To me, as a macroscopic observer of light, it appears that light moves in straight lines. If I shine a light at object A and object B moves between me and object A, the light hits, i.e. gets blocked by object B and no longer hits object A.

However, since light is a transverse wave, doesn't that mean it is oscillating back and forth in space in a dimension perpendicular to its direction of travel? If so, then perhaps it only appears to move in straight lines because of low amplitude or high frequency?

Thus, say there was a radio wave with a wavelength of 1 meter. Could this radio wave then dodge around object B and still hit object A, assuming object B is smaller than 1 meter, say a basketball?

  • $\begingroup$ Just as an experimental remark, diffraction and interference of light are easily visible even to the "macroscopic observer" if one knows suitable experimental techniques. Early experiments that could demonstrate it with simple means go back to the early 19th century, if I am not mistaken. Interference on thin films has probably been known since soap bubbles and oil films have been around, i.e. for thousands of years... people just didn't know what they were looking at. Nature, by the way, has used interference for millions of years to produce saturated colors e.g. in butterfly wings. $\endgroup$ – CuriousOne Oct 26 '15 at 2:05
  • $\begingroup$ I have edited your original title because photons have fixed energy given by E=h*nu . The light wave emerges from zillions of photons but photons are not light waves . see motls.blogspot.com/2011/11/…. Individual photons move in straight lines as they are elementary particles , until they interact or are absorbed. It is the light wave that has amplitude and can diffract $\endgroup$ – anna v Oct 26 '15 at 4:42
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    $\begingroup$ "since light is a transverse wave" means that the electric field (and magnetic field) oscillate in a direction perpendicular to the direction of motion. Nothing is physically moving perpendicular to the direction of motion. (This is one of the ways these waves are different from transverse waves in telephone cords, springs, and ropes.) $\endgroup$ – Eric Towers Oct 26 '15 at 6:47

Thus, say there was a radio wave with a wavelength of 1 meter. Could this radio wave then dodge around object B and still hit object A, assuming object B is smaller than 1 meter, say a basketball?

Waves of large wave lengths can indeed 'wash around' an object that is sufficiently smaller than the wave lengths, a phenomenon called diffraction.

Have a look at the animation embedded in this link and play around with object size to see the effect. For large wavelength to object size ratios the waves down stream from the object are practically undisturbed. It's as if the wave 'doesn't see' the object.

We exploit the inverse effect in electron microscopy to see very small objects, much smaller than the wavelengths of visible light. Electron waves allow to achieve much smaller wavelengths and allows to see much smaller objects.

The importance of Long Wave radio waves for similar reasons is explained in this link.

Whether the waves are of transversal or longitudinal character is irrelevant here: sound waves (longitudinal) show diffraction too.

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    $\begingroup$ You should probably put more emphasis on the fact that while diffraction is a thing, it doesn't really work anything like the "dodging" described in the question. The "dodging" mechanism in the question would lead to effects like radio waves passing near a surface "dodging into" it and being stopped cold. It would also require amplitude as well as wavelength proportional to the size of an obstacle to pass around it. $\endgroup$ – user2357112 supports Monica Oct 26 '15 at 6:21

Aragos or Poisson spot

During the 19th century scientists were still undecided if light is a wave phenomenon or consisting of particles. Poisson thought he refuted the wave theory by predicting correctly that a bright point should appear in the middle of a shadow if the light and the object are prepared in a specific way. Arago actually tried this and saw the "impossible" spot:

Arago spot


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