Light
demonstration | history
| religious beliefs | rainbows
| extra | links
Demonstration: Young's
double slit experiment
Thomas Young's 1801 double slit experiment can be easily shown using a laser.
Some distance
between the slits and the screen is needed to allow the waves to constructively
or destructively interfere with each other causing the light bands that
show that light is indeed a wave.
History
Day 1. Genesis 1:3 – And God said, “Let there be light,”
and there was light.
~AD1380? Sayana
(c. 1315-1387). Max
Müller's translation* of Sayana's commentary on the Rig Veda states:
"Thus it is remembered: [O Sun] you who traverse 2202 yojanas in
half a nimesa." This corresponds to a speed of about 302,073 km/s,
which is close to the speed of light (299,792 km/s).
*It seems it may actually have been the work of an unnamed German scholar but Müller took full credit for it.
1660s. Pierre
Gassendi proposed a particle theory of light while Robert
Hooke published a wave theory of light.
1675. Isaac
Newton published Hypothesis of Light.
1678. Christian
Huygens thought light was a wave (Treatise on Light published
in 1690).
1704. Newton published his final version of his theory in Opticks (in English; the scholarly Latin version followed in 1706).
He said light was particles. (We now call these particles photons.) Newton's
reputation helped this theory dominate during 18th century (the 1700s).
1801 (or maybe as late as 1805). Thomas
Young's double slit experiment, which was such good evidence for wave
theory of light that the wave theory held sway during most of the 19th century (the 1800s). Thomas Young also came up with the idea that different colours
were different wavelengths of light. Some think Thomas Young was the last
person to "know everything." As well as his scientific work
he was also the first person (in recent history, obviously) to translate
Egyptian hieroglyphs, using the Rosetta Stone.
1845. Michael
Faraday discovered that the angle of a polarised light beam could
be changed by a magnetic field, so light and magnetism are connected somehow.
In 1847 he proposed that light is a self-propagating high-frequency electromagnetic
vibration, which (unlike sound) does not need a medium.
1865. Faraday's work got James
Clerk Maxwell into light in a big way, and he figured out all sorts
of things, such as (in 1865) the speed of light is constant. Unfortunately that
contradicted the laws of motion which said that all speeds were relative
to the observer.
Wikipedia says "Many physicists regard Maxwell as the 19th-century scientist having the greatest influence on 20th-century physics. His contributions to the science are considered by many to be of the same magnitude as those of Isaac Newton and Albert Einstein."
1905. Albert Einstein figured out that light has a dual wave-particle
nature and explained the photoelectric effect, where electrons get liberated
after being hit by light, making solar panels possible. If the photoelectric
effect had been caused by a wave, the maximum energy of the dislodged electrons would be proportional to the intensity of the incident light. Instead
they were proportional to the frequency. So particles were back.
Einstein also discovered stimulated emission while researching the photoelectric
effect.
Einstein also figured out the solution to the speed of light being constant,
and physics students have been muttering about relativity ever since (special
theory of relativity in 1905, general theory of relativity in 1914).
1915. After ten years of work Robert
Andrews Millikan experimentally confirmed Einstein's work, all the
while trying to disprove Einstein. (This is how science should be!) Einstein got the Nobel Prize for it
in 1921 and Millikan got it in 1923, partly for that and partly for his
famous 1909 oil-drop
experiment in which he measured the charge of a single electron, showing that charge is quantised.
1974. Two slit experiment done with one electron at a time and
it STILL worked.
Since then the double slit experiment has been performed with other particles as large as buckyballs (C60) and it still works even one buckyball at a time.
Religious beliefs of above historical figures
- Pierre Gassendi – Catholic priest and astronomer/astrologer.
- Sir Isaac Newton – believed in God but not in the Trinity. He has also
been accused of astrology and numerology.
- Thomas Young – Quaker (a Christian sect).
- Michael Faraday – devout Christian, Sandemanian Church (an offshoot of the Church of Scotland).
- James Clerk Maxwell – devout Christian, an evangelical Presbyterian.
- Albert Einstein – humanist, non-observant Jew (but attended a Catholic
elementary school).
- Robert Millikan – Christian.
Note that simply because many of these scientists were Christians does
not mean that they would be happy attending our own churches. They were
mostly very much more conservative than we are.
Rainbows and other atmospheric phenomena
No two people see exactly the same bow because they depend on the relative
positions of sun, viewer, and the water drops or ice crystals causing
the refraction of light.
Rainbow.
A large continous bow 40°-42° out from its centre, the point
directly opposite the sun from the viewer (the antisolar point).
A secondary bow at 50°-53° can also form outside the main one,
with its colours reversed (red on the inside). This is not the same as
supernumary bands, shown at far right. Supernumary rainbows was evidence
that light is a wave (proved by Thomas Young).
A fogbow is
an "ordinary" rainbow that appears in fog, except that it is
colourless (or yellowish) thanks to the small size of the water droplets.
It is also called a sea dog.
Click on the pictures for larger versions. Photographs copyright ©
2004-2006 Ian Mander, except where otherwise noted.
Glory.
A small continuous bow surrounding the point directly opposite
the sun, when fog is beneath you but not above you (between you
and the sun) and so very unusual to see without a shadow. Their
size varies from 5° to 20° depending on the size of the
drops. Commonly seen from aircraft flying over clouds.
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Sun dog or moon dog.
A discontinous bright patch 22° away from sun on the horizontal
or the vertical. Made by light passing through tiny (horizontal)
hexagonal plate shaped ice crystals in thin cirrus clouds. They
are sometimes coloured (apparently if the ice crystals are all about
the same size). Also known as a parhelion (plural parhelia) or mock
sun, or paraselene or mock moon. |
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Icebow or halo.
A continuous bow, sometimes coloured, surrounding the sun or moon,
also at 22° out. They are caused by pencil shaped ice crystals,
which might be at any angle, and so diffract light at any angle,
making a full bow.
Extra
Neon lights.
When covering Module 13 – Nuclear force
we discussed electron shells and neon and I mentioned we'd get back to
it. Neon is a inert gas because its outermost electron layer is full.
This means it doesn't easily interact with other atoms. However, one of
those electrons in the outermost shell can be excited into a higher energy
electron shell. When it falls back to its normal shell it gives off a
photon with a suitable amount of energy for us to be able to see it. Hence
we get a neon light. Simple.
De Broglie wavelength.
On the subject of single electrons behaving as waves, we could add another
event to 1923: Louis de Broglie came up with the idea that matter actually
consists of waves. He assumed that any particle (electron, atom, bowling
ball, whatever) had a "wavelength" that was equal to Planck's
constant divided by its momentum. He knew that the momentum and the wavelength
of a photon are related in this way. (Photons don't have mass, but they
do have energy, and so have momentum.)
And so we have this interesting
quote:
Physical objects from quarks to planets have wavelike attributes.
The quantum nature of a bowling ball, unfortunately, is not manifest since
its equivalent quantum (or de Broglie) wavelength is so tiny that interference
effects (for example, the left part of the ball negating the right part
of the ball) cannot be detected in a practical experiment.
– American
Institute of Physics Bulletin of Physics News Number 453 (summarising
Arndt et al, Nature, 14 October 1999).
This
web site also has easy-to-understand information about it:
According to de Broglie, the wavelength is equal to Planck's constant
divided by the object's momentum; Planck's constant is very, very, very
tiny, and the momentum of a bowling ball, relatively speaking, is huge.
If you had a bowling ball with a mass of, say, one kilogram, moving
at one meter per second, its wavelength would be about a septillionth
of a nanometer. This is so ridiculously small compared to the size of
the bowling ball itself that you'd never notice any wavelike stuff going
on; that's why we can generally ignore the effects of quantum mechanics
when we're talking about everyday objects. It's only at the molecular
or atomic level that the waves begin to be large enough (compared to
the size of an atom) to have a noticeable effect.
Links
Java-based
wave tank. Sonic boom (see Module 14),
two slit experiment etc.
Some more pictures of rainbows,
halos and sundogs.
This
pic is an impressive set of halos and sundogs at the south pole.
The Atmospheric
Optics site has great photos of a whole bunch of interesting bows
and halos.
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