DrPhotoMoto PRO 3:13am, 5 October 2011
Like many of the black swallowtails, the Pipevine Swallowtail has pretty orange spots. The orange color is caused by deposits of pigment. The Pipevine also glows a blue color when viewed in bright sunlight. This is not caused by pigment but rather by a process called Thin-Film Interference (TFI).

The process of TFI is dependent on the angle of the light hitting the butterfly. I have found through trial and error that early morning and late afternoon are the best times to catch this blue color. I assume it is because the sun is lower in the sky at these times and hits the wings near to the perpendicular. Mid day it is harder to catch the blue color with as much intensity. The above shot was taken close to 10:00 am which is about when the butterflies really begin to appear and, coincedentally, is about when I have had enough coffee to venture outdoors.

When light reflects off a membrane with many layers, thin-film interference (TFI) can occur. TFI is a process that causes certain frequencies of incident light to be magnified (constructive interference) while other frequencies cancel out (destructive interference) during the reflective process. When the thickness of the membrane is just right (close to the wavelength of visible light), the process of TFI can turn white light into the colors of the rainbow.

TFI is the same phenomenon that causes color in soap bubbles and oil films. To view the phenomenon in the Pipevine it is handy for them to land with wings facing the sun.

This link is to the photo from which the group icon was made. In this photo are a number of explanations and links about TFI. I am not an expert on this subject so if there are any physics majors out there who can add to the discussion please do so!
DrPhotoMoto PRO 7 years ago

This article in wikipedia discusses iridescence. I can think of examles such as the color in glistening sea shells and the red on the neck of ruby throated humminbird. I hope people are interested in this phenomenon and find more examples to display in the group.
DrPhotoMoto PRO Posted 7 years ago. Edited by DrPhotoMoto (admin) 7 years ago
If you are more of a mathematical person then you can look at this reference: In it you will see the math and physics explanation for TFI.

Basically one theory of light is that it is an electromagnetic wave. Just like under the right conditions sound waves can cancel each other out, light waves can also cancel each other out. Noise canceling earphones work by generating waves that cancel out noise. The physics term for this is destructive interference.

White light contains a broad spectrum of light waves. In other words each color has a differnt wave length and frequency. The speed of a wave equals the wave length times the number of waves per second (frequency). In a vacuum, all light waves move at the same velocity. Therefore as the wave length gets smaller, the frequency has to get higher.

A film is a reflective layer that has an inner and outer surface. If the distance between these layers is in the range of a wavelength of light then the conditions to cause TFI, i.e. wave cancellation or amplification, is ripe. Since reflected light has to travel into and out of the mebrane, it travels twice the distance of the membrane thickness. That is called 2d in the reference. If this distance is equal to one wavelenth (or an even number of wavelenths) then the waves will add together and be brighter. If this distance is exactly half a wavelength (or an even multiple of this) then the waves will cancel each other out.

If you understand the above you are ready to look at the referece about the mathematics of TFI. It is not necessary to understand the math to understand what happens. All you have to do is look at some examples and your experience will intuitively take over. It is nice to get a handle on the mathematics and physics of TFI.

For example when you realize that only some waves will have the right wavelength to cause cancellation then it makes sense that certain colors will not reflect off of a membrane. Therefore what does reflect will no longer be white light but will be white light minus the frequencies that are cancelled.

The frequencies that don't get cancelled will be missing the ones that do get cancelled. Some of the frequencies that get reflected will get amplified. When you add together the colors that get reflected it is like mixing paint dyes at the hardware store. We all know that if you mix certain colors together you get a new color. Now realize that TFI is also dependent on the angle of the reflected light. These are the factors that cause the irriidescent glow of certain membranes.

Good luck reading and understanding the math.

Be thinking about where you have seen this phenomenon. We have mentioned soap and oil films, butterfly scales, and feathers. Industry has harnessed the principles of TFI and everday you can see examples of this.
Bushra & Sons 7 years ago
Great info thanks :)
DrPhotoMoto PRO Posted 7 years ago. Edited by DrPhotoMoto (admin) 7 years ago
I was looking at a shot of a CD that was very colorful and found this interesting reference that points out the source of the color in light reflection off a cd is due to TFI. I can't help but think that diffraction from the grooves in the CD also plays a role.

The next comment by Robert of 60 EOS D shows the color produced by white light reflecting off a CD. Thanks Robert for sharing your marvelous shot. I also love the refractions produced by the beautiful water drops.
Robert~EOS~60D 7 years ago
Cd Colour bubbles
DrPhotoMoto PRO Posted 7 years ago. Edited by DrPhotoMoto (admin) 7 years ago
Martin's Refrain

There is color produced as light reflects off the arms of this seed parachute. I am not sure but I believe this is TFI. Some would say diffraction plays a role and so maybe it does.
DrPhotoMoto PRO Posted 7 years ago. Edited by DrPhotoMoto (admin) 7 years ago
When light reflects off of something and a color (or colors) appear that is not noticable in low light, then you are probably witnessing thin film interference. The flash brings out the red in the wings of this pretty fly.
TFI in Wings
DrPhotoMoto PRO Posted 7 years ago. Edited by DrPhotoMoto (admin) 7 years ago
Thin Film Interference is just one way of making color. You can easily think of more ......

prisms ....
pigments ...
pruinescence ...
glowing hot objects (black body radiation) ...
diffraction ...
to name the few I can think of.

Of all these diffraction is the most difficult for me to understand and so close to interference as occurs in TFI that I don't ususally look for it. The color seen in a spider web is said to be from diffraction.

I would like to start a group for each of these methods of color production:
1. pruinescence
2. prisms body radiation.

Pigments might be interesting but I don't think I would take this one on. Let me know if you are interested. I would love to have some help if there are any color fanatics out there. Math is not a requirement. Just interest.
Here is a link to my set on pruinescence:

Here is a short article about diffracton from Wikipedia:

Here is one on black body radiation. I have graduate degrees in science and I have trouble getting through this math. I mainly placed the article here so you can see that when objects heat up they radiate light. In fact the yellow in a candle flame is from elemental carbon (soot) that has not yet burned but is hot enough to glow yellow or blue. Fascinated?
DrPhotoMoto PRO Posted 7 years ago. Edited by DrPhotoMoto (admin) 7 years ago
Here is a shot of a jumping spider.
Happy St. Patrick's Day
Notice there is a band of green across the brow.

In this angled view only a glimmer of green is seen from the back of the abdomen. The head looks pretty black. At this angle the head is absorbing all light.
Jumping Spider

Finally in this angle green shows up quite well. This is the right angle for thin film interference to take place. Green is being contructively enhanced while the other colors are destructively interfered with (like sound reducing headphones).
Irish Jumper
When viewed with the eyes, the green seemed even more noticible though it was iridescent (wavering due to angle effects).
DrPhotoMoto PRO Posted 7 years ago. Edited by DrPhotoMoto (admin) 7 years ago
The Ebony Jewelwing is one of the two "Broadwing" damselfies we see in the US. This is the male and he has coal black wings and a shiney green body.

Color Green

The green seen on the body of this male Ebony Jewelwing damselfy contrasts nicely with the pigment of the leaf. There is a metallic lustre that resembles what you might see if green light was reflected off of tinfoil. Why is this?

The surface of the jewelwing's body is composed of multiple layers of partially reflective film. As light waves penetrate and then reflect off of this house of mirrors, the reflected waves "bounce back" out of phase with one another. Because of the interesting physics that takes place as these reflected waves interact with each other, some waves get canceled out while others are seemingly amplified. In this example we see an iridescent green that glimmers with the neighboring frequencies of yellow and blue.

This process of color production is called thin film interference or TFI for short. All colors can be produced by this process depending on the distance separating the reflective layers. An oil slick or a soap bubble is more fluid than the exoskeleton of a bug. For this reason we can see the entire range of colors in a soap bubble.

In the leaf, by contrast, we see green as a result of the molecule chlorophyll. The Great Designer of the Universe created chlorophyll to convert the energy in sunlight to a storable energy that sustains life (glucose.) Just as the wooden body of a guitar resonates with the sound of the strings, the electrons that bind the carbon atoms of chlorophyll resonate with visible light. Only one frequency escapes the clutches of this light magnet. That color would be green.

There are other ways to produce color but I have touched on two mechanisms seen in this photograph.
Sienna62 7 years ago
Very interesting information! Thank you for posting
DrPhotoMoto PRO 7 years ago
Thanks for visiting.
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