Restatement of title:

Would an obstacle still cause diffraction of light if it has the same refractive index as the surrounding material?


1 Answer 1


Generally speaking we use the term diffraction when we have some apparatus that blocks part of the light. So for example a diffraction grating absorbs light except at the slits in it. When no light is being absorbed we normally use the term refraction. They're both the same physics, but the distinction is often convenient. Your question implies the obstacle doesn't absorb light, so your question is about refraction not diffraction.

Anyhow, refraction relies on different regions of the light encountering media with different refractive indices. The differing refractive index causes a path dependant phase change in the light that leads to refraction. In the case you describe where the refractive index is everywhere constant there will be no refraction.

This is used in the measurement of refractive indices of powders. If you do first year crystallography at university you are likely to do a practical where you measure the refractive index of a powdered mineral by placing the powder in liquids of different refractive indices and measuring the light absorption. When the refractive index of the powder is the same as the refractive index of the liquid the powder becomes virtually invisible and the light absorption falls to almost zero.

  • $\begingroup$ Couldn't put it better. You put my own answer to shame xD $\endgroup$
    – Dayman75
    Commented Apr 29, 2015 at 14:56
  • $\begingroup$ Great answer and can't up vote you answer due to reputation. Suppose now there is a slight increase in the obstacle's refractive index. I now assume there are both refraction and diffraction simultaneously by the incident waves. Is there a something that tells me when refraction vs. diffraction dominate the light impinging on this transparent obstacle? This is really the question that I was trying to ask and didn't realize until your well explained answer. I hope this is clearer. $\endgroup$
    – Los
    Commented Apr 29, 2015 at 20:04
  • $\begingroup$ Is it okay to change the question title and modify the question itself once it has been asked? $\endgroup$
    – Los
    Commented Apr 29, 2015 at 20:06
  • $\begingroup$ @Los: it's generally frowned on if you change the question so much that the existing answers become invalid. But if you're just going to change diffraction to refraction then go ahead. $\endgroup$ Commented Apr 30, 2015 at 5:09

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