Know and understand optical discs including CD, DVD, Blu-ray

3 Storage devices and media

Optical Discs: CD, DVD, Blu‑ray

Optical discs are like the “books” of the digital world. They store data on a thin layer of material that a laser reads. Let’s explore the three main types: CD, DVD and Blu‑ray. 📀

Compact Disc (CD) 💿

A CD is the oldest optical format. Think of it as a small library shelf that can hold about 700 MB of data. It uses a laser with a wavelength of 780 nm, which is visible to the eye (red light). Typical uses include music, software installers and small data backups.

• Capacity: 700 MB (≈ 80 minutes of audio)
• Laser wavelength: 780 nm (red)
• Read/write speed: 1–2× (1× = 150 kB/s)
• Typical uses: music CDs, software, small data archives

Digital Versatile Disc (DVD) 📀

A DVD is like a bigger shelf that can hold up to 4.7 GB (single‑layer) or 8.5 GB (dual‑layer). It uses a shorter laser wavelength of 650 nm (orange light), which allows more data to be packed into the same space. DVDs are perfect for movies, large software and game installations.

• Capacity: 4.7 GB (single) / 8.5 GB (dual)
• Laser wavelength: 650 nm (orange)
• Read/write speed: 1–8× (8× = 1.2 MB/s)
• Typical uses: movies, game discs, large data sets

Blu‑ray Disc (BD) 🔵

Blu‑ray is the “high‑definition” shelf. It can hold 25 GB (single) or 50 GB (dual) thanks to a laser with a wavelength of 405 nm (blue/violet). This shorter wavelength means the laser can focus on a smaller spot, packing more data into the same area. Blu‑ray is ideal for 4K movies, large software and high‑resolution backups.

• Capacity: 25 GB (single) / 50 GB (dual)
• Laser wavelength: 405 nm (blue/violet)
• Read/write speed: 1–16× (16× = 2.4 MB/s)
• Typical uses: 4K video, large game installs, high‑res backups

Comparison Table

How a Laser Reads Data

1. Laser emits light at a specific wavelength.
2. Light reflects off the disc surface.
3. Reflected light passes through a sensor.
4. Sensor converts light patterns into digital data.

The shorter the wavelength, the smaller the spot the laser can focus on. That’s why Blu‑ray (405 nm) can store more data than a CD (780 nm). Think of it like using a finer pencil to write more words on the same page. ✏️

Practical Example: Songs on a CD

If one song averages 4 MB, you can fit:

$700 \div 4 \approx 175$ songs

That’s enough for a whole album plus a few extras! 🎶

Common Mistakes & Tips

• ❌ Forgetting to label the disc: Always write the title and date on the label side.
• ?? Cleaning the disc: Use a soft, lint‑free cloth and wipe from the centre outwards.
• ❌ Using the wrong drive: Some older drives can’t read Blu‑ray discs.
• ?? Backing up data: Store a backup on a hard drive or cloud to avoid loss.

Key Takeaways

• Optical discs use lasers to read/write data.
• CD, DVD and Blu‑ray differ mainly in capacity and laser wavelength.
• Shorter wavelengths (Blu‑ray) allow more data per area.
• Always label, clean, and backup your discs.