This is an example of an audio amplifier meant to let you "hear" when there is light present. A photo-transistor is used to detect ambient light levels.
The circuit is discussed in this forum post
A phototransistor (modeled here as just a sinusoidal current source) is used to detect ambient light levels. The current going into the phototransistor will be proportional to the ambient light, this current flows through R7 which essentially turns the current signal into a voltage signal. This is then amplified by the op-amp before going to the speakers. The capacitors C2 and C3 are chosen to filter for signals roughly in the audio range.
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