# Can light be diffracted the same way other waves can?

According to what my teacher at school taught me, light can be diffracted just like any other wave. A friend of mine, however, says that light doesn't diffract, and that it reflects so that it seems as though it is diffracting.

He pointed me toward a certain experiment using a pinhole in paper and a source of light in a dark room, saying that it proves light doesn't diffract.

I, since my friend's explanation of the experiment was very rudimentary, am looking for a final say in the matter. Can light be diffracted (normal speeds, no medium change)?

• here is the wiki page for diffraction, with a picture of red light diffracting in the top left – By Symmetry Apr 19 '17 at 19:33
• The top right but otherwise spot-on :) – NickD Apr 19 '17 at 19:40
• @Nick - but is it a diffraction spot-on? – Jon Custer Apr 19 '17 at 19:49
• @JonCuster: spot-on, spot-off: what's the diffraction? – NickD Apr 20 '17 at 6:35

# Your friend is wrong: light can be diffracted

Here is an awesomely simple experiment you can use to prove your friend wrong: a DVD and a laser/LED pointer.

http://www.sciencebuddies.org/science-fair-projects/project_ideas/Phys_p011.shtml

We do not often see the effects of light diffraction in the same way we can see with other waves though because seeing diffraction requires two things:

1. coherent waves
2. openings / obstacles that are sized near the wavelength of the wave

Light is a problem for both point 1 and 2 in that most light sources are not coherent; you get waves of many different wavelengths, and they come in a jumble out of phase with each other. Also there are very few items in our daily lives that have a structure that are of micrometer size. The pin hole your friend used is too large by a factor of about 10 to 100.

But if you do get hold of a coherent light source (laser, LED pointer) and something that is of micrometer size (the information-carrying grooves of a CD/DVD), then it is easy to show diffraction effects.

• Shine a laser by the edge of anything and it will show diffraction. – Lambda Apr 21 '17 at 1:32

Diffraction occurs when a certain condition is met. That is- the size of the obstacle is close to the wavelength of light. Tell your friend that the pin hole experiment that he did isn't sufficient to disprove the diffraction of light because the pin hole is too large compared to the wavelength of visible light.

Also, light is special in a way that we can't say if it is a wave or a particle or whatever. We can just study the different properties of light from different experiments. If we change experimental conditions then we may observe different effects. It doesn't change the fact that light does behave like waves under certain conditions.

• Everything is a wave and a particle. Even you are. It is just that you are so hugely massive that any movement you perform at faster than $20 mm/s$ will make your wavelength shorter than the Plank constant. :) – MichaelK Apr 20 '17 at 7:31
• @a25bedc5-3d09-41b8-82fb-ea6c353d75ae Yeah.. sure everything is a wave and a particle but light is special since light is neither a wave nor a particle. The dual nature of light is just a convenient way of writing scientific literature. Well of course this is due to QFT and Copenhagen interpretation and beyond any individual's imagination – Sad_lab_rat Apr 20 '17 at 9:13
• "Ceci n'est pas une pipe". Light can be modelled both as waves and as particles. What light actually is is anyone's guess. As with everything in physics, we are always dealing with models and theories. For as long as reality plays nice and behaves according to our models and theories, then we are happy. When reality breaks away from the models and theories, we are even happier, because then we get to make new models and theories. :) – MichaelK Apr 20 '17 at 9:17
• @a25bedc5-3d09-41b8-82fb-ea6c353d75ae I somehow feel that we both are saying the same thing with different words :D Cheers! – Sad_lab_rat Apr 20 '17 at 9:19