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:
- The angles of dip are a measure of the inclination of the Earth's magnetic field with respect to the horizontal plane.
- The apparent angles of dip, $\theta_1$ and $\theta_2$, are measured in two vertical planes that are perpendicular to each other.
- The true angle of dip is the resultant of these two apparent measurements, which in essence combines the effects seen in both vertical planes.
- 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:
\[\cot^2 \theta = \cot^2 \theta_1 + \cot^2 \theta_2\]