  The green house effect is caused by the gases which absorb infrared ray (IR)
emitted by the earth. It is worthwhile if we can adjudicate on which gas causes
the greenhouse effect in our class. For this purpose, one of our authors,
Kaneko has designed an educational tool for testing greenhouse effect
\cite{Kaneko}. This system (hereafter abbreviated PAS) is constructed based on
photo acoustic effect. Without difficulty and high cost, we can build PAS and
check the IR absorption of gas. In this paper we give the simple theoretical
basis for this PAS. The amplitude of sound observed in PAS depends on the
modulation frequency of IR pulse. Its dependence can be explained by this
simple model. Further we show the sound amplitude does not depend on the
thermal diffusion, which provides the accuracy of amplitude as the IR
absorption rate of the gas. According to this model, sound signal is not the
sinusoidal function and it has higher harmonics. The theory and experiment are
compared in the third harmonics by spectrum analysis. From this apparatus and
theory, students can study not only the greenhouse effect but also the basics
of physics.
