A and I bands?

Actin and myosin behave differently in polarised light. Actin is isotropic (appearing clear) and myosing is anisotropic (appearing dark) and the bands that can be seen in the microgaph (above) have been called I- and A-bands, accordingly. The I-band contains mainly actin and is isotropic hence (I-band) and the A-band contains mainly myosin which is anisotropic (hence A-band).