Igniting paper with an overhead projector

Demonstration no.: 105
Purpose: To demonstrate focussing of light and heat.
Summary: The head of an overhead projector is removed and a piece of paper is placed in the focus of the light. The paper will burst into flames.
Keywords: Diffraction, interference, light, optics, heat, heat radiation.

Description:

An ordinary overhead projector contains a powerful light bulb which radiates light through a converging Fresnel lense and into a projection lense located above in the head of the projector. From here, the light (and thus the image on the transparency) is projected onto the projections screen. If the projection head is removed, the light from the Fresnel lense will be focussed in the air above the projector. In the focus area, the light and heat from the light bulb is so intense that a piece of paper can catch fire.

The light and heat from an overhead projector can ignite a piece of paper.

To find the exact position of the focus, the paper can be moved up and down inside the light cone until the light spot on the paper is as small as possible. This is the focus point of the Fresnel lense, and this is were the heat is most intense.

It is important to notice that white paper probably doesn't work since most of the light and heat is simply reflected from it. If the paper, however, is dark or has some pictures or text printed on it, the paper will typically start smoking after a few seconds in the focus.

Focused light from an overhead projector ignites a piece of paper. Watch the movie in the references.

The smoke can afterwards be used as an indicator to show where the light from the projector is focussed and defocussed:

The smoke from the burned paper indicates clearly where the light from the overhead projector i focussed

That it is nessecary to use a piece of dark paper or paper with black ink to absorb the heat can be seen clearly by holding the paper into the light for only a short time. The first burn appear exactly where the letters are located on the paper:

This piece of paper has been in the focus of an overhead projector. Notice that the paper is only scorched where the letters are printed.

For the same reason, it is important that the printed letters are on the side facing down towards the projector.

Obviously, when a piece of paper can catch fire in the focus of the projector, there is a lot of light and heat in that area. Consequently, do not put your hand or anything else into the focus. Especially, make absolutely sure that you never put your head into the focus. Your eyes will be permanently damaged!

Questions and Answers:

Why doesn't the paper catch fire?

First, you have to be absolutely certain that the paper is positionen exactly in the focus of the light. Move the paper up and down until the light spot is as small as possible. Also, make sure that the paper is not white. Try printing a black square on a piece of paper or use paper with a lot of printed text. If this doesn't work, try using different types of paper. Finally, it is possible that the light bulb in the projector simply doesn't radiate enough power.

Equipment and materials:

Overhead projector
Paper

References:

Link to demonstration in database at University of Maryland.
Link to demonstration in database at Montana State University.
Quick Time video of paper bursting into flames (2,50 MB).
Windows Media Video of the demonstration performed at the University of Aarhus on January 27 2006 (1,15 MB).

PIRA DCS: 6A20.60 (Optics: Geometric optics)

Updated: 12 August 2007

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