) Once you understand it, other color phenomena will begin to make sense, too.

A few drops of gasoline fell on wet pavement. Since gasoline is lighter than water, the gasoline floated. Because gravity tried to pull all the gasoline to the same level, it spread out. It stopped spreading when the tightness of its skin finally balanced the force of gravity. The resulting film was incredibly thin, because gasoline is even waterier than water.

Nevertheless, the middle of the film was thicker than the edge, because the skin tightness kept the film mound-shaped by a few millionths of an inch. And that's why the film makes a rainbow.

(Pay attention, now. We're getting to the good part.)

Some of the light waves hitting the film pass through and bounce off the bottom, but when they come back out they are canceled by other waves arriving behind them. Whether a wave is canceled or not depends on its wavelength (color) and the thickness of the film. In the middle of the film pictured above, long red waves are canceled, letting the remaining short blue waves dominate. That's why the middle looks blue. Farther out, as the film grows thinner and thinner, shorter and shorter waves are canceled, letting longer and longer waves dominate. That's why the color gradually changes from blue to violet to red to orange as you travel toward the edge.

In short, a gasoline rainbow performs the same selective filtering of colors as a rainbow in the sky. The main differences are that the medium is gasoline instead of water and the colors are filtered by interference instead of bending.

And of course there's no pot of gold at the end.

There. That wasn't so bad, was it?

About this page: Precisely at noon each day, I step out of my office for a 3.5-mile walk around my Connecticut neighborhood. I carry a Panasonic Lumix DMC-TX5 pocket camera with a Leica 10x optical zoom lens. My object is to make an interesting photograph of at least one thing that is different that day. I post the results here, hoping they will inspire you and your friends to walk, too.  —Terry Dunkle, DietPower founder and CEO.

Click here to return to today's page

All My Yesterdays

Friday, May 1: The Duck and the Bashful Maiden

Thursday, April 30: A Poison Ivy Sandwich

Wednesday, April 29: The Very Picture of Spring

Tuesday, April 28: A Busy Bumblebee

Monday, April 27: Electric Pink

Sunday, April 26: Saturday Night Special

Saturday, April 25: An Old Oak Falls

Friday, April 24: How an Ant Sees a Daffodil

Thursday, April 23: The Nameless Brook

Wednesday, April 22: Weeding Time

Tuesday, April 21: Wet Apple Buds

Monday, April 20: Mr. Allen and the Crew Team

Sunday, April 19: Bloodroot II

Saturday, April 18: Green Jellybeans

Friday, April 17: Bloodroot

Thursday, April 16: Skunk Cabbage III

Wednesday, April 15: Find the Critter

Tuesday, April 14: Blessing of the Animals

Monday, April 13: The Crow Who Said "Wow!"

Sunday, April 12: A Quirky Church

Saturday, April 11: Self-Portrait in a Pothole

Friday, April 10: Easter flowers

Thursday, April 9: Dumb as a Squirrel

Wednesday, April 8: April Snow

Tuesday, April 7: Egg Trees, Connecticut Style

Monday, April 6: I Carry My Lunch

Sunday, April 5: A Tree in Spring

Saturday, April 4: Pigs with Drivers Licenses

Friday, April 3: Forsythia

Thursday, April 2: Skunk Cabbage II

Wednesday, April 1: Mystery of the Hanging Shoes

Tuesday, March 31: Downy Woodpecker

Monday, March 30: 300-Year-Old House

Sunday, March 29: The Broken Fence

Saturday, March 28: "You're Such a Delight"

Friday, March 27: Skunk Cabbage

Return to today's page