Microtubules are filamentous structures in cells that maintain cell shape and facilitate the movement of molecules within the cell. They are long, hollow cylinders with a diameter of about \(25 \mathrm{~nm}\). It is possible to incorporate fluorescent molecules into microtubules; when illuminated by an ultraviolet light, the fluorescent molecules emit visible light that can be imaged by the optical system of a microscope. If the emitted light has a wavelength of \(500 \mathrm{~nm}\) and the NA of the microscope objective is 1.4, can a biologist looking through the microscope tell whether she is looking at a single microtubule or at two microtubules lying side by side? Light that enters your eyes is focused to form an image on your retina. The optics of your visual system have a total refractive power of about \(+60 \mathrm{D}\)-about \(+20 \mathrm{D}\) from the lens in your eye and \(+40 \mathrm{D}\) from the curved shape of your cornea. Surgical procedures to correct vision generally do not work on the lens; they work to reshape the cornea. In the most common procedure, a laser is used to remove tissue from the center of the cornea, reducing its curvature. This change in shape can correct certain kinds of vision problems.