# Solutions to Homework Problem set #7

Textbook, Chapter 5, Page 158: P2, P6, PM1

P2.  Accommodation is the process that the eye uses to focus on objects at different distances. It is achieved by changing the focal length of the eye-lens. The eye-lens is normally focused on a very distant object when the muscles are relaxed. As an object moves closer to the eye, the muscles that do this increase the strength of the eye-lens and therefore shorten its focal length until the object is in focus on the retina.

P6.  Black light contains ultra-violet light in addition to some visible light in the blue and purple regions of the spectrum. The light appears fuzzy because of chromatic aberration in the eye. The eye-lens cannot focus simultaneously on the blue and ultra-violet portions of the spectrum because the index of refraction of the eye-lens changes with wavelength (This is called chromatic aberration).

PM1. The speed of a lens is the ratio of the focal length to the diameter. Using the values in the problem, the focal length is given as 16 mm and the diameter is given as 4 mm. Therefore the speed is 16/4 = f/4.