Physics Photo of the Week

April 28, 2006

Light Interference

Photo and discussion by Finn Haverkamp

Diagram by Joseph Alward, University of the Pacific

The photo for this week is a rainbow puddle, which can usually be found after a thunderstorm. The composition is a water puddle topped with a thin layer of motor oil.  The Rainbow Puddle illustrates light interference.

Light interference occurs when multiple wavelengths of light either constructively or destructively meet. Light travels at different speeds depending on the material it’s going through. Light’s speed through a given material is called its refractive index. As oil has a higher refractive index than air or water, light travels more slowly through it. Light interference is dependent on three things: the angle of the light relative to the surfaces, the wavelength of the light, and the distance between the two surfaces (air and water). The angle of light is the location of the sun in the sky and the direction of it’s light. The distance between the two surfaces is the thickness of the oil, the space between the air above the oil and water below the oil. The oil layer is thicker in the middle and lighter on the edges, this is important to the interference of the light rays. When light hits the oil, some of the light bounces right off of it and is reflected upwards. Other light continues through the oil and bounces off the water, continuing back through the oil and into the air. Because of the oil thickness variation, all of the light rays meet back up at different angles and interfere in different ways. A set of rays may have constructive interference, while another set may have destructive interference. All of these different combinations causes the light to separate, when the rays come back together, the light combines in different ways so that our eyes perceive different colors. The diagram below shows how light interferes.

The interference of light proves beyond a shadow of doubt that light is a wave phenomenon.  More discussion in the future...

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to

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