Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): Sextant is a modern navigational instrument, which is used for the measurement of vertical and horizontal angles at sea.
Reason (R): It is an instrument of double reflection by means of two mirrors and is capable of measuring angles up to 120\(^\circ\).
In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

Remember that a sextant uses a 60\(^\circ\) physical arc (one-sixth of a circle) to measure angles up to 120\(^\circ\) because of the principle of double reflection. This optical rule is a common topic in navigation exams.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Celestial navigation relies on measuring the angle between a celestial body (such as the sun, moon, or stars) and the horizon.
The primary optical instrument used to make these measurements at sea is the sextant.

Step 2: Detailed Explanation:
Let us evaluate the Assertion and Reason individually:
Assertion (A):
A marine sextant is an essential navigational instrument used to measure the angle between the horizon and a celestial body, or the horizontal angle between two landmarks on shore.
These angle measurements allow mariners to calculate their geographic position (latitude and longitude) at sea.
Thus, Assertion A is correct.
Reason (R):
The optical design of a sextant is based on the principle of double reflection.
It uses two mirrors: the index glass (which rotates with the index arm) and the horizon glass (which is fixed).
Based on the laws of reflection, when a ray of light is reflected consecutively from two mirrors, the total angle of deviation is twice the angle between the reflecting surfaces.
This double reflection design allows an instrument with a physical arc of 60\(^\circ\) (one-sixth of a circle, hence "sextant") to measure angles up to 120\(^\circ\).
Thus, Reason R is also correct.
While both statements are correct, Reason R describes the optical design and capacity of the instrument, rather than explaining why or how it is used for navigation.
Therefore, Reason R is not the correct explanation for Assertion A.

Step 3: Final Answer:
Both (A) and (R) are true, but (R) is not the correct explanation of (A).
This corresponds to Option (B).