Physics

Electromagnetic Waves and Spectrum

659 Questions

Electromagnetic waves and spectrum questions test a candidate's understanding of radiation frequencies, wavelengths, and the properties of different rays like infrared, ultraviolet, and visible light. Concepts also cover practical applications in astronomy and the fundamental speed of light calculations. This physics topic appears regularly in general science sections of major competitive examinations.

UV rays propertiesElectromagnetic radiation frequencyWavelength identificationSpeed of light calculationsBlack body radiation

Electromagnetic Waves and Spectrum Questions

Multiple choice

A medium is divided into regions I and II about x = 0 plane, as shown in the figure below. An electromagnetic wave with electric field E1 = 4$\widehat a$x + 3$\widehat a$y + 5$\widehat a$z is incident normally on the interface form region-I. The electric field E2 in region-II at the interface is

  1. E2 = E1
  2. 4$\widehat a$x + 0.75$\widehat a$y + 1.2 5$\widehat a$z
  3. 3$\widehat a$x + 3$\widehat a$y + 5$\widehat a$z
  4. - 3$\widehat a$x + 3$\widehat a$y + 5$\widehat a$z
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Multiple choice

Two identical antennas are placed in the $\theta$= $\dfrac{\pi}{2}$ plane as shown in figure. The elements have equal amplitude excitation with 180° polarity difference, operating at wavelength $\lambda$. The correct value of the magnitude of the far-zone resultant electric field strength normalized with that of a single element, both computed for $\phi$= 0, is
  1. 2 cos $\left( \dfrac{2\pi s}{\lambda} \right)$
  2. 2 sin $\left( \dfrac{2\pi s}{\lambda} \right)$
  3. 2 cos $\left( \dfrac{\pi s}{\lambda} \right)$
  4. 2 sin $\left( \dfrac{\pi s}{\lambda} \right)$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Multiple choice

Two 4-ray signal constellations are shown. It is given that $\phi_1$ and $\phi_2$ constitute an orthonormal basis for the two constellations. Assume that the four symbols in both the constellations are equiprobable. Let $\dfrac{N_0}{2}$ denote the power spectral density of white Gaussian noise.

If these constellations are used for digital communications over an AWGN channel, then which of the following statements is true?

  1. Probability of symbol error for constellation 1 is lower.

  2. Probability of symbol error for constellation 1 is higher.

  3. Probability of symbol error is equal for both the constellations.

  4. The value of 0 N will determine which of the two constellations has a lower probability of symbol error.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Noise power is same for both which is $\dfrac{N_0}{2}$

Thus probability of error will be lower for the constellation 1 as it has higher signal energy.

Multiple choice

Two 4-ray signal constellations are shown. It is given that $\phi_1$ and $\phi_2$ constitute an orthonormal basis for the two constellations. Assume that the four symbols in both the constellations are equiprobable. Let $\dfrac{N_0}{2}$ denote the power spectral density of white Gaussian noise.

The ratio of the average energy of constellation 1 to the average energy of constellation 2 is

  1. 4a2

  2. 3.5

  3. 2

  4. 8
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

By using solution we get                        Energy of constellation 1 = 14 a^2                      Energy of constellation 2 = 4 a^2                                                  Ratio = 14 a^2/4 a^2 = 3.5                      

Multiple choice

A medium of relative permittivity $\epsilon_{r2}$ = 2 forms an interface with free-space. A point source of electromagnetic energy is located in the medium at a depth of 1 metre from the interface. Due to the total internal reflection, the transmitted beam has a circular cross-section over the interface. The area of the beam cross section at the interface is given by
  1. 2$\pi$ m2
  2. $\pi$2 m2
  3. $\frac{\pi}{2}$ m2
  4. $\pi$m2
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Multiple choice

Consider the following statements regarding the complex Poynting vector $\overrightarrow{P}$ for the power radiated by a point source in an infinite homogeneous and lossless medium Re $(\overrightarrow{P})$ denotes the real part of $\overrightarrow{P}$. S denotes a spherical surface whose centre is at the point source, and $\hat{n}$ denotes the unit surface normal on S. Which of the following statements is TRUE?
  1. Re $(\overrightarrow{P})$ remains constant at any radial distance from the source
  2. Re $(\overrightarrow{P})$ increases with increasing radial distance from the source
  3. _ $∯_s Re$$(\overrightarrow{P})$.$\hat{n}$dS remains constant at any radial distance from the source
  4. $∯_s Re$$(\overrightarrow{P})$.$\hat{n}$dS decreases with increasing radial distance form the source
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Powe radiated from any source is constant.