The nature of electromagnetic waves - class-XII
the nature of electromagnetic waves
Questions
Choose the correct answer from the alternatives given.
Maxwell in his famouse questions of eletromagnetism introduce the concept of:
- ac current
- displacement current
- impedance
- reatance
Which of the following has/have zero average value in a plane electromagnetic wave?
- Both magnetic and electric fields
- Electric fields only
- Magnetic field only
- None of these
Which of the following ray is not electromagnetic wave?
- X-rays
- $\gamma$-rays
- $\beta$-rays
- Heat rays
Which of the following is not true for electromagnetic waves?
- They transport energy.
- They have momentum.
- They travel at different speeds in air depending on their frequency.
- They travel at different speeds in medium depending on their frequency.
An electromagnetic wave of frequency $\upsilon = 3$ MHz passes from a vacuum into a dielectric medium with permittivity $ = 4$, Then
- Wavelength and frequency both become half.
- Wavelength is doubled and frequency remains unchanged.
- Wavelength and frequency both remains unchanged.
- Wavelength is halfved 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
- $8.33\ \times { 10 }^{ -8 }\hat { k }\ T$
- $18.9\ \times { 10 }^{ -8 }\hat { k }\ T$
- $4.1\ \times { 10 }^{ -8 }\hat { k }\ T$
- $2.1\ \times { 10 }^{ -8 }\hat { k }\ T$
Which one of the following is a property of a monochromatic, plane electromagnetic wave in free space?
- Electric and magnetic fields have a phase difference of ${\pi}/{2}$
- The energy contribution of both electric and magnetic fields are equal
- The direction of propagation is in the direction of $\overline { B } \times \overline { E } $
- The pressure exerted by the wave is the product of its speed and energy density.
The ultra high frequency band of radiowaves in electromagnetic waves is used as in
- Television waves
- Cellular phone communication
- Commercial FM radio
- Both (b) and (c)
Which of the following electromagnetic wave play an important role in maintaining the earths warmth or average temperature through the green house effect?
- Visible rays
- Infrared rays
- Gamma rays
- Ultraviolet rays
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,
- $1.2\ \hat {j}$
- $3.6\ \hat {k}$
- $1.2\ \hat {k}$
- $3.6\ \hat {j}$
The electromagnetic wave travel in free space with the velocity of
- Sound
- Light
- Greater than that of light
- Greater than that of sound
The waves that are propagated by simultaneous periodic variations of electric and magnetic field intensity known as:
- Transverse wave
- Longitudinal wave
- Electromagnetic wave
- None of the above
Select wrong statement from the following:
Electromagnetic waves
- are transverse
- travel with same speed in all media
- travel with the speed of light
- are produced by accelerating charge
Electromagnetic waves do not transport
- energy
- charge
- momentum
- information
The wavelengths for the light of red and blue colours are roughly $7.8\times 10^{-7}m$ and $4.8\times 10^{-7}m$ respectively. Which colour has the greater speed in glass?
- Red light
- Violet light
- Yellow light
- All colors have same speed
What is the speed of Light $c$?
- $c=3\times 10^{-8} m s^{-1}$
- $c=3\times 10^8 km s^{-1}$
- $c=3\times 10^8 mm s^{-1}$
- $c=3\times 10^8 m s^{-1}$
The electromagnetic waves do not transport
- energy
- charge
- momentum
- information
In which form does the Light propagates?
- Electric waves
- Magnetic waves
- Electromagnetic waves
- Sound wave
As speed decreases, if we change medium of electromagnetic waves from air to water, frequency
- also decreases.
- also increases.
- remains same.
- may increase or decrease.
A plane electromagnetic wave of angular frequency ${ mm }^{ 2 }$ propagates in a poorly conducting medium of conductivity $\sigma $ and relative permittivity $\epsilon $ Find the ratio of conduction current density and displacement current density in the medium.
- ${ \epsilon \epsilon } _{ 0 }\omega /\sigma $
- $\sigma /{ \epsilon \epsilon } _{ \sigma }\omega $
- $\omega /{ \sigma \epsilon } _{ 0 }\omega $
- $\omega \sigma /{ \epsilon } _{ 0 }\epsilon $
Which among the four options is not correct ion given situation ? A charged particle oscillates about its mean equilibrium position with a frequency of $10^9, Hz.$ The electromagnetic waves produced
- Will have frequency of $10^9\, Hz.$
- Will have frequency of $2\times 10^9\, Hz.$
- Will have wavelength of $0.3\,m.$
- Fall in the region of radio waves
An electromagnetic wave with frequency $\omega$ and wavelength $\lambda$ travels in the $+y$ direction. Its magnetic field is along $-x$ axis. The vector equation for the associated electric field (of amplitude E_0)is :
- $ \vec { E } = E _0 \cos \left(\omega t -\dfrac{2\pi}{\lambda}y\right)\hat{x}$
- $\vec{ E } = -E _0 \cos \left(\omega t +\dfrac{2\pi}{\lambda}y\right)\hat{x}$
- $-\vec{ E } = E _0 \cos \left(\omega t +\dfrac{2\pi}{\lambda}y\right)\hat{z}$
- $\vec { E } = E _0 \cos \left(\omega t -\dfrac{2\pi}{\lambda}y\right)\hat{z}$
Electro magnetic waves are transverse in nature as is evident by
- polarization
- interference
- reflection
- deffraction
The maximum kinetic energy of electron if wavelength of incident electromagnetic wave is $260 ,nm$ and cut-off wavelength is $380 ,nm$ given $hc = 1237 ,nm-eV$ is
- $1.5 \,eV$
- $6.4 \,eV$
- $10 \,eV$
- None of these
If $\vec{E} = E _0 \cos (kz) \cos (\omega t)\hat{i}$ then $\vec{B}$ for electromagnetic wave is:
- $\vec{B} = \dfrac{E _0}{C} \hat{k}$
- $\vec{B} = \dfrac{E _0}{C} \sin (kz) \sin (\omega t)\hat{i}$
- $\vec{B} = \dfrac{E _0}{C} \sin (kz) \cos (\omega t) \hat{j}$
- $\vec{B} = \dfrac{E _0}{C} \cos (kz) \sin (\omega t) \hat{j}$
For the propagation of electromagnetic waves
- Medium is required
- no medium is required
- E and B are in mutually opposite phase
- E and b do not contriute
On the basis of the following features identify correct option:
I. They require no material medium.
II. They are always transverse.
II. They are produced by accelerating charged particles
- Mechanical waves
- Electro-magnetic waves
- Both (1) and (2)
- Neither (1) and (2)
In which of the following media can non-mechanical waves travel?
- In vacuum as well as in a medium
- In vacuum but not in a medium
- In a medium but not in a vacuum
- Neither in a medium nor in vacuum
Which of the following have zero average value in a plane electromagnetic wave?
(i) Magnetic field
(ii) Electric field
(iii) Electric energy
(iv) Magnetic energy.
- (i) and (ii)
- (i) and (iv)
- (ii) and (iii)
- (iii) and (iv)