Tag: physics

Questions Related to physics

Which of the following has/have zero average value in a plane electromagnetic wave?

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. Both magnetic and electric fields

  2. Electric fields only

  3. Magnetic field only

  4. None of these

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Correct Option: A
Explanation:

Both electric and magnetic field have sinusoidal nature in a plane electromagnetic wave. As we know, the average value of a sinusoidal wave is zero, so both magnetic and electric fields have average values zero.

Which of the following ray is not electromagnetic wave?

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. X-rays

  2. $\gamma$-rays

  3. $\beta$-rays

  4. Heat rays

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Correct Option: C
Explanation:

Cosmic rays, γ-rays, and X-rays are part of electromagnetic spectrum, while β-rays are emitted by radioactive elements. Hence β-rays is not electromagnetic waves.

Which of the following is not true for electromagnetic waves?

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. They transport energy.

  2. They have momentum.

  3. They travel at different speeds in air depending on their frequency.

  4. They travel at different speeds in medium depending on their frequency.

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Correct Option: C
Explanation:

Electromagnetic wave have constant velocity in a particular medium and it is independant of frequency and it is equal to $V=\sqrt{\dfrac{1}{\mu _0 \epsilon _0}}=3 \times 10^8 \ m/s$ 

Option C

An electromagnetic wave of frequency $\upsilon = 3$ MHz passes from a vacuum into a dielectric medium with permittivity $ = 4$, Then

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. Wavelength and frequency both become half.

  2. Wavelength is doubled and frequency remains unchanged.

  3. Wavelength and frequency both remains unchanged.

  4. Wavelength is halfved and frequency remains unchanged.

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Correct Option: D
Explanation:

Frequency remains constant during refraction.

$v _{med}=\dfrac{1}{\sqrt{\mu _0 \epsilon _0 *4}}$=$\dfrac{c}{2}$

$\dfrac {\lambda _{med}}{\lambda _{air}} = \dfrac{v _{med}}{v _{air}}$

= $\dfrac{\dfrac{c}{2}}{c}$ =$\dfrac{1}{2}$
​$\therefore$ wavelength is halved and frequency remains unchanged.

A plane electromagnetic wave of frequency 50 MHz travels in free space along the x-direction . At a particular point in space and time, $\overrightarrow { E } $ = 6.3 $\hat j$ V ${m}^{-1}$. At this point $\overrightarrow { B } $ is equal to

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. $8.33\ \times { 10 }^{ -8 }\hat { k }\ T$

  2. $18.9\ \times { 10 }^{ -8 }\hat { k }\ T$

  3. $4.1\ \times { 10 }^{ -8 }\hat { k }\ T$

  4. $2.1\ \times { 10 }^{ -8 }\hat { k }\ T$

Show answer
Correct Option: D
Explanation:
Given: The Electric field intensity due to electromagnetic waves is, $E _0=6.3\ V/m$

To find: The direction and intensity of the magnetic field of Electromagnetic wave.

The magnetic field intensity due to the wave is given by
$B _0=\dfrac{E _0}{C}$

$\Rightarrow B _0=\dfrac{6.3}{3\times 10^8}=2.1\times 10^{-8}$ Tesla

Since the propagation of the wave is along $\hat i$ (X-direction) and the electric field vector is along $\hat j$, the magnetic field of the wave should lie perpendicular to both the direction of propagation and direction of electric field.

So, $\vec B$ should be along $\vec k$ such that $\vec E\times \vec B$ should give $\hat i$ ( propagation in X-direction).

$\Rightarrow \vec B=B _0\hat k$
$\vec B=2.1\times 10^{-8}\ T\ \hat k$

Which one of the following is a property of a monochromatic, plane electromagnetic wave in free space?

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. Electric and magnetic fields have a phase difference of ${\pi}/{2}$

  2. The energy contribution of both electric and magnetic fields are equal

  3. The direction of propagation is in the direction of $\overline { B } \times \overline { E } $

  4. The pressure exerted by the wave is the product of its speed and energy density.

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Correct Option: B
Explanation:

The energy contribution of both electric and magnetic fields are equal, Energy density is written as $E=\dfrac 12\epsilon _0E^2=\dfrac{B^2}{2 \mu _0}$

And the direction of propagation of wave is written as $\hat V= \hat E\times \hat B$
Option B

The ultra high frequency band of radiowaves in electromagnetic waves is used as in

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. Television waves

  2. Cellular phone communication

  3. Commercial FM radio

  4. Both (b) and (c)

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Correct Option: B
Explanation:
Radio Waves
  • Radio waves are usually in the frequency range from $500\ kHz$ to $1000\ MHz$.
  • TV waves range from $54\ MHz$ to $890\ MHz$.
  • The FM (frequency modulated) radio band is from $88\ MHz$ to $108\ MHz$.
  • Cellular phones also use radio waves to transmit voice communication in an ultra-high frequency $(UHF)$ band.
As given in last point radio waves are used in cellular phone communication. 

Which of the following electromagnetic wave play an important role in maintaining the earths warmth or average temperature through the green house effect?

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. Visible rays

  2. Infrared rays

  3. Gamma rays

  4. Ultraviolet rays

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Correct Option: B
Explanation:

Infrared radiation plays an important role in maintaining the earth's warmth or average temperature through the greenhouse effect. It is the part of the spectrum that is detected by the human eyedetected by the human eye.

A plane electromagnetic wave travels in free space along X-direction. If the value of $\overrightarrow { B } $ (in tesla) at a particular point in space and time is $1.2 \times {10}^{-8} \hat {k}$, the value of $\overrightarrow { E } $ (in V ${m}^{-1}$) at that point is,

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. $1.2\ \hat {j}$

  2. $3.6\ \hat {k}$

  3. $1.2\ \hat {k}$

  4. $3.6\ \hat {j}$

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Correct Option: D
Explanation:
Given: The magnetic field of the plane electromagnetic wave is $1.2×10^{-8} \hat k\ T$
The direction of propagation of the electromagnetic wave is along X-direction.

The magnitude of  $\vec E$ is given by:
$E\, = \, B\cdot c\\ \ \ \ \ = (1.2 \, \times \, 10^{-8} T)(3 \, \times \, 10^{-8} m \,  s^{-1})\\ \ \ \ \ = 3.6 \,  V/m$

Since the magnetic field is along $Z-$ direction and the wave propagates along $X -$ direction. Therefore $\vec E$  should be in a direction perpendicular to both $X$ and $Z$ axes.

Using vector algebra  should be along X-direction.

Since $(+\hat j) \times (\hat k )= \hat i$

$\vec E$ is along the $Y-$direction.
Thus,  $\vec E= 3.6\hat j\ Vm^{-1}$

The electromagnetic wave travel in free space with the velocity of 

the nature of electromagnetic waves space exploration and forms of light observing space: telescopes electromagnetic waves physics

  1. Sound

  2. Light

  3. Greater than that of light

  4. Greater than that of sound

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Correct Option: B
Explanation:

Electromagnetic waves travel at the speed of light ($3 \times 10^8\ m/s$) in vacuum.