# How to bend light?

As we all know that light travels in rectilinear motion. But can we bend light in parabolic path? If not practically then is it possible in paper? Has anyone succeeded in doing that practically ?

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## 4 Answers

Light does not, in general circumstances, travel in straight lines (although it does do so in the ones we usually encounter).

For one, light is really a wave and can only approximately be thought of as consisting of independently-propagating rays. This happens when the wavelength of the light is much smaller than the distances it is propagating over, which is usually the case for light (whose wavelength in the visible range is $0.4$ to $0.7\,\mu\textrm{m}$) but is not necessarily the case e.g. for radio waves and when nanoparticles are involved.

In this short-wavelength limit, wave propagation gives way to ray propagation (which is a special, approximate case of the former), and specifically to Fermat's principle for the mathematical description of light. This principle states that light rays starting at $A$ and ending up at $B$ will follow the path that minimizes the travel time $$S=\int_A^B n(s)\textrm{d}s,$$ where $n(s)$ is the (possibly spatially dependant) refraction index along the path.

For a homogeneous medium, this does indeed give straight lines for propagation. For a planar interface between two different media it gives Snell's law for refraction and it also describes reflection. (However, because it does not account for the actual nature of light as an oscillating electric field, this description cannot predict transmission or reflection coefficients.

However, if the medium is not homogeneous, then light will not travel on a straight line, and for complicated inhomogeneities the path can be correspondingly difficult to calculate. For an example, see the formation of mirages or more generally atmospheric refraction. Conversely, if one has a path one wishes a given light ray to take, then it is possible to engineer a refractive index spatial dependence that will make light bend that way. (Of course, whether such a dependence is physically reasonable is another matter; if the path bends too sharply then it may not be possible to find materials with the correspondingly large index and index gradients necessary.)

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To generalize all the nice answers here, we can bend light in almost any shape using optical fibres or photonic crystals. Although it may look artificial, it is basically equivalent to all other methods because is governed by the same laws of physics.

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We bend light all the time - using lenses.

Light bends when going from one material to another, due to conservation of momentum.

Snell's law describes how light bends.

Light is also bent when traveling past massive objects - look into "gravitational lensing" if you are interested.

Light can be effectively bent into a parabolic path using materials that have changing index of refraction. This is done in fiber optics using "graded-index fiber."

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sorry I forgot to add parabolic path to it! I have now done that! – Pranit Bauva Jun 16 '12 at 14:26
I wouldn't say that light is anyhow (effectively or not) bent in the graded-index fibres. It's as misleading as to draw light ray reflection in a step-index fibre. In a straight graded-index single-mode fibre, light propagates along a straight line because it forms a standing wave in the transverse direction, so there is no propagation. – texnic Jun 18 '12 at 14:49
@texnic : but in multimode graded-index fibres the light indee follows a sinusoidal path. – Frédéric Grosshans Jun 18 '12 at 16:14

I agree to 16BitTons. Its stated that light travels in straight line,but owing to the huge number of optical instruments we use today,namely lenses,mirrors,prisms,etc. we are able to change the direction of the light's motion,deviate it from its actual path and hence'bend'it.Here I would also suggest that like in geometry we know that circle is a combination of a number of small straight lines joined together to form the final shape.Try on taking triangle,square,pentagon.....,icosagon,... and as you go up higher and higher the shape tends to become a circle more. If a similar experimental setup can be arranged to prepare a part of the curve using a combination of a number of mirrors, and then the light is made incident upon from one end, then we may be able to view the "bending" of the light from the other end.

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