---
title: "Propagation of electromagnetic energy"
subject: "Surveying I"
chapter: 2
key: "semester_3/surveying_i:propagation-em-energy"
source: https://www.bctengineer.com/q/semester_3/surveying_i/propagation-em-energy
site: BCT Engineering
---

# Propagation of electromagnetic energy

Surveying I · Chapter 2: Distance Measurements

- Asked in: 2074 Asoj, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/propagation-em-energy
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-2

## Question

Write the propagation of electromagnetic energy.

## Short answer

Electromagnetic energy travels as transverse waves with velocity v = c/n in air, in straight lines. It undergoes reflection (needed for prisms), refraction, absorption and scattering. In EDM, the carrier is modulated with a known frequency; the round-trip time or phase shift yields the distance.

## Answer

Electromagnetic (EM) energy is transmitted as waves consisting of electric and magnetic fields perpendicular to each other and to the direction of travel. Light, infrared, microwave and radio waves are parts of the EM spectrum.

#### Characteristics of propagation
1. **Velocity.** In vacuum, EM waves travel at $c = 299{,}792{,}458$ m/s. In air, the velocity is $v = c/n$, where $n$ is the refractive index, slightly greater than 1 (about 1.0003). It varies with temperature, pressure and humidity.
2. **Frequency and wavelength:**
$$
c = f\,\lambda \quad\text{or}\quad v = f\,\lambda
$$
3. **Straight-line travel.** In a uniform medium the wave travels in a straight line. In real air, refraction bends the path slightly.
4. **Reflection, refraction, absorption and scattering.** At a prism the wave is reflected back parallel to its path; fog, rain, dust and haze absorb or scatter it and reduce the range.
5. **Modulation.** In EDM the carrier wave is modulated in amplitude (or frequency) with a known frequency, so that the measuring wave has a known wavelength.

```text
  Transmitter ~~~~ wave ~~~~> Reflector
              <~~~ return ~~~~
  wavelength  | lambda |   one cycle = 360 degrees
```

#### Use in distance measurement
The wave travels the double distance $2D$, which equals $N\lambda$ plus a fraction of a wavelength. Atmospheric corrections are applied to the velocity because it changes with temperature and pressure. Microwaves are not affected by poor visibility, whereas light and infrared waves are; however, light and infrared waves give more accurate results.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is the velocity in air related to that in vacuum?
   A. v = c n
   B. v = c / n, with n slightly greater than 1
   C. v = c + n
   D. v = c
   Answer: B. The refractive index of air (about 1.0003) slows the wave slightly. (Key point: Velocity in air)

2. Which relation links velocity, frequency and wavelength?
   A. v = f / lambda
   B. v = f lambda
   C. v = lambda / f^2
   D. v = f + lambda
   Answer: B. v = f lambda, so lambda = v / f. (Key point: Wave relation)

3. Which waves are unaffected by poor visibility?
   A. Microwaves
   B. Visible light
   C. Infrared
   D. Laser
   Answer: A. Microwaves pass through haze and rain, while light and infrared are affected but more accurate. (Key point: Weather effects)

4. In EDM the wave travels a total path of
   A. D
   B. 2D
   C. D/2
   D. 4D
   Answer: B. It goes to the reflector and returns, so 2D = N lambda + a fraction. (Key point: Double path)

## Related in Surveying I

- [EDM classification; electromagnetic energy](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aedm-classification-em-energy) — Surveying I, chapter 2

## Related in other subjects

- [Electromagnetic waves and propagation](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aemw-propagation) — Basic Electronics Engineering, chapter 5, asked 2×
- [Define antenna and EMW propagation](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Adefine-antenna-emw) — Basic Electronics Engineering, chapter 5
- [Energy transfer rate of em wave](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Aem-wave-energy-transfer-intensity) — Engineering Physics, chapter 9