An experiment with spray paint
Positioned a mask with two slits in front of a wall and use a paint gun to spray paint through the slits onto the wall. What do you expect to see?
We expect to observe two lines of paint on the wall, which are images of the two slits through which the paint particles have traveled. If we make the slits very narrow and put them very close together, we expect the lines to overlap. The figure below shows the patterns we expect to observe when paint is sprayed through two closely spaced slits with only one slit uncovered or with both slits open.
An experiment with light
If we shine light onto a screen through a mask with two large holes, whose dimensions and separation are much bigger than the wavelength of light, we expect to observe sharp shadows of the edges of the holes on the screen. But as we decease the size and separation of the holes, the shadows begin to wash out and a diffraction or interference pattern appears. That pattern is a characteristic signature of a wave. The wave travels through both slits. Wavelets emanating from each slit constructively and destructively interfere behind the obstacle with the two slits.
If we let the light fall onto a screen behind the obstacle, we will observe a pattern of bright and dark stripes on the screen. This pattern of bright and dark lines is known as a fringe pattern. The bright lines indicate constructive interference and the dark lines indicate destructive interference. The bright fringe in the middle of the diagram above is caused by constructive interference of the light from the two slits traveling the same distance to the screen. It is known as the zero-order fringe. Crest meets crest and trough meets trough. The amplitude of the wave increases. The intensity is proportional to the square of the amplitude and therefore increases. The dark fringes on either side of the zero-order fringe are caused by destructive interference. Light from one slit travels a distance that is 1/2 wavelength longer than the distance traveled by light from the other slit. Crests meets troughs at these locations and the amplitude of the wave, and therefore the intensity, decreases. The dark fringes are followed by the first-order fringes, one on each side of the zero-order fringe. Light from one slit travels a distance that is one wavelength longer than the distance traveled by light from the other slit to reach these positions. Crest again meets crest.
The diagram shows the geometry for the fringe pattern. If light with wavelength l passes through two slits separated by a distance d, we will observe constructively interference at certain angles. These angles are found by applying the condition for constructive interference, which is
dsinq = ml, m = 0, 1, 2, ….
Observe single and double slit diffraction with water waves.
Diffraction patterns can be produced by a single slit or by two slits. When light encounters an entire array of identical, equally-spaced slits, called a diffraction grating, the bright fringes, which come from constructive interference of the light waves from different slits, are found at the same angles they are found if there are only two slits. But the pattern is much sharper. The figure below shows the interference pattern for various numbers of slits. The width of all slits is 50 micrometers and the spacing between all slits is 150 micrometers. The location of the maxima for two slits is also the location of the maxima for multiple slits. The single slit pattern acts as an envelop for the multiple slit patterns.
In classical physics light is an electromagnetic wave. Electromagnetic waves are categorized according to their frequency f or, equivalently, according to their wavelength l = c/f. Visible light has a wavelength range from ~400 nm to ~700 nm. Violet light has a wavelength of ~400 nm, and a frequency of ~7.5´1014 Hz. Red light has a wavelength of ~700 nm, and a frequency of ~4.3´1014 Hz.
Visible light makes up just a small part of the full electromagnetic spectrum. Electromagnetic waves with shorter wavelengths and higher frequencies include ultraviolet light, X-rays, and gamma rays. Electromagnetic waves with longer wavelengths and lower frequencies include infrared light, microwaves, and radio and television waves.
Type of Radiation
Frequency Range (Hz)
Wavelength Range
gamma-rays 1020 - 1024 < 10-12 m x-rays 1017 - 1020 1 nm - 1 pm ultraviolet 1015 - 1017 400 nm - 1 nm visible 4-7.5´1014 750 nm - 400 nm near-infrared 1´1014 - 4´1014 2.5 mm - 750 nm infrared 1013 - 1014 25 mm - 2.5 mm microwaves 3´1011 - 1013 1 mm - 25 mm radio waves < 3´1011 > 1 mm
Light transports energy across space. The intensity (energy per unit area and unit time) is proportional to the square of the amplitude of the light wave. This energy, however, arrives at a receiver not continuously but in discrete units called photons. The energy transported by an electromagnetic wave is not continuously distributed over the wave front. It is transported in discrete packages. Photons are the particles of light.
Properties of photons:
We will observe two-slit interference, one photon at a time using an apparatus made by TeachSpin.
How is it possible for light to propagate as if it were a wave and yet to be detected as if it were a particle? How can a single particle interfere with itself? This paradox is the central theme in Richard Feynman's introduction to the fundamentals of quantum mechanics:
"We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery. We cannot make the mystery go away by explaining how it works . . . In telling you how it works we will have told you about the basic peculiarities of all quantum mechanics."
Photons are particles of light. But they do not behave like macroscopic particles. We cannot use the laws of classical physics to predict the behavior of individual photons. We have to use the rules of quantum mechanics.
