In about 1801 Thomas Young performed a splendid experiment that demonstrated the wave nature of light. He let a ray of sunlight into a dark room, placed a dark screen in front of it, pierced with two small pinholes, and beyond this, at some distance, a white screen. On the screen Young observed a pattern of lines. I repeated Young’s experiment with the use of a halogen bulb as a primary light source and a double slit cut with a Stanley blade, into a black lacquered piece of glass, thus providing two secondary light sources. The result is a colourful line pattern caused by the interference of light arriving at the screen from both sources. Slit distances should be in the order of a fraction of a millimeter. The colours are caused by the overlap of line patterns with different line frequencies for each wavelength. The shorter the wavelength, the higher the line frequency (the finer the pattern). http://en.wikipedia.org/wiki/Double-slit_experiment

Young’s experiment: interference of light waves

Interference of light waves Small angle: Low line frequency Large angle: High line frequency The line frequency increases with the angle

Thomas Young’s experiment The brightness of the interference fringes has a sinusoidal variation: this is characteristic of all optically created interference patterns and this has significant implications for the security value of diffractive structures, because sinusoidal structures can be copied by optical copying methods.

Young’s famous experiment can be simply repeated in the following manner. Spray a piece of glass with black lacquer or add soot to it in a candle flame. Then cut two very narrow slits through the black layer with a Stanley blade, about one half millimeter apart. Or better, cut two slits with a varying distance. Hold the slits right in front of your eye and look at a candle flame at a distance of a few meters. The next slide illustrates the experiment, which can be performed for a complete class room if you make the effort to produce the required number of pieces of glass with slits.

Thomas Young’s experiment Cover a piece off glass with a black lacquer, then, using a Stanley blade, cut two fine slits in the lacquer You will observe interference fringes in the candle flame Hold the slits right in front of your eye and look at a candle flame at a distance of a few meters

Interference of water waves The principle of the interference of waves can be nicely demonstrated by blowing two streams of air on a water surface in a dinner plate through two straws. The traveling waves that are invoked by the air streams move fast enough to be difficult to see. By blinking our eyes or waving spread fingers in front of our eyes, these waves become visible. Wherever both traveling wave systems overlap, they interfere to create a standing wave pattern.

Standing waves standing waves have points of minimal amplitude (nodes) and points of maximum amplitude (anti-nodes). anti-nodes nodes As the name “standing wave” implies, it does not move as travelling waves do, but it has a fixed location in the field of both interfering travelling waves. This is illustrated in the next slide.

Interference of travelling waves to create a standing wave pattern node antinode

Interference of water waves For a class, the experiment can be demonstrated by placing a transparent tray on the platen of an overhead projector. Place a round piece of paper of some 4-6 cm diameter, on the projection lens to provide for better visualization of the waves (dark field projection). This simple set-up is sufficient for most purposes and lets you avoid buying expensive equipment.

Interference of water waves The three principal types of holography can be schematically indicated in the interference field of both wave sources s1 and s2: 1. In-line holography (Gabor, 1948) 1 2. Off-axis holography (Leith and Upatnieks, 1962) 2 3. Volume-reflection holography (Denisyuk, 1963) 3 S1 S2 End of presentation Return to 1st slide