Type 3: Optical Storage (CD / DVD / Blu-ray)

Type 3: Optical Storage (CD / DVD / Blu-ray)

How It Works:

Uses a laser to read (and sometimes write) data encoded as microscopic pits and lands on a reflective disc surface. The laser shines on the disc, and a sensor detects reflections to read binary data.

Types and Capacities:

• CD (Compact Disc): ~700 MB capacity, red laser (780nm), invented 1982
• DVD (Digital Versatile Disc): ~4.7 GB single-layer, 8.5 GB dual-layer, red laser (650nm)
• Blu-ray: 25 GB single-layer, 50 GB dual-layer, 100 GB triple-layer, blue-violet laser (405nm)

Types by Write Ability:

• ROM (Read-Only): CD-ROM, DVD-ROM - written once at factory, cannot change (commercial software, movies)
• R (Recordable): CD-R, DVD-R, BD-R - can write once, cannot erase (good for permanent archives)
• RW (Rewriteable): CD-RW, DVD-RW, BD-RE - can write, erase, and rewrite ~1,000 times

Advantages:

• Very cheap per disc (blank CD-Rs cost £0.20-0.50 each)
• Portable and lightweight - easy to mail, distribute, carry
• Good for backups and archives (if stored properly, lasts 10-25 years)
• Universal compatibility - most devices can read CDs/DVDs
• No power needed to retain data (unlike SSD which can lose charge over years)

Disadvantages:

• Very slow - 1-8 MB/s read speeds (50-200x slower than HDD)
• Low capacity compared to HDD/SSD (even Blu-ray only 50-100GB)
• Fragile - can be scratched, cracked, or degraded by sunlight
• Requires optical drive (many modern laptops don't have one)
• Writing is slow - burning a full DVD can take 10-20 minutes

Best Used For:

• Software distribution (games, applications)
• Movie/music distribution (though streaming is replacing this)
• Long-term archives (family photos, important documents)
• Bootable discs (OS installation, recovery discs)
• Situations where physical portability without power is needed