
Various position vectors are placed in the three-dimensional geometry that denotes the location or position of a point concerning the origin. A 3D space has an infinite number of such position vectors. Thus, in a 3D space, we will get a plane with a specific number of position vectors.
The angle between two planes can be calculated with the help of a normal. Thus, we can say that the angle between two planes and the normal is between the planes.
Let n1 and n2 be the two normal to the planes aligned to each other at an angle θ. The equation of two planes can be given by:
r n₂ = d₂
r n₂ = d₂
The angle between two planes is given by-
A dihedral angle refers to the angle that is between two intersecting planes. It is most commonly used in chemistry. It refers to the angle between planes through two sets of three atoms, which have two atoms in common. In solid geometry, we define it as the union of a line and two half-planes that have this line as a standard edge.
Let us consider A₁ x + B₁ y + C₁z + D₁ = 0 and A₂x + B₂y + C₂z + D₂ = 0 be the equation of two planes at an angle θ where A₁, B₁, C₁ and A₂, B₂, C₂ are the direction ratios of the normal to the planes. From the cosine of the angle, we can find the angle between the two planes, and is given by:
Example
Find the angle between the planes whose vector equations are given by r. (2i + 2j – 3k) = 5 and r. (3i – 3j + 5k) = 3.
Solution
Comparing the equation given in the question with the general equation of a plane in vector form, we get,
n₁ = 2i + 2j – 3k and n2 = 3i – 3j + 5k
| n₁ | = (2² + 2² + (-3)²)½ = 17½ and | n₂| = (3² + (-3)² + 5²)½ = 43½
Thus, Cos = (2i + 2j – 3k). (3i – 3j + 5k) / 17½. 43½
Cos = | 2×3 + 2 x (-3) + (-3) x 5 | / 17½. 43½
Cos = | 6 -6 – 15 | / 17½. 43½
Cos = | -15 | / 731½
Cos = 15 / 731½
Therefore, we get the angle as Cos-1 (15 / 731½).
Example
Calculate the angle between two planes given by the equations 2x + 4y - 4z - 6 = 0 and 4x + 3y + 9 = 0.
Solution
In these equations of the plane,
A1 = 2, B1 = 4, C1 = - 4, D1 = - 6
A2 = 4, B2 = 3, C2 = 0, D2 = 9
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