Thin Film Interference

SKILL LEVEL

Elementary School, Middle School, High School

TRY IT!

For all of its complexity and mystery, light is surprisingly limited in what it can actually do.  In short, light really only do two things.  It can bounce (reflection) and it can bend (refraction).  When multiple light waves are parallel to each other, they can also add or subtract from each other in a process called interference.

In this experiment, you will use reflection, refraction, and interference to create a permanent rainbow pattern so awesome you'll want to frame it and hang it in your room, right above your periodic table and to the left of your Albert Einstein portrait.

Refraction:

You may have heard before that the speed of light is a constant, which is true (thanks Einstein!).  However, this is not necessarily true when light interacts with matter.  When light passes through a transparent or translucent solid, liquid, or gas, it slows down.  Because the light must maintain all of its wave properties, this slowing down causes the light wave to pivot when it enters a new medium at an angle.

Imagine you are in your car, driving on an asphalt road.  The road is washed out ahead due to a storm.  There is thick mud crossing your path at an angle.  The car is going to enter the mud, it's unavoidable, but the angle of the mud means your right front tire will hit the mud before your left front tire.

What will happen?

Your right tire will start to spin as it loses traction in the thick mud.  Thus, the right side of your car will 'slow down'.  Does that cause your car to rip apart? Of course not! In the same way that light will maintain it's wave properties when it enters a new medium, your car will maintain its 'car' properties when it moves from asphalt to mud at an angle.  The car will pivot, just like light!

Interference:

When multiple light waves are parallel, they can interfere with each other constructively (adding together to become more intense) or destructively (subtracting from each other to become less intense).

In this experiment, you'll be creating a thin transparent film using nail polish top-coat.  When light passes through the film, it will reflect off of both the bottom surface and the top surface.  In between, the thin film will slow down the light, causing it to refract.  When the two reflected waves leave the film, they will interfere in such a way to produce only certain colors in the visible spectrum.  In fact, whenever you see a different color, you are actually observing tiny (reallllly tiny) differences in the thickness of the film.  Very cool!

You've probably seen this same type of thin film interference in a parking lot when leaked oil from a car floats on the surface of a puddle or in a soap bubble floating in the air.

 

SUPPLIES, TOOLS, MATERIALS

  • Clear nail polish or top coat
  • Shallow disposable pan or tray
  • Card stock (we used black)
  • Water
interference

INSTRUCTIONS

  1. Fill a shallow pan with water to a depth of about 1 inch.
  2. Submerge a piece of cardstock paper.  We used a round pie-pan and thus had to cut the paper so that it would fit.
  3. Using an eye dropper, add a single drop of clear nail polish or top coat.  Notice how it rapidly spreads across the surface.
  4. Allow the layer of polish to dry on the water for 2-3 minutes, then very carefully left the paper so that the thin layer transfers to the paper.
  5. Observe your newly formed thin film interference rainbow in various lights and at various angles. Bask in the coolness of knowing this permanent rainbow is yours to keep for as long as you like!
Thin Film Interference

QUESTIONS TO CONSIDER

  1. Look up and define the following terms:
    Reflection
    Refraction
    Interference
  2. List all of the colors you observed.
  3. What can you tell about the thickness of your film from the various colors you observe?
  4. Look up an image of a blue morpho butterfly.  Can you guess what causes the brilliant blue shimmering color of its wings?