Which scientific device or instrument is commonly used to measure the depth of the ocean or sea bed?

Introduction / Context:

This general knowledge physics question asks about the specialised instrument used to measure the depth of the ocean or sea bed. Measuring sea depth is important for navigation, submarine operations, laying underwater cables, and understanding ocean topography. Different scientific instruments are used to measure length, density, or very small sizes, so it is important to connect each instrument name with its correct application.

Given Data / Assumptions:

• We are interested in measuring the depth of the ocean or sea bed below the water surface.
• The options list four instruments: Lexometer, Nanometer, Fathometer, and Hydrometer.
• Each instrument is associated with a different type of physical measurement.
• Standard textbook definitions of these devices are assumed.

Concept / Approach:

A fathometer is an instrument that measures depth using the echo sounding principle. It sends ultrasonic or sound pulses downward into the water. These pulses reflect from the sea bed and return to the instrument. By measuring the time taken for the echo to return and knowing the speed of sound in water, the depth of the ocean is calculated. Other instruments in the list are used in very different contexts and are not suitable for depth sounding in oceans.

Step-by-Step Solution:

Verification / Alternative check:

Ship navigation charts and descriptions of marine exploration often mention echo sounders and fathometers as standard equipment. Historical depth units included the fathom, which gives the instrument its name. Modern sonar systems are based on the same principle, confirming that fathometers are indeed depth measuring devices. The other instruments have well established uses that do not match ocean depth measurement, which supports the conclusion.

Why Other Options Are Wrong:

• Lexometer: This name is rarely used in standard physics contexts for ocean depth and is not the recognised echo sounding instrument.
• Nanometer: Linked with measuring or indicating nanometre scale lengths, used in optics or materials science, not in marine depth measurement.
• Hydrometer: Designed to measure the relative density of liquids such as acids, milk, or battery electrolyte, not vertical distance below water surface.

Common Pitfalls:

A common confusion is to pick hydrometer because it is clearly related to liquids. However, hydrometer measures density, not depth. Another error is to be distracted by the word nano and think about precision, but nanometer refers to extremely small lengths, which is the opposite scale from ocean depths. Remember that ocean depth is measured by echo sounding, and fathometer is the classic name for such an instrument on ships.

Final Answer:

The device used to measure the depth of the ocean is the fathometer.