We have already learnt about trigonometry in previous units in detail. Trigonometry has its own applications in mathematics and in physics. One such application of trigonometry in mathematics is “height and distances”. To know about height and distances, we have to start from the most basic part of that, which is “angle of elevation” and “angle of depression”. The first and foremost angles that we are going to study about here is angle of elevation. In this part of height and distances we will be discussing about angle of elevation in detail.
Definition of Angle of Elevation:
The angle of elevation of an object as seen by the observer is defined as the angle between the horizontal and the line from the object to the observer’s eye. The line in which observer’s eye is there is known as the line of sight.
Let us assume an example where an observer is standing on the ground in front of a pole at a distance of ‘x’ meters from the bottom of the pole. Let us assume that height of pole is ‘y’ meters. If the observer is seeing the top most point of the pole from the ground level, and the angle made by the observer’s eye and the top most point of pole be ‘theta(θ)’ in the given figure:
In the above figure, let
P be the top most point of the pole.
Q be the bottom point of the pole.
R be the position of the observer’s eye.
PQ be the pole of height ‘y’ units;
QR be the distance between the bottom of pole and observer’s eye of ‘x’ units.
PR be the line of sight or the line along which observer is observing the top of the pole of ‘h’ units.
The angle ‘θ’ is the angle of elevation, and it can be found using following formulae:
sin θ = y/h; cosec θ = h/y
cos θ = x/h; sec θ = h/x
tan θ = y/x; cot θ = x/y.
depending upon the data given in the question, corresponding formula is applied to find out the angle of elevation.
Let us look at the following examples to see how to find out the angle of elevation:
1. The height of a pole is 30 m. A man is standing at a distance of 20 m from the foot of the pole. The man looks at the top most point of the point from the place where he is standing. Find out the angle made by the man’s eye with the top most point of the pole.
The above problem can be visualized as:
From the given problem:
PQ = height of the pole = 30m
QR = distance between the man and the foot of the pole = 20m
We have to find angle ‘θ’ which the angle made by the man’s eye with the top most point of the pole and is angle of elevation.
We know that, tan θ = PQ/QR
⟹ tan θ = 30/20
⟹ θ = tan-1 (30/20)
⟹ θ = tan-1 (3/2)
⟹ θ = 56.3o.
2. A ladder of length 30 m is kept in against a wall of length 20m such that their topmost point is in contact with one another and their bottom point are at certain distance as shown in the figure. Find the angle subtended by the ladder on the floor.
Solution: the length of ladder is BA = 30 m
The height of wall is BC = 20 m
We have to find angle BAC = angle subtended by ladder on the floor.
Let angle BAC = α
We know that,
sin α = BC/BA
⟹ sin α = 20/30
⟹ α = sin-1 (20/30)
⟹ α = sin-1 (2/3)
⟹ α = 41.810.
3. A man is standing in front of a wall and looking at its topmost point. If the angle of elevation is 60o. If the height of wall is 40m, then find the distance between the foot of the man and the wall.
The given problem can be visualized as:
Here, angle of elevation, θ = 60o
Height of wall, y = 40 m.
Distance between foot of man and the wall = x
We know that,
tan θ = y/x
⟹ tan θ = 40/x
⟹ x = 40/tan θ
⟹ x = 40/tan 60o
⟹ x = 40/1.732
⟹ x = 23.09
Hence distance between foot of man and wall is 23.09 m or 23.1 m.