How Things Work - Chapter 15 Demonstrations

# Section 15.1 Cameras

In a large lecture hall, you may want to observe the images that are formed in these demonstrations with a camera and display them on a monitor.
Demonstration 15.1.1:  Forming a Real Image
Description: While the light from a light bulb alone causes a diffuse illumination of a white screen, the addition of a converging lens can form an inverted image of the light bulb on the screen.
Purpose: To show how a converging lens forms a real image.
Supplies:
1 light bulb (or another bright, identifiable object)
1 white screen or ground-glass screen
1 converging lens (about 50 mm in diameter, with a focal length of about 250 mm or so)
1 optics bench (optional—otherwise just use lens and component holders)
Procedure: Place the light bulb and the white screen about twice the focal length of the lens apart and turn on the light bulb. Only a diffuse illumination will appear on the screen. Now insert the converging lens midway between the bulb and screen and move the lens back and forth until a sharp real image of the bulb appears on the screen. Point out how refraction at the surfaces of the lens is bending all the light from one point on the bulb together to a single point on the screen. Show that blocking part of the lens with your hand simply dims the image—because each portion of the lens is sending light rays from all parts of the bulb to their appropriate positions of the screen.
Explanation: The converging lens takes the diverging light rays that emerge from a particular point on the bulb and bends them so that they converge to a particular point of the screen. But even if the screen weren't there, the real image would form in space and you could find it with your hand or with a piece of photographic film.
Demonstration 15.1.2:  How a Lens Works
Description: You use black board optics (or camera table optics) to show how a converging lens bends light rays together and can thus form a real image of an object.
Purpose: To show how refraction at the surfaces of a converging lens bends light rays toward one another.
Supplies:
1 set of black board optics or camera table optics
Procedure: Begin with several parallel light rays heading from left to right. Show that inserting a converging lens into these rays causes them to bend toward one another. Point out that there is one point at which all the bent rays meet.
If possible, repeat this experiment with a spray of diverging light rays that come from one point in the light source. Show that the converging lens still brings these rays together, but the convergence is delayed.
Explanation: A converging lens brings formerly parallel light rays together at its focal length. It can also bring formerly mildly diverging light rays together somewhat beyond its focal length.
Demonstration 15.1.3:  Depth of Focus
Description: A large-diameter converging lens forms real images of three different light bulbs at three different distances from the lens. Only one of these images is sharply focused on a screen at a time. But when a small aperture is inserted over the lens, allowing only its central portion to form the image, all three images are in focus at once.
Purpose: To show how a lens's aperture affects its depth of focus.
Supplies:
3 light bulb (or other bright, identifiable objects)
1 white screen
1 large diameter converging lens—a magnifying lens will work (about 100 mm in diameter, with a focal length of about 200 mm or so)
1 optics bench (optional—otherwise just use lens and component holders)
1 cardboard aperture, about 20 mm in diameter (or even less; a variable aperture is even better)
Procedure: Place the three light bulbs at various distances from the white screen (none closer than 1 m) and insert the lens about 100 mm away from the screen. Move the lens back and forth until a sharp real image of the middle distance bulb appears on the screen. Point out that the images of the other two bulbs also appear, but that they are out of focus. Shift the lens to bring the other light bulbs into focus, one at a time, and note that you can't bring all three into focus at once.
Now insert the narrow aperture in front of the lens and show that, while the images have become dimmer, they are all essentially in focus on the screen.
Explanation: When only rays that pass through the center of the lens are allowed to reach the screen, they are so nearly converged well in front of and behind the true focus, that the precise distance between the lens and the screen isn't very important. Everything appears in focus. But when the whole lens is active, rays converging from the edges of the lens are badly out of place in front of and behind the true focus and the patterns of light observed before or after the true focus are badly blurred.
Demonstration 15.1.4:  The Importance of a Lens's Focal Length
Description: The real image formed on a screen by a long focal length lens is larger and dimmer than the real image formed by a short focal length lens of the same diameter.
Purpose: To show how a lens's focal length affects the brightness and size of the image it forms.
Supplies:
1 light bulb (or another bright, identifiable object)
1 white screen
1 short focal length converging lens (about 50 mm in diameter, with a focal length of about 25 mm or so)
1 long focal length converging lens (about 50 mm in diameter, with a focal length of about 100 mm or so)
1 optics bench (optional—otherwise just use lens and component holders)
Procedure: Place the light bulb and the white screen about 2 m apart and insert the short focal length lens about 25 mm away from the screen. Move the lens back and forth until a sharp real image of the bulb appears on the screen. Point out how small that real image is and how bright it is.
Now remove the short focal length lens and replace it with the long focal length lens. Place this new lens about 100 mm from the screen and move it back and forth until a sharp image forms. Point out how much larger that real image is than the previous one and how much dimmer it is. Discuss the decreased curvature of the surfaces of the long focal length lens and how this delays the focus.
Explanation: With more distance over which to travel before it focuses, the light can spread farther away from the axis of the lens and create a larger (and dimmer) real image.
Demonstration 15.1.5:  Mixing the Primary Colors of Pigment
Description: You show 4 transparent sheets containing fractions of a color image. One sheet bears magenta pigments, another cyan, another yellow, and the last black. When these sheets are stacked, they reveal a full color image.
Purpose: To show how the primary colors of pigment can be combined to form full color images (as is done in photographic film).
Supplies:
1 set of color separation transparencies from a newspaper print shop (or generated with a color printer and software that can make color separations)
Procedure: Show the four separate sheets of transparent material. Point out that the magenta sheet contains a dye that absorbs green light wherever green light isn't wanted in the final image, that the yellow sheet contains a dye that absorbs blue light wherever blue light isn't wanted in the final image, that the cyan sheet contains a dye that absorbs red light wherever red light isn't wanted in the final image, and that the black sheet helps to darken parts of the image that should be particularly dark (black dye isn't present in color photography, but saves the printers from having to use extremely large quantities of costly colored dyes). Now overlap all the sheets and show that they combine to form a full color image. By controlling where you see red, green, and blue lights, these dye layers can make you see any possible color.
Explanation: Each layer absorbs one of the primary colors of light. Together, these layers can turn white light into any specific arrangement of red, green, and blue lights and thus make you perceive any possible color.

# Section 15.2 Optical Recording and Communications

Demonstration 15.2.1:  Putting Sound on Light (Analog Version)
Description: The signal from a tape player is used to modulate the light outputs of a flashlight and a laser pointer. This light strikes an optical sensor that's connected to an amplifier and speaker, and the sound is heard.
Purpose: To show that light can carry sound information (albeit in analog form in this demonstration).
Supplies:
1 flashlight
1 laser pointer
1 large inductor (a 5 cm diameter coil of about 100 turns of insulated copper wire will do)
1 tape player
1 light sensor that's AC coupled to an amplifier and speaker
Procedure: Modify the flashlight's circuit so that, in addition to passing through its batteries and bulb, its current must also pass through the inductor. Attach the output wires from the tape player's headphone jack to the two sides of the inductor. Shine the light from the flashlight onto the light sensor and turn on the tape player. You will hear sound from the speaker. Repeat this same procedure with the laser pointer. Some laser pointers have pocket clips that also act as their switches. All you have to do in this case is to insert the inductor between the clip and the case of the pointer.
Explanation: The inductor carries the DC current needed to maintain operation of the light bulb or laser. But the tape player superimposes an AC current onto the DC current passing through the bulb or laser and its light output fluctuates accordingly. The light sensor detects these fluctuations and uses them to reproduce the sound.
Demonstration 15.2.2:  Total Internal Reflection
Description: You use black board optics (or camera table optics) to show how light that tries to escape from a clear medium into air at a glancing angle is totally reflected from the interface.
Purpose: To show how total internal reflection works.
Supplies:
1 set of black board optics or camera table optics
Procedure: Send a light ray into the wide face of a right-angle prism. Show that as this light tries to exit the prism through the narrow face, it's at least partially reflected and that the emerging beam is bent dangerously close to the surface of that face. When the prism's angle is adjusted far enough in one direction, the emerging beam vanishes altogether and the beam is perfectly reflected from the surface.
Explanation: When light speeds up as it moves from one medium to another, it bends away from the normal to the surface. As the angle of incidence on the interface becomes more shallow, the outgoing beam bends more and more toward the surface between the media. At a shallow enough angle, the light no longer emerges from the first medium at all—it's totally internally reflected.
Follow-up: Send light rays upward through a container of water and watch how they exit from the water's surface. The light rays that travel almost directly upward escape without difficulty, but those that strike the water a glancing blow simply reflect—they experience total internal reflection.
Demonstration 15.2.3:  Light Following a Stream of Water
Description: A beam of laser light shines into the stream of water leaving a container. The light follows the water as the water arcs downward and illuminates the spot where the water hits a basin.
Purpose: To show that light can become trapped in a medium by total internal reflection.
Supplies:
1 laser or laser pointer (a flashlight and a converging lens will also work)
1 clear beaker with a pipe attached to its side near its bottom
1 cork for the pipe
1 basin to catch the water
water
Procedure: Insert the cork in the beaker's pipe and fill the beaker with water. Shine the laser beam through the beaker so that it travels through the pipe and hits the cork. Now remove the cork and allow the water to flow in an arc into the basin. The laser light will follow the water all the way to the basin.
Explanation: The laser light encounters the surfaces of the water stream at such shallow angles that it experiences total internal reflection. Unable to escape from the water, the beam travels with it all the way to the basin.
Demonstration 15.2.4:  Send Sound Via Light in an Optical Fiber
Description: Words spoken into the microphone of a transmitter travel through a long optical fiber and are reproduced by the speaker of a receiver.
Purpose: To show that light can carry information through an optical fiber and follow all of its bends and turns.
Supplies:
1 audio transmitter with microphone (this entire demonstration is available in kit form from Carl's Electronics
1 optical fiber
Procedure: Connect the transmitter and receiver via the optical fiber and turn both on. Talk into the transmitter's microphone and the sound of your voice will be reproduced by the receiver's speaker.
Explanation: The transmitter AM modulates light in reponse to your voice and sound information is conveyed in analog form to the receiver, where it is used to reproduce the sound of your voice. The optical fiber carries the light (probably infrared) with little loss because the light undergoes total internal reflection throughout its passage.
Demonstration 15.2.5:  Birefringence in Calcite
Description: You place a piece of cardboard with a small hole in it on an overhead projector. One circle of light appears on the screen. But when you put a calcite crystal on top of the hole, two separate circles of light are visible. With a polarizing sheet, you determine that the two circles of light have different polarizations. The calcite crystal is handling those two polarizations differently.
Purpose: To show that some materials allow the two polarizations of light to travel at different speeds.
Supplies:
1 screen
1 cardboard sheet with a small (3 mm) hole punched in it
1 calcite crystal
1 polarizing sheet
Procedure: Place the cardboard sheet on the overhead projector and form a clear image of the hole on the screen. Now place the calcite crystal on top of the hole and observe that two different circles of light are present on the screen. Rotate the calcite crystal and see that they move relative to one another—their separation is evidently related to the structure and orientation of the calcite crystal. Now use the polarizing sheet to show that the two circles of light have different polarizations.
Explanation: The electrons in a calcite crystal can move more easily in some directions than in other directions. As a result, calcite slows one polarization of light more than the other. It thus bends one polarization of light more than other upon entry and exit, and this different bending leads to the spatial separation of the two circles of light.
Follow-up: If you have a calcite-based polarizing beam splitter, show how it uses both calcite's ability to bend the two polarizations differently and total internal reflection to separate the two polarizations of light from one another completely.