The Optics of Iridescence:
Why Carnival Glass is So Hard to Photograph

Highlights on Fenton Vintage Carnival Glass Bowl
photo courtesy of payrollgal

Introduction

          All carnival glass collectors know that the essence of carnival is the ever-changing rainbow play of light on the glass, the effect called iridescence.  Like a peacock feather or the rainbow hues of a soap bubble, a carnival glass piece seems to change color as you watch it -- the colors darken and brighten, shimmer and intensify, if you simply tilt it back and forth.  And, as any carnival glass seller will tell you, this effect is nearly impossible to capture in a photograph.

HOW IRIDESCENCE WORKS

           What causes carnival glass to be iridescent?

           Ordinary white light is really made up of a spectrum of colors that run from red to violet.  In optics, a color is called a "wavelength".  So, in the picture below, you see white light striking a shiny surface (like a mirror), and various colors -- such as red, green and blue "wavelengths," reflecting off the surface:

White light is made up of a spectrum of colors.
Notice that the red light's waves are long and spread out,
while the blue's are very short. 

If you were standing to the right of the picture above, what you would see is reflected white light -- all the colors of the spectrum would bounce back in a unified beam that looked like ordinary white light to you.  Notice, for the time being, that the red light's wavelengths in the above picture are long and spread out, while the blue's are short, and the green's in between.  You may have heard of "short-wave infra-red" light or "long-wave ultraviolet" light:  these phrases refer to the fact that different colors have different lengths of light waves.

          Now, let's move on to iridescence.  Iridescence is an intense play of light on the surface of something:  carnival glass, a soap bubble, an oil slick, a peacock feather, and even the breast feathers of a pigeon.  When you look at a piece of iridescent carnival glass, what happens is that the light wavelengths are reflected in a way that makes them seem to flicker and pulse.  Iridescence is caused by something called interference.

          So what's interference?  It's what happens to light when it falls on a thin semi-transparent layer of any substance.  What causes interference in carnival glass is that a thin layer is sprayed onto the surface after the molten glass cools.  Whenever light falls on a thin semi-transparent film -- like an oil slick on a water puddle, or the thin coating on carnival glass, some light is reflected off the top of the layer, and some is reflected off the surface below:
 

    

Red light wavelengths reflecting off the top and bottom
of the thin layer applied to carnival glass

 
In the picture above, the red arrows are the red wavelengths of the light hitting a thin semi-transparent layer.  Some of the red light bounces off the top of the layer.  The rest travels through it and bounces off the bottom of the layer. The two reflected wavelengths now travel back together to meet your eye.  If the two wavelengths are in sync, this intensifies the red light you see:  if you were looking at the double reflection of the red light, you would see the brilliant red iridescence of the carnival bowl shown below:

Detail of red Fenton Stag & Holly carnival bowl
photo by antiquersfind

          What does it mean to say that the two wavelengths are in sync?  Well, see how the light wavelengths are wavy?  They have crests and troughs like ocean waves.  When the crests on one wavelength match up exactly with those on the other, then the light from the two combines and becomes more intense.  Thus, in the picture below, the blue wavelengths in row A would combine to make intense, blue iridescent light:

The blue light wavelengths above are in sync,
while the red are not; the green are almost in sync:
If you looked at these you'd see strong blue, 
subtle green and no red highlights.

In B, the green wavelengths don't meet up exactly, and so the light would be weakened:  when the wavelengths met your eye, you'd see green, but it wouldn't be a very strong color.  And in C, the red light waves would cancel each other out, because the troughs of the first ray coincide with the crests of the second ray.  As a result, you wouldn't see red at all.  If you were looking at carnival glass, what you'd see is intense blue iridescence with subtle green highlights and no red highlights -- like the ring of blue-green iridescence around the center of this bowl:

A ring of greenish-blue iridescence
around the center of a green Fenton Vintage carnival bowl
photo courtesy of payrollgal

          What determines whether the rays will be in sync?  This is not so hard to understand -- think of a ball thrown at you that bounces on the ground before getting to you.  The angle at which the ball first hits the ground will determine the angle at which the ball then bounces off to reach you.  In iridescence, the angle at which the original light rays hit the glass (called angle of incidence) determines the best angle from which you might see the reflected light (called angle of observation). Together, the two angles have a big effect on whether two reflected light rays bouncing off the iridescent layer on carnival glass will be in sync when they meet your eye:

The angle of the white light (rainbow-colored arrows)
is called the angle of incidence.  The angle of the reflected
blue wavelengths is called the angle of observation.

In optics, the angle of the white light hitting the surface (the rainbow-colored rays) or angle of incidence affects how far the light travels before it hits the bottom edge of the layer.  Light (the blue rays above) then bounces back upward and travels a second distance through the layer to meet your eye at the angle called the angle of observation.  The science of optics uses a very simple formula based on these two distances the light travels.  If you subtract the distance from c to d from the distance the light travels from a to b to c through the layer, the number you get is called the phase difference. 

          Once you have this number, you apply a mathematical formula:  if the phase difference equals exactly the size of the blue light's wavelength (from the top of one crest to the top of the next), or is a multiple of that number, the two beams will be in sync and produce intense blue light.  In the above picture, the blue light's short wavelengths are much shorter than the phase difference.  (Assume, in this example, that the phrase difference also does not equal a multiple of the exact length of a blue light wavelength.)  As a result, the two blue light waves are out of sync and will not produce blue iridescence.

          If you think about it, the thickness of the reflecting layer (that is, of the coating on the carnival glass), will make the distance light travels from a to b and then to c shorter or longer too.  Thus, the thickness of the layer also will affect which color wavelengths are "in phase".  As a result, if the coating on carnival glass varies in thickness from one place to the next, this will produce different iridescent colors on a single piece:   
  

Blue, green, gold, pink and violet iridescence
on surface of Fenton Heart & Vine bowl
photo by lo_rock

PHOTOGRAPHING CARNIVAL GLASS

          Think again about the fact that the angle from which the light strikes the glass (angle of incidence), and the angle from which you're looking at it (angle of observation) affect how far the light travels from a to b and from b to c.  This is why the iridescence on carnival glass seems to flicker -- to appear and disappear -- as you hold a bowl or plate up and move it to see it at different angles.  By moving the bowl, you're changing the angle from which you're looking at the glass -- in other words, your angle of observation.  It's this very aspect of carnival color -- its constantly shifting quality, that makes the glass so beautiful and magical:

        

By shifting a carnival bowl slightly in your hand,
you change your angle of observation, and thus,
the very colors of the iridescence.

photos courtesy of payrollgal

          This is why carnival glass is so hard to photograph:  you have to photograph the bowl at exactly the right angle to get the perfect angle of observation, and thus the phase difference that produces that incredible flicker of color you know you just saw a second ago.  You shift the bowl to change the angle of observation, but even this doesn't quite do it, because what you really would like to show is how the bowl changes from one beautiful intense color to the next -- what it looks like from several angles of observation.  You can't, however, move the bowl in front of the camera to show how the colors shimmer and change -- you'll just get a blurred picture.  A camera lens can snap a picture from only one angle of observation at a time.  And yet, it's the shimmering, changing effect that is the essence of iridescence and of carnival glass.

          So there it is -- now you know why carnival glass iridesces, and why it's so hard to capture carnival glass on film.

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          Many thanks to E-Bayers  antiquersfind,  lo rock  and  payrollgal,  for generously contributing their superb photographs.  Rights to photos belong to the photographers, and photos should not be used without their permission.  Text is (c) 2007 curculiosglass, all rights reserved.  To locate any E-Bay seller mentioned here,  just click on "Site Map" at the bottom of your E-Bay screen, and then click on "Feedback Forum" at the right top corner of the large menu that pops up.  Type or copy the seller's name into the Feedback Forum's search blank. 

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