Young's double slits
Young’s experiment is one of the classic physics experiments and was one of the first
to prove the ‘wave theory of light’ i.e. light is a wave. Previously light was thought to
be a stream of particles – the ‘corpuscular theory’.
If two waves have the same frequency and wavelength, and a constant
phase relationship, then they are said to be coherent. Coherence is essential to
produce a steady interference pattern. Separate lamps are not coherent wave sources,
and cannot therefore be used to produce an interference pattern.
In 1801 Thomas Young got around this problem by passing the light from a
monochromatic source through an initial slit, and then allowing it to pass through a
pair of narrow parallel slits.
The light diffracted through the pair of slits interferes and produces a series of bright
and dark bands. Bright fringes are lines of constructive interference where light from
the two slits arrives in phase and reinforce each other, and dark fringes are lines of
destructive interference where the waves cancel each other out.
This pattern of bright and dark lines is known as a fringe pattern, and is easy to see on
a screen. The bright fringe in the middle is caused by light from the two slits
travelling the same distance to the screen; this is known as the zero-order fringe, A.
The dark fringes B on either side of the zero-order fringe are caused by light from one
slit travelling half a wavelength further than light from the other slit. These are
followed by the first-order fringes (one on each side of the zero-order fringe), caused
by light from one slit travelling a wavelength further than light from the other slit, and
so on.
The diagram above shows the geometry for the fringe pattern. For two slits separated
by a distance d, and emitting light at a particular wavelength, light will constructively
interfere at certain angles. These angles are found by applying the condition for
constructive interference, which in this case becomes:
The angles at which dark fringes occur can be found be applying the condition for
destructive interference:
The Young Equation is usually written as:
λ = ay / D
where
λ = wavelength of light
a = slit separation
y = fringe separation of adjacent bright or dark fringes
D = slit to screen distance
Note: Factors which affect the fringe separation.
(i)
(ii)
(iii)
(iv)
Increasing D increases the fringe separation
Decreasing a increases the fringe separation
Increasing the slit width produces a more intense but a more blurred
pattern
Moving the source closer to the slits increases the intensity of the pattern
but has no effect on the separation.
Thomas Young was a not only a physicist but also a physician and Egyptologist, who
was responsible for deciphering the Rosetta stone.
Diffraction Grating
When there is a need to separate light of different wavelengths with high resolution,
then a diffraction grating is most often the tool of choice. This "super prism" aspect of
the diffraction grating leads to application for measuring atomic spectra in both
laboratory instruments and telescopes. A large number of parallel, closely spaced slits
constitute a diffraction grating. The condition for maximum intensity is the same as
that for the double slit or multiple slits, but with a large number of slits the intensity
maximum is very sharp and narrow, providing the high resolution for spectroscopic
applications. The peak intensities are also much higher for the grating than for the
double slit.
When light of a single wavelength, like the
632.8nm red light from a helium-neon laser
at left, strikes a diffraction grating it is
diffracted to each side in multiple orders.
Orders 1 and 2 are shown to each side of the
direct beam. Different wavelengths are
diffracted at different angles, according to the
grating relationship
Remember the equation ! WJEC uses the following notation:
nλ = d sin Ө
Web sites.
Single slit applet:
http://webphysics.ph.msstate.edu/javamirror/ipmj/java/slitdiffr/index.html
Young’s slit notes:
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
http://www.eiu.edu/~mediasrv/davis/chapter_20/ch20_2.htm
http://tonydude.net/physics197/phys197.html
Theory of slits:
http://www.jcphysics.com/toolbox_indiv.php?page=27
Slit diffraction paterns:
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
http://online.cctt.org/physicslab/content/PhyAPB/lessonnotes/twosourceinterference/d
oubleslitinterference.asp
Applets at:
http://physics.uwstout.edu/physapplets/Northwesten/www.physics.nwu.edu/vpl/optics
/diffraction.html
http://micro.magnet.fsu.edu/primer/java/doubleslitwavefronts/
http://www.colorado.edu/physics/2000/schroedinger/two-slit2.html
Information on light phenomenon:
http://www.exploratorium.edu/xref/phenomena/interference.html