# Explain how rainbow are form in term of photon

1. Mar 19, 2005

### ArielGenesis

Can some one explain how rainbow are form in term of photon. and also how photon are related into frequency.

2. Mar 19, 2005

### SpaceTiger

Staff Emeritus
Rainbows form by refraction of light from the sun off of small water droplets in the atmosphere. This is why they tend to occur after rainstorms. Refraction is basically a phenomenon in which a material "bends" light that passes through it. Common examples of refraction are when you shine light through a prism or lens, the former case creating a similar "rainbow" pattern to what you're talking about.

Why does bending of light form a rainbow? It turns out that these materials (prisms, lenses, and water droplets), bend different frequencies of light by different amounts (that is, red gets bent less than blue), so sending the light through the material acts to split it up into its component colors. Isaac Newton was the first to realize this and he did the experiment over 300 years ago.

As for the connection to photons, there isn't a particularly interesting one. Photons are the particle manifestation of light, but you can describe this phenomenon classically (that is, in the wave picture of light).

3. Mar 20, 2005

### NEOclassic

Individual photons have lengths only.

Each length is inversely proportional to its energy/color; i.e., lambda in meters multiplied by energy in MeVs is equal to ~1.24 E -12 MeV-meters. It has been that way for a few centuries; then in the late 19th century the distance traveled by a photon in a single second was accurately measured as "L" meters. How convenient it was to divide "L" by lambda to get a mere population number and then to wrongly term it a "frequency" which implies that photons are connected end-to-end in some kind of rigid straight string of Sausage-links!!!!!
Alternating electric current is a man-made phenomenon that shows a particular behavior of the flow of electrons in a looped wire circuit - for an explanation of this Phenomenon, James Maxwell deserves major credit and there can be no argument with the validity of his work. Even with the primative pre-television oscilloscopes of the WWII era, the sinusoidal undulation of Maxwell's waves were shown to be continuous as was the radio frequency extension to the alternating current phenomena. However; SFAIK, no experiment, even with coherent laser photons, has yet been devised to show the direct connection of any two photons. Cheers, Jim

Last edited: Mar 20, 2005
4. Mar 20, 2005

### gptejms

Here's something,word for word, I wrote on rainbows two years back for physics-zone,a yahoo group(hope it's useful):-

I've read a wonderful review of a book on rainbows and
I'll share what I've read.There is a primary (rain)bow
below a secondary bow---in between there is a dark
band called Alexander's band.The order of colours in
the primary bow is reverse that of the order of the
colours in the secondary bow.Just below the top of the
primary rainbow are pastel fringes called
supernumerary bows.The primary and secondary bows are
easily explainable by geometrical optics--reflection
and refraction (inside and at the surface of a water
droplet).
Supernumerary bows are explained by considering the
wave theory of light.It turns out the primary bow is
the first interfernce maximum,2nd,3rd etc. maxima
correspond to the supernumerary bows---try to figure
this out considering a wavefront incident on a droplet
and following its evolution inside and out of the
droplet.
The transition to the dark bank is not an abrupt one
due to diffraction effects.All this was wonderfully
worked out by Airy.Airy's function is the solution of
the equation
y'' + xy = 0
The solution is an oscillating function with
decreasing
amplitudes of maxima--the first corresponds to the
primary bow and subsequent ones to the supernumerary
bows.The transition to the dark band also turns out to
be a smooth one.
Even Airy's theory is an approximate one and can not
explain some of the laboratory generated rainbows.Mie
worked out this problem by considering scattering of
light off homogeneous spheres.Peter Debye also worked
on the problem.The scattering amplitudes are expressed
as an infinite sum(partial wave expansion).The no. of
terms that must be retained in the sum(before the
series can be truncated turns out to be quite high---a
lot of computer time would be required to to arrive at
a numerical solution!
A complete analysis of the problem requires
introducing a complex angular momentum variable---the
contributions to the partial wave series are then
redistributed to a set of points in the complex
plane(called Regge poles in particle physics).
There are many more aspects in this complex analysis
like 'collision of two real saddle points' resulting
in 2 complex saddle points being born!As far as the
physics is concerned,Airy's theory suffices.

Jagmeet Singh

5. Mar 20, 2005

### ArielGenesis

Firt of all, I am a secondary student. So please don't expect me to understand math and ~~~ thing.

NEOclassic, may you explain this in a simpler way ^^

the distance traveled by a photon in a single second was accurately measured as "L" meters. How convenient it was to divide "L" by lambda to get a mere population number and then to wrongly term it a "frequency" which implies that photons are connected end-to-end in some kind of rigid straight string of Sausage-links!!!!!

gptejms, it's nice actually if i cud understood it. i only understod the first paragraph.

6. Mar 21, 2005

### NEOclassic

Hi Ariel,
Assume a Sodium "D" photon whose length is ~5890 Angstroms

Its length in time is 0.00000000000000196 seconds = ~ 2 x 10^-15 earth seconds.
Cheers, Jim

7. Mar 22, 2005

### ArielGenesis

hi Jim, I still don't get it ...