Showing posts with label Light. Show all posts
Showing posts with label Light. Show all posts

Refraction: Where’s the Test Tube?

I have been waiting nearly two years to share this with you, because I was hoping to find the appropriate lab-ware to borrow so I could photograph this VERY cool demonstration.  But, it hasn't happened yet and doesn't look like it will in the near future, so I've decided to share it with you, without any photographs.  I hope you'll try it.  If you do, maybe you'll take some pictures and be interested in sharing them!

Pyrex has the same index of refraction as corn oil. As such, this demonstration will only work with corn oil, not with any other kind of oil. 

Put some CORN oil in a beaker.

Place an empty test tube in the corn oil - it will appear bent due to refraction. Then, begin to fill the test tube with corn oil - the test tube will disappear! This could also be done with a small beaker.

If you have a mature audience, try this trick… place a broken test tube (be VERY careful, Pyrex is SHARP) into the beaker of oil (which already contains the hidden test tube), telling the students that it’s a restorative potion. Then pull out the whole, hidden test tube!

Light: Refraction: Flip an Arrow

This is a great way to kick off your study of refraction and lenses.

Provide students with a cylinder of water (without lines, words, etc. on it - check your recycling bin for empty glass bottles) and an index card with an arrow printed on it. 

Have the students observe the arrow, looking through the cylinder of water.  What happens as the move the arrow away from the water? 

How about when the move the arrow closer to the water. 

The cylinder of water is acting as a lens.  When the arrow is close to the lens, it is magnified, but pointing in the original direction.  As the arrow is moved away from the lens, it flips. 

Provide students with plenty of time to play with these simple objects, allowing them to make as many observations as possible.  Some ideas to try out:
  • watching the arrow through the water while the rotate the paper
  • holding the arrow vertically instead of horizontally
  • observing other shapes or letters through the lens
Once students have gained some hands-on experience with this simple lens, head back to the desks to work on explaining what was witnessed with words and diagrams. 

Glue Stick Magnifying Glass

Did you know a clear glue stick (for a hot glue gun) can be used as a magnifying glass?  The way the light bounces around the cylinder and refracts because of the change in material causes the glue stick to act as a lens.  Lay it over some small type and see how much larger it appears. 

Some other objects that can be used in the same way:
--a glass stirring rod
--a test tube filled with water (sealed with a stopper)

Crooked Pencil

Fill a glass with water.

Place a pencil in the water.

Look through the side of the glass at the pencil - what do you see?


The pencil looked bent because light travels slower through water than it does through air.  This causes the light rays to bend - refraction.

Light: Seeing Color

[The nature of this activity makes it difficult to photograph (at least for one with limited skills in that area, such as myself).  Try it yourself for the best results.]

A little refresher on color and light...
--White light is composed of all the colors in the spectrum (red, orange, yelllow, green, blue, indigo, violet).

--An object appears red because it reflects the red light and absorbs all the other colors.
--An object appears white because it reflects all the colors in the spectrum, and absorbs none. 
--An object appears black because it absorbs all the colors in the spectrum, and reflects none. 

What you'll need...
--a red toy car
--a blue (or other non-red color) toy car
--a red light (this could be a laser pointer - be careful!, a filtered flashlight, a toy - mine came from a cereal box)


You'll need to do this demonstration in a darkened room.  If you can't get your room dark enough, you can set it up inside a large cardboard box - but only a few people will be able to view it at a time.


To perform the demonstration...
Before darkening the room, show the students the two cars, so they can see that one appears red and the other blue.

Turn off the lights.  Shine the red light on the red car.

The car will still appear red because it is reflecting the red light (it's not absorbing anything since you didn't shine any of the other spectrum colors on it, but that doesn't change what you see).

Now shine the red light on the blue car.

The car will appear black (or at least a dark shade - to get black you'd need a totally dark room and a pure red light).

Here's why... 
Your light is only emitting red light.  The blue car only reflects blue light.  There is no blue light for the car to reflect.  The red light is being absorbed by the car.  The result, for our eyes, is black, or the absence of color.

Why is the Sky Blue?

Fill a glass with water, then add a few drops of milk so it becomes a little cloudy.

Darken the room and shine a flashlight through the milky water.

Look into the glass from above.
T
The picture exaggerates the color a bit - it looks blue, but not neon blue!

The Earth's atmosphere is filled with dust and water drops.  These particles bend the light from the sun, causing it to appear blue.  In this demonstration, the milk provided the particles to bend the light from the flashlight.

At sunset, the angle of refraction changes and you're able to see colors from the other end of the spectrum - reds and oranges.

And, if you've ever watched a sunset in a region where there have been forest fires, you'll notice even more intense colors - more particles (smoke from the fire), causing more refraction. 
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