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Can light bend around corners?
Category: Physics
Published: February 7, 2014
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
Yes, light can bend around corners. In fact, light always bends around corners to some extent. This is a basic property of light and all other waves. The amount of light that bends around a corner depends on the exact situation. For visible light on the human scale, the amount of light that bends around corners is often too small to notice unless you know how to look for it. The ability of light to bend around corners is also known as "diffraction". There are two mechanisms that cause light to bend around corners.
1. Internal Diffraction.
Light is a lot more complex than many people realize. The ray picture of light, which describes light as a bunch of arrows traveling in straight lines and bouncing off objects, is an understandable and even useful picture, but it is greatly over-simplified. In reality, light is a quantized waving of the electromagnetic field. Light is always waving against itself, leading to internal interference of the different wave components in what we call internal diffraction. This diffraction causes a beam of light to slowly spread out as it travels, so that some of the light bends away from the straight line motion of the main part of the wave. Even seemingly perfect laser beams spread out as they travel because of internal diffraction. This turning away of some of the light from the forward direction is a form of "bending around corners" even when a corner may not exist. The tendency of beams to spread out via diffraction makes it so that a light beam can never be focused to a perfect point, and, in consequence, light microscopes cannot perform infinite magnification. Many textbooks imply that all diffraction is caused by light interacting with an object. This is not strictly true. A finite beam of light traveling through free space where no objects are present will still spread out because of internal diffraction. Other names for simple internal diffraction are "beam spreading" or "beam divergence". Note that when a system involves creating multiple beams, diffraction can lead to beautiful patterns of rings or stars, but the basic mechanism is still the same: light interferes with itself.
In general, a light beam spreads out more (turns the corner more) if the beam has a narrow beam width compared to its wavelength. Light can therefore be made to spread out more by reducing the beam width or by increasing the wavelength of the light. The wavelength of visible light is so small that you have to use very narrow beams of visible light in order to notice its diffraction. Such narrow beams are typically obtained by running light through a very narrow slit. For large-wavelength light such as radio waves, the bending of the wave around human-scale objects is much stronger. Note that the light from a flashlight spreads out not because of diffraction. It spreads out because the mirror in a flashlight is specifically designed to bounce light in different directions. Also, note that the fuzziness of shadows in everyday life is not caused by diffraction, but is instead caused by the fact that an extended light source creates many, slightly-offset, shadows of the object which blur together.
2. Interaction with Objects
Light can also interact with objects in such a way that its ability to bend around corners is enhanced. Light passing through a simple slit and diffracting could be described as light interacting with an object, but such a situation is more a case of internal diffraction. The slit simply creates a narrow beam and then does nothing more, so that the diffraction in such a case results internally from a narrow beam interfering with itself. In contrast, there are cases where the interaction of light with an object does more to the light than just change its beam width. If light hits an object made out of conducting material (such as metal), the electromagnetic fields in the light exert a force on and accelerate the free charges in the conductor, thereby inducing electric currents in the surface of the conducting object. These oscillating electric currents create more light, and this light induces more currents. The end result is that part of the light that hits an electrically conductive material couples to the surface of the object and travels as a surface wave. Light can therefore bend around the corner of an object by riding the curved surface of the object. For a smooth surface, the light can travel along the surface for a relatively long distance. However, roughness, irregularities, cracks, bumps, and seams on the object's surface interrupt the coupling between light and the electric currents in the surface, so that the surface waves tends to scatter off into space at such obstacles instead of continuing to ride the surface. In optics, light waves riding the surface of a conductive object are called "surface plasmons". In radar, such waves are called "creeping waves" or simply "surface waves". In radar images, this creeping wave effect can lead to physically important ghost images or echo images of the object, because it takes longer for the creeping wave to return to the receiver than the main reflected wave.