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Light Conversation

Science Snack
Light Conversation
The talk is simple, but illuminating.
Light Conversation
The talk is simple, but illuminating.

Experiment with an automatic night-light that has a built-in photocell sensor—and put two night-lights face to face for a little “light” conversation.

Tools and Materials
  • Two incandescent night-lights with photocell sensors Note: LED night-lights are now widely available and can be used for some of these activities; see requirements below. Night-lights with manual switches, however, will not work.
  • Two extension cords at least 6 feet (2 meters) long
  • Two electrical outlets near each other or one electrical outlet and a power strip or multiplug extension cord (not shown)
  • Small handheld mirror
  • Credit card, or other opaque card about the same size
  1. Remove the plastic shade from each light so you can see the bare bulbs (see photo below).
  2. Plug each light into a different extension cord and plug both cords into electrical outlets.
To Do and Notice

Investigation 1: Photocell Coverup (incandescent or LED)

Hold the photocell sensors of both lights (the small window below the bulb on the base of the light) so that bright light shines on them. Are the night-lights on or off?

Now use your hand to completely cover the photocell sensor of one of the lights. What does the light do?

Investigation 2: Lights in Motion (incandescent or LED)

Darken the room. One or both of the night-lights should turn on. Experiment with the night-lights by moving them around. Watch the behavior of the lights, and use your observations to predict when they will go on and off.

Investigation 3: Reflecting the Light (incandescent only; LED will not work)

Darken the room and unplug one of the night-lights. The night-light that’s still plugged in should go on. Hold the mirror so the light from this night-light shines into its own photocell. Adjust the position of the mirror until you get the light to flicker. Can you explain what’s happening?

Investigation 4: Flip-Flop (incandescent or LED)

Keep the room dark and plug in the second night-light again. Place the lights on a table so that each bulb shines into the other light’s photocell, as shown. Initially, both lights may go out for an instant, but one or the other should come back on. (The photo below, and the four that follow, show how this investigation is done. Note, however, that these photos were taken with the room lights on, and don’t show how the night-lights turn on and off.)

Slowly slide an opaque object, such as a credit card, between the unlit bulb and the photocell opposite it. Keep sliding until the card has passed beyond both lights (see photos below). Then slowly slide the card back between the two lights in the opposite direction. What happens?

Repeat the four steps in the process, but this time start by sliding the card between the lit bulb and the photocell opposite it. What happens?

What's Going On?

Photocell Coverup

A photocell is a light-sensitive semiconducting diode that works something like a switch: When enough light falls on it, it allows current to flow, activating whatever circuit it controls—in this case, a night-light.

Lights in Motion

As you can see, it’s possible to arrange two night-lights so that they control one another. The light from one can cause the other to turn off, and vice versa.

Reflecting the Light

Using the mirror, you can see how a single night-light can affect its own behavior. Light reflected to the photocell causes the incandescent bulb to turn off. Then, when the light is no longer reflected, the bulb turns back on. There is a small but noticeable time delay in the cooling and heating of the filament in the bulb, so you see this rapid on-off-on-off sequence as flickering. An LED doesn't show the flickering because an LED has almost no delay time and is therefore perceived as always on.

This on-off cycle is an example of a feedback loop, which occurs whenever the output of a device is fed back into the device as input. If the feedback increases the output of the device, it’s called positive feedback; if it decreases the output, it’s called negative feedback.

Your flickering night-light is an example of negative feedback: When the output is positive (on) it causes a negative result (the light going off). Some other devices that use negative feedback are the cruise control on a car, the thermostat on a heating system, and a weather vane. A well-known example of positive feedback is the loud screech heard when sound from the speakers in an auditorium gets fed back into the microphone.


When you slide the card past the unlit blub, no change occurs. When the card goes past the lit bulb, though, its light is blocked and no longer falls on the photocell opposite it. This turns on the bulb that was originally unlit, and its light falls on the photocell across from it, turning off the bulb that was originally lit.

This on-off switching effect of sliding a card between the two lights is another a form of feedback, known to computer scientists as a flip-flop, or bistable gate. Each time you slide the card between the bulbs, the bulb that was unlit turns on and the lit bulb turns off. In computers, a similar switching process is at work behind the binary logic that underlies all computer operations. Computers use flip-flops to store and process information as a series of bits—ones and zeros—equivalent to on and off.

When you initially slide the card past the lit bulb, its light is blocked and the unlit bulb goes on, which causes the first bulb to go off. As you continue to slide the card past the second bulb, its light is blocked, causing the first bulb to go on and the second to go off. The system is then back to its original state.

Going Further

Gather more night-lights—the more the better—and have them "talk” in a dark room. Try different locations and orientations of the lights, and see what happens. This experiment works well when a different person holds and manipulates each light. Alternatively, you can mount the lights on a table or board in such a way that their positions and orientations can be easily adjusted.