As the name implies, the Blu-ray disc has the same diameter and refractive index as a DVD or CD, but the laser light used is in the blue–violet region with a wavelength in air of 405 nm. The index of refraction of the plastic coating on the disc is n = 1.55. 

(a) What is the wavelength of this light within the disc?

(b) How deep are the pits in a Blu-ray disc? Are they deeper or shallower than the pits in a CD or DVD? (The laser light used by a CD has a wavelength of 780 nm while for a DVD it is 650 nm.) 

(c) Tracks (adjacent rows of pits) are separated by 1.6 mm in a CD, 0.74 mm in a DVD, and 0.32 mm in a Blu-ray disc. Assuming each pit edge encodes one bit and using the number of pit edges in Figure 26.30B as a reference, calculate the approximate amount of memory in megabytes (1 byte  8 bits, and 1 Mb = 10⁶ bytes) per square centimeter of disc area for each format. This calculation of storage capacity is only a rough estimate because it ignores several important factors such as the optics used to focus the laser light, and (more importantly) the need to allow some space between pits to allow for alignment errors, etc. This is why your answers are a bit larger than the actual storage capacities of CDs, DVDs, and Blu-ray discs.