Question

The thickness of a CD ROM Is..........mm approximately.

• A. 1.22
• B. 1.5
• C. 0.66
• D. None of these

Hint:

The physical base dimensions of standard optical discs (CDs, DVDs, Blu-rays) are universally identical to ensure backward compatibility across different optical disk drives: 120 mm in overall diameter and 1.2 mm in thickness.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The topic is Computer Hardware Specifications. This question asks for the physical dimension (thickness) of a standard Compact Disc Read-Only Memory (CD-ROM). These specifications are part of the rigid international standards (the Rainbow Books) that ensure any disc produced by any manufacturer can fit into and be read by any drive made by any other manufacturer. Precise knowledge of these measurements is often required in technical engineering and hardware exams.
Step 2: Key Formulas and approach:
The approach involves referencing the "Red Book" standard for Compact Discs. These are fixed physical constants:
1. Diameter = 120 mm.
2. Thickness = 1.2 mm.
3. Center Hole Diameter = 15 mm.
We compare the standard measurement (1.2 mm) against the choices provided in the question to determine if a match exists.
Step 3: Detailed Explanation:

According to the international standard for optical discs, the thickness of a standard CD is exactly 1.2 mm.

This 1.2 mm thickness consists of the polycarbonate plastic substrate, the reflective metallic layer, and the protective lacquer coating.

Let's look at the options: (A) 1.22 mm is close but mathematically distinct from the 1.2 mm standard. (B) 1.5 mm is far too thick. (C) 0.66 mm is approximately half the thickness (closer to a single layer of a DVD).

Because the industry-standard value is 1.2 mm and it is not explicitly listed among the numerical options, the most accurate choice is "None of these".

Step 4: Final Answer:
The standard thickness of a CD ROM is 1.2 mm. Consequently, the correct choice is (D) None of these.

Answer 2

A useful way to judge the thickness options for a CD-ROM is to compare them against everyday objects whose thickness is easy to picture, rather than relying purely on memorized specification numbers.

1. 1.22: Picture a stack of about six to seven sheets of ordinary printer paper, each roughly 0.1 mm thick; that stack would measure close to 0.7 mm, noticeably thinner than 1.22 mm. A real CD is very slightly less than 1.22 mm, so this figure overshoots by a hair.
2. 1.5: A standard plastic credit card is about 0.76 mm thick. A CD feels only marginally thicker than one credit card, not as thick as two stacked credit cards would suggest for 1.5 mm, so a genuine CD is thinner than this comparison implies.
3. 0.66: This is close to the thickness of a single credit card. Holding an actual CD, it clearly feels sturdier and more rigid than a flexible credit card, so 0.66 mm understates its real bulk.
4. None of these: Since a real CD sits at exactly 1.2 mm, between these comparison points, and none of the three numeric choices equal that value, this is the option that correctly reflects reality.

Using familiar objects as a ruler shows that none of the three specific numbers offered actually lines up with a real CD's true thickness.

Therefore, the correct answer is None of these.