Hot Image

Instructions for: Hot Image

We can use a small lens to make an image on a screen of the filament of a clear incandescent light bulb. As shown in the picture above, the lens needs to be a lot closer to the light bulb than the screen, and it has to be exactly the right place (most easily found by moving the lens around). For the case of the picture, the filament is less than 1/2" tall, and the image on the screen is several inches tall. This is already interesting, because it reveals that the filament is a coil.

In this picture the screen has been replaced by a temperature "liquid crystal" temperature sensitive sheet. It turns green or blue when it is warm. The energy delivered by the light warms the sheet, and makes a temporary photograph of the filament.

Now a filter has been put in the way. It is a sandwich of a red color filter and blue color filter. Very little visible light gets through. But the temperature sensitive sheet still changes color, because the sandwich still transmits infrared light.

This illustrates why incandescent light bulbs are becoming obsolete: for every dollar's worth of electricity you put in, an incandescent light gives you less than two cents worth of light and 98 cents of heat and infrared light. Fluorescent light bulbs give you 7 times as much light for the same dollar.

This activity makes several interesting points:

Light carries energy
Light can be focused to make an image.
There are forms of light that our eyes cannot see, and incandescent lights make a lot of these.

Problems with this activity:

It requires electricity.
It only works with a clear incandescent light bulb, which get too hot to touch (that's related to the point that is being made).
Kids move the lens and the screen. They don't know how to make the image display, and without the image the activity is pointless.
Even when everything is working, most people don't understand what they are seeing (but it is pretty, so maybe it doesn't matter).

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