This demo shows the shifting colors as a function of filament temperature. Also, this demo shows prism dispersion, radiation intensity as a function of wavelength, and infrared radiation. The black body radiation curve can be demonstrated by using a radiation sensor hooked to a digital millivoltmeter. The carbon disulfide prism (see Dispersion [1]) is used to spread out the light of a slide projector lamp onto a screen . As you scan across the spectrum with the radiation sensor, the millivoltmeter shows the peak of the 3000 K tungsten filament in the infrared with the tails of the curve in the visible spectrum and further infrared.
The same demonstration can be done more qualitatively. Turn down the room lights and show the spectrum of "white" light on the wall. As you reduce the voltage to the lamp with the variac, the blue color dies away, and then the green, leaving only dull red of low intensity. (Of course, the 3000 K tungsten filament already peaks in the infrared so the initial "white" light is already quite red. Infrared itself can be demonstrated; see Infrared, Radiometer, and Maxwell's Spectrum [2])
The applet below shows the blackbody curve and colors corresponding to the given temperature. |
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A laser beam is arranged to pass through the slits and be reflected onto the overhead screen. Standard demonstrations are single slit diffraction, double slit interference, and diffraction from a circular opening. Two lasers are arranged so that single and multiple slits can be shown simultaneously, one pattern above the other.
We have precision slits etched in metal foil. The slit widths and spacings are marked. The most useful single and double slits have a width of .04 mm. The double slit spacings are .125, .250, and .5 mm. There is also 3, 4, and 5 slits with the same width (0.04 mm) and spacings of .125 mm.
Hair, CD's and DVD's can be used as diffraction gratings. Simple measurements of the first maximum gives the track spacing. Comparing the pattern from CD's and DVD's gives the ratio of the track spacing for the higher density DVD.
The diffration pattern can be observed with both a red and a green laser simultaneously to show the effect of wavelength. The red laser wavelength is 0.6328 micron and the green laser is 0.532 micron
The Cornell plate, diagrammed below can also be used for these demonstrations. It is clipped to a special stand so that successive slits in each column can be brought into the laser beam by adjusting a rack and pinion knob.
Column (a) Successively narrower single slits
Column (e) Successively wider double slits
Column (b) Single slit starts narrow, becomes wider,
becomes double slit, becomes narrower.Column (d) Go from one slit to two slits to three to four to ten to show sharpening
of the fringe maxima.
The Cornell plate was originally designed to be used (and can still be used) with the individual eye to view a straight filament bulb.
A very simple demonstration of the photoelectric effect is performed with a zinc plate as the electrode of an electroscope. An ultraviolet lamp covered with glass is arranged to shine on the plate. The plate is charged negative with an electrophorus, and the electroscope needle diverges indicating the charge. The blue light of the lamp will not knock out electrons from zinc, but if the glass (opaque to UV) is removed from the lamp, the needle quickly falls as electrons are kicked away from the plate. The zinc plate must be cleaned with steel wool within an hour or so of the demonstration to remove the oxide.
A variation of this experiment has a spiral electrode with a positive voltage in front of the zinc plate with a sensitive current meter to measure the small current of the photoelectrons through the air.
The photoelectric effect is also done as experiment 4 in the 8E lab. The stopping voltage is measured as a function of wavelength (color) of the exciting light, and Planck's constant determined from the slope of the line.
Links:
[1] https://demoweb.physics.ucla.edu/node/293
[2] https://demoweb.physics.ucla.edu/node/207
[3] http://webphysics.davidson.edu/Applets/Applets.html