{"id":555,"date":"2011-03-01T07:30:52","date_gmt":"2011-03-01T07:30:52","guid":{"rendered":"http:\/\/occamstypewriter.org\/rpg\/?p=555"},"modified":"2011-02-28T22:06:28","modified_gmt":"2011-02-28T22:06:28","slug":"on-the-hairy-nature-of-light","status":"publish","type":"post","link":"https:\/\/occamstypewriter.org\/rpg\/2011\/03\/01\/on-the-hairy-nature-of-light\/","title":{"rendered":"On the hairy nature of light"},"content":{"rendered":"

One of the least<\/em> strange physical phenomena is that light behaves like a wave. Some of the time, at least. (The Younger Pawn asked if she could surf on light, if it was a wave. I said only if she was really really small.)<\/p>\n

Physics teachers demonstrate the wave nature of light using the double-slit, or Young’s, experiment. This is quite easy to understand, if you’re at all familiar with the movement of waves upon water: when a wave (of water or light) hits a barrier in which there is a small aperture, you get waves spreading out from the other side of the barrier.<\/p>\n

But if you have two apertures (or slits) in the barrier, then the two waves interfere with each other, and you get a well-known pattern. I was going to draw a diagram but as usual, somebody on the internet has already done it for me<\/a>. Even in three dimensions<\/a>.<\/p>\n

\"interference\"
\nOlaf Wucknitz<\/a><\/em><\/small><\/p>\n

For this to work however, you generally need a coherent light source. Traditionally this involves a pinhole and an incoherent source<\/a> (which is not what necessarily you put on your kebab after a hard night’s drinking)\u2014hence the S<\/em>0<\/sub> in the above diagram\u2014and a lens to refocus the whole shebang. In high school physics lessons they tend to use a laser as the coherent source, and highly expensive engineered twin slit gizmos that the physics teacher gets really upset about if it goes missing.<\/p>\n

A couple of weeks ago I was at a film-making workshop and somebody showed a clip that was meant to demonstrate the problems of lighting labs for camera. One of the problematic experiments had a rather large static laser, some smoke to see the beam, a screen and lots of what you might call ‘proper’ equipment. The actual experiment was incidental, but I caught a glimpse of it and was intrigued. <\/p>\n

It was, in fact, a variation on the double-slit experiment. The experimenter shone the laser at a wall, and then moved a wire into the beam. Naively, you’d expect the wire to cast a shadow on the wall. But what actually happens is that the wire acts as the partition between two virtual slits: the laser beam is split in two, and simulates a coherent light source shining simultaneously through two slits. And you get the laddering pattern on the wall.<\/p>\n

Cool, I thought.<\/p>\n

Wait a minute, I thought.<\/p>\n

I’ve got a laser. I’ve got several in fact: weedy 1 milliWatt laser pointers with the old F1000 branding. I also, because every kid knows lasers are cool and I’m just a big kid at heart and it was there<\/em>, have a green 50 mW laser which is a whole heap of fun. I mainly use it for intimidating the dirty pigeons in our garden.<\/p>\n

So I raided the hairbrush, held a hair over the end of my laser pointer, and demonstrated the wave nature of light:<\/p>\n

\"wave<\/a>
\nPhoto by J. Rohn<\/em><\/small><\/p>\n

The distance from the central spot to the middle of the second maximum is about 4 cm. A human hair is about 100 \u00b5m diameter. Now, constructive interference occurs when<\/p>\n

<\/p>\n

and assuming θ is less than about 10°<\/p>\n


\n(from Wikipedia<\/a>)<\/p>\n

then 2 x λ = (0.04 x 0.0001)\/4 = 1 x 10-6<\/sup> = 1 µm.<\/p>\n

So the wavelength of my laser is about 500 nm. Seeing as it’s rated as 532 nm and I’m a biologist, I’d call that a result.<\/p>\n

Postscript<\/em><\/p>\n

A couple of years ago I wrote a poem, called Morning<\/a>. It contains the line,<\/p>\n

and sunlight rainbows through Fresnel hair:<\/em><\/p>\n

which annoyed a certain Stephen Curry<\/a>, who was upset at the thought of hair diffracting anything on the wavelength of light. I took this as a sign that the good professor has no poetry in his soul, but I now feel scientifically vindicated, in that I have been able to use hair to do this experiment and Fresnel expanded Young’s famous experiment (and came up with an experimentally better version<\/a>, lasers not being available to him), supporting the wave theory of light. So there.<\/p>\n","protected":false},"excerpt":{"rendered":"

One of the least strange physical phenomena is that light behaves like a wave. Some of the time, at least. (The Younger Pawn asked if she could surf on light, if it was a wave. I said only if she … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[34],"tags":[94,93,95],"class_list":["post-555","post","type-post","status-publish","format-standard","hentry","category-science","tag-laser","tag-light-wave","tag-pawns"],"_links":{"self":[{"href":"https:\/\/occamstypewriter.org\/rpg\/wp-json\/wp\/v2\/posts\/555","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/occamstypewriter.org\/rpg\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/occamstypewriter.org\/rpg\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/occamstypewriter.org\/rpg\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/occamstypewriter.org\/rpg\/wp-json\/wp\/v2\/comments?post=555"}],"version-history":[{"count":0,"href":"https:\/\/occamstypewriter.org\/rpg\/wp-json\/wp\/v2\/posts\/555\/revisions"}],"wp:attachment":[{"href":"https:\/\/occamstypewriter.org\/rpg\/wp-json\/wp\/v2\/media?parent=555"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/occamstypewriter.org\/rpg\/wp-json\/wp\/v2\/categories?post=555"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/occamstypewriter.org\/rpg\/wp-json\/wp\/v2\/tags?post=555"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}