Final answer:
In a confocal scanning microscope, short-wavelength incident light is absorbed by the specimen and reemitted at a longer wavelength. The principle is based on the use of high-energy excitation light that, once absorbed, is reemitted at a lower energy and hence a longer wavelength. Option number 2 is correct.
Step-by-step explanation:
In a confocal scanning microscope, short-wavelength incident light is absorbed by the specimen and reemitted at a longer wavelength. When we consider the principles of fluorescence microscopy, we understand that short-wavelength, high-energy light such as ultraviolet (UV) is typically used to excite the fluorophores labeled on a specimen. These fluorophores then emit light at a longer, lower energy wavelength, which is seen as the fluorescence from which we obtain our image.
Two-photon microscopes represent an advancement in this technology. They utilize long-wavelength light (infrared) for excitation, where two photons are needed to excite the fluorochrome due to the low energy of the long-wavelength light. Nonetheless, in the context of traditional confocal microscopy, the sequence involves excitation with short, high-energy wavelengths of light, which is absorbed by the specimen and then emitted at a longer wavelength. Hence, the correct answer to the question is option 2: short, longer.