A new optical disc format known as the Blu-ray Disc (BD) is becoming popular. In what way is it different from the traditional DVD? 1. DVD supports standard definition video while BD supports high definition video. 2. Compared to a DVD, the BD format has several times more storage capacity. 3. Thickness of BD is 2.4 mm while that of DVD is 1.2 mm. Which of the statements given above is/are correct?
Contents16
- A1 only
- B1 and 2 only
- C2 and 3 only
- D1, 2 and 3
Show answer
Answer: (B) 1 and 2 only
Statements 1 and 2 are correct.
Statement 3 is WRONG.
Statement 1 (✓):
DVD supports Standard Definition (SD) video (480p/576p resolution), while Blu-ray supports High Definition (HD) video (1080p and even 4K/UHD in newer versions).
This is one of the main reasons Blu-ray was developed — for HD movies.
Statement 2 (✓):
A single-layer DVD holds 4.7 GB, while a single-layer Blu-ray holds 25 GB — more than 5 times the capacity.
Dual-layer Blu-ray holds 50 GB vs DVD's 8.5 GB.
This huge storage is what enables HD video.
Statement 3 (✗):
This is a TRICK statement.
Both Blu-ray and DVD discs have the SAME thickness — 1.2 mm.
The question deliberately swaps the numbers (says BD = 2.4 mm, DVD = 1.2 mm) to confuse.
The real difference is in the laser used:
DVD uses a red laser (650 nm wavelength), while Blu-ray uses a blue-violet laser (405 nm) — hence the name 'Blu-ray.'
The shorter wavelength allows reading smaller, more densely packed data pits.
Blu-ray discs store 5+ times more data than DVDs (25 GB vs 4.7 GB) and support high-definition video while DVDs only support standard definition.
The key technology difference is laser wavelength: DVDs use red lasers while Blu-ray uses blue-violet lasers, allowing smaller data pits and higher storage density.
UPSC tests whether students can identify the deliberate trick in statement 3 - both disc formats have identical 1.2 mm thickness, not different thicknesses as stated.
Blu-ray Disc Technology
Science And Technology Blu-ray Disc BD optical disc
Blu-ray Technology: HD Storage & Blue-Violet Lasers
Blu-ray uses blue-violet laser (405 nm) vs DVD's red laser (650 nm)
Single-layer Blu-ray holds 25 GB vs DVD's 4.7 GB (5x capacity)
Blu-ray supports HD video (1080p) vs DVD's SD video (480p/576p)
Both discs have same thickness of 1.2 mm
Blu-ray Disc (BD) is an optical storage format developed to store high-definition video and large amounts of data. The name comes from the blue-violet laser it uses to read data.
Blu-ray vs DVD Comparison
Feature | DVD | Blu-ray Disc |
|---|---|---|
Laser Type | Red laser (650 nm) | Blue-violet laser (405 nm) |
Storage (Single Layer) | 4.7 GB | 25 GB |
Storage (Dual Layer) | 8.5 GB | 50 GB |
Video Resolution | Standard Definition (480p/576p) | High Definition (1080p, 4K) |
Disc Thickness | 1.2 mm | 1.2 mm |
Data Track Pitch | 0.74 micrometers | 0.32 micrometers |
Development Year | 1995 | 2006 |
Why Blue-Violet Laser?
The shorter wavelength of blue-violet laser (405 nm) allows reading much smaller data pits than DVD's red laser (650 nm). Smaller pits mean more data can be packed into the same disc area, resulting in 5x higher storage capacity.
Key Technical Advantages
Higher data density: Track pitch reduced from 0.74 to 0.32 micrometers
Better error correction: Advanced error correction codes prevent data loss
Regional coding: Uses different region codes than DVD for content protection
Backward compatibility: Many Blu-ray players can read DVDs and CDs
Trap: Statement 3 falsely claims BD thickness is 2.4 mm - both BD and DVD are 1.2 mm thick
Confusion: Students mix up laser wavelength numbers (405 nm vs 650 nm) with disc thickness
Common error: Thinking physical disc size differs - both are 120 mm diameter, 1.2 mm thick
Trick question pattern: UPSC often swaps correct technical specifications between similar technologies
DVD Technology
Science And Technology DVD traditional DVD
DVD: Digital Versatile Disc Technology
DVD uses red laser (650 nm wavelength) to read data
Storage capacity: 4.7 GB (single layer), 8.5 GB (dual layer)
Supports Standard Definition (SD) video up to 480p/576p
Replaced VHS tapes and CDs for video storage in 1990s
Digital Versatile Disc (DVD) revolutionized home entertainment by storing digital video on optical media. Developed in 1995, it became the dominant format for movies and data storage before Blu-ray.
Technical Specifications
Physical dimensions: 120 mm diameter, 1.2 mm thick (same as CD)
Data layer structure: Single or dual-layer options for different capacities
Track pitch: 0.74 micrometers between data tracks
Video compression: Uses MPEG-2 compression for storing movies
Regional encoding: Different regions (1-6) to control content distribution
DVD Storage Variants
DVD Type | Layers | Capacity | Usage |
|---|---|---|---|
DVD-5 | Single, Single-sided | 4.7 GB | Standard movies, software |
DVD-9 | Dual, Single-sided | 8.5 GB | Long movies, games |
DVD-10 | Single, Double-sided | 9.4 GB | Data storage |
DVD-18 | Dual, Double-sided | 17 GB | Professional applications |
Question Context
This PYQ tests understanding of optical disc evolution from DVD to Blu-ray. The key distinction is laser technology - DVD's red laser limits data density, while Blu-ray's blue-violet laser enables HD video storage.
Optical Laser Technology in Storage
Science And Technology
Laser Technology in Optical Storage: Wavelength & Data Density
Shorter wavelength lasers can read smaller data pits = higher storage density
CD uses infrared (780 nm), DVD uses red (650 nm), Blu-ray uses blue-violet (405 nm)
Data pit size and track spacing decrease with shorter wavelengths
Optical storage works by using lasers to read microscopic pits and lands on disc surfaces. The wavelength of laser light determines how small the data features can be - shorter wavelengths enable reading tinier pits, dramatically increasing storage capacity.
Laser Evolution in Optical Media
Format | Laser Color | Wavelength | Min Pit Length | Track Pitch | Capacity |
|---|---|---|---|---|---|
CD | Infrared | 780 nm | 0.83 μm | 1.6 μm | 700 MB |
DVD | Red | 650 nm | 0.4 μm | 0.74 μm | 4.7 GB |
Blu-ray | Blue-violet | 405 nm | 0.15 μm | 0.32 μm | 25 GB |
How Laser Reads Data
%%{init: {"flowchart": {"wrappingWidth": 460}}}%%
flowchart TD
s1["`**Laser beam focused on disc surface**
Precise wavelength laser targets specific track location`"]
s2["`**Light reflects differently from pits vs lands**
Pits scatter light, lands reflect it back strongly`"]
s3["`**Photodetector measures reflection intensity**
Strong reflection = 1 (land), weak reflection = 0 (pit)`"]
s4["`**Digital data converted to audio/video**
Binary stream processed by decoder chips`"]
s1 --> s2
s2 --> s3
s3 --> s4Physics Behind Storage Density
Diffraction limit: Laser cannot focus smaller than its wavelength, limiting pit size
Numerical aperture: Higher NA lenses focus tighter beams for smaller features
Blue-violet advantage: 405 nm wavelength allows 2.5x smaller pits than DVD's 650 nm
Track spacing: Closer tracks possible with precise shorter-wavelength focusing
Video Resolution Standards
Science And Technology standard definition high definition video
Video Resolution: From Standard Definition to Ultra HD
Standard Definition (SD): 480p/576p resolution, supported by DVD
High Definition (HD): 720p/1080p resolution, requires Blu-ray storage capacity
Ultra HD (4K): 2160p resolution, needs advanced Blu-ray or streaming
Video resolution refers to the number of pixels displayed on screen. The transition from SD to HD drove the need for higher-capacity storage like Blu-ray, since HD video files are much larger than SD.
Video Resolution Standards
Standard | Resolution | Pixels | Storage Needs | Supported By |
|---|---|---|---|---|
SD (480p) | 720×480 | 345,600 | Low | DVD, VHS |
SD (576p) | 720×576 | 414,720 | Low | DVD (PAL) |
HD (720p) | 1280×720 | 921,600 | Medium | Blu-ray, Digital TV |
HD (1080p) | 1920×1080 | 2,073,600 | High | Blu-ray, Streaming |
4K (2160p) | 3840×2160 | 8,294,400 | Very High | UHD Blu-ray, Streaming |
Storage Impact of Resolution
1080p HD video requires 4-5x more storage than 480p SD video
Compression codecs (MPEG-2, H.264, H.265) reduce file sizes significantly
DVD's 4.7 GB sufficient for 2-hour SD movie but inadequate for HD
Blu-ray's 25 GB enables 2-hour 1080p movie with high bitrates
Question Connection
This PYQ tests the core reason Blu-ray was developed: DVD cannot store HD video due to capacity limitations. The 5x storage increase from DVD to Blu-ray directly enables the 5x pixel increase from SD to HD resolution.
Don't confuse: 480p is SD, 720p/1080p are HD - not the reverse
Storage trap: Higher resolution requires higher storage capacity, not just better display
Format confusion: DVD = SD maximum, Blu-ray = HD capable - this is a hardware limitation