Question:medium

If $\theta_1$ and $\theta_2$ be the apparent angles of dip observed in two vertical planes at right angles to each other, then the true angle of dip $\theta$ is given by :

Updated On: May 8, 2026
  • $\tan^2 \theta = \tan^2 \theta_1 + \tan^2 \theta_2$
  • $\cot^2 \theta = \cot^2 \theta_1 - \cot^2 \theta_2$
  • $\tan^2 \theta = \tan^2 \theta_1 - \tan^2 \theta_2$
  • $\cot^2 \theta = \cot^2 \theta_1 + \cot^2 \theta_2$
Show Solution

The Correct Option is D

Solution and Explanation

To solve the problem of finding the true angle of dip given the apparent angles of dip in two vertical planes at right angles, we use the following concept from physics:

The relation between the true angle of dip $\theta$ and the apparent angles of dip $\theta_1$ and $\theta_2$ when measured at right angles to each other is given by:

\[\cot^2 \theta = \cot^2 \theta_1 + \cot^2 \theta_2\]

Let's understand why this is the correct expression:

  1. The angles of dip are a measure of the inclination of the Earth's magnetic field with respect to the horizontal plane.
  2. The apparent angles of dip, $\theta_1$ and $\theta_2$, are measured in two vertical planes that are perpendicular to each other.
  3. The true angle of dip is the resultant of these two apparent measurements, which in essence combines the effects seen in both vertical planes.
  4. Based on the geometry and trigonometric identities of the cotangent function, the above equation for the true angle of dip ($\cot^2 \theta = \cot^2 \theta_1 + \cot^2 \theta_2$) is derived.

Let's rule out the other options:

  • $\tan^2 \theta = \tan^2 \theta_1 + \tan^2 \theta_2$: This suggests an incorrect additive relationship for tangent, not typically used in the scenario of dip angles.
  • $\cot^2 \theta = \cot^2 \theta_1 - \cot^2 \theta_2$: This implies a negative cotangent relationship, which contradicts the principle of combining perpendicular components.
  • $\tan^2 \theta = \tan^2 \theta_1 - \tan^2 \theta_2$: Similar to the second option, but again incorrectly applies a subtraction for tangents.

Therefore, the correct relationship that provides the true angle of dip in terms of the apparent angles in perpendicular planes is indeed:

\[\cot^2 \theta = \cot^2 \theta_1 + \cot^2 \theta_2\]

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