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 electromagnetic spectrum electromagnetic waves physics

Which is having minimum wavelength ?

  1. X-rays

  2. Ultraviolet rays

  3. y - rays

  4. Cosmic rays
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Cosmic Rays             ${{10}^{-14}}m\,\,\,to\,\,{{10}^{-12}}m$

$\gamma $ Rays                        ${{10}^{-12}}m\,\,\,to\,\,{{10}^{-10}}m$

$x$ Rays                        ${{10}^{-10}}m\,\,\,to\,\,{{10}^{-09}}m$

Ultraviolet Rays         ${{10}^{-07}}m\,\,\,to\,\,\,4\times {{10}^{-07}}m$ 

Hence, Cosmic Rays have smallest wavelength.

Multiple choice electromagnetic spectrum electromagnetic waves physics

An electromagnetic radiation has an energy 14.4 eV. To which region of electromagnetic spectrum does it belong?

  1. Ultraviolet region

  2. Visible region

  3. X-ray region

  4. Y-ray region
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

$ E=\dfrac{hc}{\lambda } $

$ \lambda =\dfrac{hc}{E}=\dfrac{6.626\times {{10}^{-34}}\times 3\times {{10}^{8}}}{14.4\times 1.6\times {{10}^{-19}}}=0.86\times {{10}^{-09}} $

Cosmic Rays                ${{10}^{-14}}m\,\,\,to\,\,{{10}^{-12}}m$

$\gamma$ Rays                        ${{10}^{-12}}m\,\,\,to\,\,{{10}^{-10}}m$

$x$ Rays                        ${{10}^{-10}}m\,\,\,to\,\,{{10}^{-09}}m$

Ultraviolet Rays         ${{10}^{-7}}m\,\,\,to\,\,\,4\times {{10}^{-7}}m$

Hence, it lies in $x$ Rays region.

Multiple choice electromagnetic spectrum electromagnetic waves physics

Electromagnetic wave of intensity $1400\ W/m^{2}$ falls on metal surface on area $1.5\ m^{2}$ is completely absorbed by it. Find out force exerted by beam.

  1. $14\times 10^{-5}N$
  2. $14\times 10^{-6}N$
  3. $7\times 10^{-5}N$
  4. $7\times 10^{-6}N$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation
For a perfectly absorbing surface,
$F=\dfrac{IA}{C}$

$=\dfrac{(1400w/m^2\times 1.5m^2)}{(3\times 10^8m/s)}$

$=7\times 10^{-6}N$.
Multiple choice deflection of electron beam by magnetic field observing the force and electron beam tubes charged particles electromagnetic forces physics

Monochromatic light of wavelength 440 nm is produced. The power emitted by light is 18 mW. The number of photons emitted per second by light beam is $(h=6.6\times { 10 }^{ -34 }Js)$

  1. $3\times { 10 }^{ 16 }$
  2. $4\times { 10 }^{ 16 }$
  3. $3\times { 10 }^{ 18 }$
  4. $4\times { 10 }^{ 18 }$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Given,

$\lambda=440nm$
$h=6.6\times 10^{-34} Js$
$I=18m W$
The energy of monochromatic light,
$E=\dfrac{hc}{\lambda}$
$E=\dfrac{6.6\times 10^{-34}\times 3\times 10^8}{440\times 10^{-9}}$
$E=0.045\times 10^{-17}J$
The number of photon emitted per second by the light beam,
$n=\dfrac{I}{E}$ 
$n=\dfrac{18\times 10^{-3}}{0.045\times 10^{-17}}$
$n=4\times 10^{16}$
The correct option is B

Multiple choice deflection of electron beam by magnetic field observing the force and electron beam tubes charged particles electromagnetic forces physics

Out of the following transitions, the frequency of emitted photon will be maximum for 

  1. $n = 5$ to $n = 3$
  2. $n = 6$ to $n = 2$
  3. $n = 2$ to $n = 1$
  4. $n = 1$ to $n = 2$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The frequency of an emitted photon is proportional to the energy difference of the transition (E = hf). The energy difference is greatest for the n=2 to n=1 transition in the hydrogen atom, as the energy levels are spaced further apart at lower quantum numbers.

Multiple choice energy in wave motion oscillation and waves waves physics

The maximum potential energy / length increases with:

  1. Amplitude

  2. Wavelength

  3. Frequency

  4. Velocity
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

If $y= A \sin (\omega t- kx)$ is the equation of a wave through a string, then the slope of the wave is $\dfrac{dy}{dx}=- Ak \cos (\omega t- kx)$. 

The maximum potential energy will be $T \times \Delta x \times (\dfrac{dy}{dx})^2A^2k^2=A^2k^2 \cos ^2(\omega t -  kx)T \Delta x$

The maximum potential energy will be obtained if cos (\omega t - kx)=1. Thus, maximum potential energy = $4 \pi^2A^2T f \times T \times \Delta x $; T is the tension in the string

We also know that $T=\mu v^2$. Substituting, we get, 

Maximum potential energy = $4 \pi^2 f^2A^2 \mu$
Thus maximum potential energy depends on frequency and as frequency increases, potential energy also increases

The correct option is (c)

Multiple choice physics kirchhoff's law circuit problems kirchoff's law and problems on it equivalent resistance in series and parallel connection

"A good absorber of a given wavelength of radiation is also a good emitter of that wavelength." This is a statement of:

  1. Stefan-Boltzmann's law

  2. Wien's Law

  3. Kirchoff's Law

  4. The First Law of Thermodynamics
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Kirchhoff's Law of radiation states that for an arbitrary body emitting and absorbing thermal radiation in thermodynamic equilibrium, the emissivity is equal to the absorptivity.

Multiple choice physics dual nature of matter and radiation davisson and germer experiment and its conclusion matter waves wave nature of matter

The wavelength of $L _\alpha$ line in $X-ray$ spectrum of $Pt^{78}$ is $1.32\mathring { A } $ then wavelength of $L _\alpha $ line $X-ray$ spectrum of another unknown element is $4.17\mathring { A } .$ If screening constant for $L _\alpha $ line is $7.4,$ then atomic number of the unknown element is -

  1. $78$
  2. $47$
  3. $40$
  4. $35$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Moseley's Law for X-rays: 1/lambda = R * (Z-sigma)^2 * (1/n1^2 - 1/n2^2). Since the transition is the same (L_alpha), (Z1-sigma)^2 * lambda1 = (Z2-sigma)^2 * lambda2. Plugging in Z1=78, sigma=7.4, lambda1=1.32, lambda2=4.17, we solve for Z2.

Multiple choice relativistic mechanics option a: relativity physics

Radiation with energy that is easily detected as quanta _______________.

  1. $1$ eV
  2. $1$ KeV
  3. $1$ MeV
  4. $10^{-10}$ eV
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Radiation with energy in the MeV range (gamma rays) is typically detected as discrete quanta using devices like scintillation counters. Lower energies are often treated as waves or are harder to detect as individual quanta in standard laboratory settings.

Multiple choice perceive colours resolution of optical instruments lenses option c: imaging physics

The dispersion of a medium for wavelength $\lambda$ is D. Then the dispersion for the wavelength $2\lambda$ will be:

  1. $(D/8)$
  2. $(D/4)$
  3. $(D/2)$

  4. D
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

As we know, Cauchy's Dispersion formula is :
$ \mu $= $A + \dfrac{B}{ \lambda^{^{2}}} $
And dispersion
D= - $ \dfrac {d\mu}{d\lambda}$
Therefore, from the above 2 equations:
D = $ -(-2\lambda ^{3})B $= $ \dfrac {2B}{\lambda^{3}}$
This implies that
D $ \alpha \dfrac{1}{\lambda^{3}}$
Hence,
$ \dfrac {{D}'}{D}$ = $( \dfrac{\lambda}{{\lambda}' })^{3}$
As $ {\lambda}' = 2\lambda$
Therefore,
$ {D}' =D/8$

Multiple choice physics nuclei gamma decay change in nucleus due to radioactive decay alpha, beta and gamma particles (rays) and their properties

A beam of ultraviolet radiation having wavelength between $100nm$ and $200nm$ is incident on a sample of atomic hydrogen gas. Assuming that the atoms are in ground state, which wavelengths will have low intensity in the transmitted beam? 

  1. $104nm$
  2. $103nm$
  3. $105nm$
  4. $100nm$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Energy corresponding to wavelength 100nm, $\dfrac{1242eV}{100}=12.42 eV$

Energy corresponding to wavelength 200nm, $\dfrac{1242eV}{200}=6.21 eV$
Energy required from ground state to first excited stage:
$E _2-E _1=13.6-3.4=10.2 eV$
Energy required from ground state to second excited stage:
$E _3-E _1=13.6-13.6-1.5=12.1  eV$

Energy required from ground state to third excited stage:
$E _3-E _1=13.6-0.85=12.75  eV$

At 10.2 eV,
$Wavelength 1=\dfrac{1242}{10.2}=122nm$

At 12.1 eV,
$Wavelength 2=\dfrac{1242}{12.1}=103nm$



Multiple choice gamma decay change in nucleus due to radioactive decay alpha, beta and gamma particles (rays) and their properties

A gamma ray photon creates an electron-positron pair. If the rest mass energy of an electron is $0.5MeV$ and the total kinetic energy of the electron-positron pair is $0.78 MeV$, then the energy of the gamma ray photon must be

  1. $0.78MeV$
  2. $1.78MeV$
  3. $1.28MeV$
  4. $0.28MeV$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Energy of $\gamma $-rays photon $=$ Rest mass energy $+$ $K.E$ 

                                         $=2\left( 0.5 \right) +0.78\ = 1.78\ MeV$

Multiple choice gamma decay change in nucleus due to radioactive decay alpha, beta and gamma particles (rays) and their properties

The wavelength of emitted $\gamma $ rays are in the other

  1. ${ \lambda } _{ \gamma 2 }\quad >\quad { \lambda } _{ \gamma 3 }\quad >{ \quad \lambda } _{ \gamma 1 }$
  2. ${ \lambda } _{ \gamma 3 }\quad >\quad { \lambda } _{ \gamma 2 }\quad >{ \quad \lambda } _{ \gamma 1 }$
  3. ${ \lambda } _{ \gamma 1 }\quad >\quad { \lambda } _{ \gamma 2 }\quad >{ \quad \lambda } _{ \gamma 3 }$
  4. ${ \lambda } _{ \gamma 3 }\quad >\quad { \lambda } _{ \gamma 1 }\quad >{ \quad \lambda } _{ \gamma 2 }$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

As the frequencies of $\gamma-rays$ are in the order:

$\nu _{\gamma 1} > \nu _{\gamma 3} > \nu _{\gamma 2}$
Thus as wavelength is inversely proportional to frequency.
$\lambda _{\gamma 2} > \lambda _{\gamma 3} > \lambda _{\gamma 1}$
Hence option A is correct

Multiple choice capacitance of an isolated spherical conductor capacitance of isolated bodies capacitance physics

The inductance of the oscillatory circuit of a radio station is 10 milli henry band its capacitance is $0.25 \mu F$. Taking the effect of the resistance negligible, wavelength of the broadcasted waves will be (velocity of light = $3.0  \ 10 ^4 \ m/s, \pi = 3.14$):

  1. $9,42 \times 10^4 m$
  2. $18.8 \times 10^4 m$
  3. $4.5 \times 10^4 m$
  4. $none\ of\ these$
Reveal answer Fill a bubble to check yourself
C Correct answer
Multiple choice kirchoff's laws black body radiation heat transfer thermal properties physics

Kirchoffs law states that

  1. a body absorbs radiation of shorter wavelengths and emits radiation of higher wavelength

  2. a body absorbs radiation of any wavelength but emits radiation of specific wavelengths

  3. a body absorbs and emits radiation of same wavelengths

  4. none of these
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

In thermodynamics, Kirchhoff's law of thermal radiation refers to wavelength-specific radiative emission and absorption by a material body in thermodynamic equilibrium, including radiative exchange equilibrium.