Physics

Electromagnetic Waves and Spectrum

670 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 laws of heat transfer heat and thermodynamics physics

Two black metallic spheres of radius 4m, at 2000 K and 1m at 4000 K will have ratio of energy radiation as

  1. 1 : 1

  2. 4 : 1

  3. 1 : 4

  4. 2 : 1
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Power P = sigma * A * T^4. P1 = sigma * 4 * pi * (4)^2 * (2000)^4. P2 = sigma * 4 * pi * (1)^2 * (4000)^4. Ratio P1/P2 = (16 * 2000^4) / (1 * 4000^4) = 16 / 16 = 1. The ratio is 1:1.

Multiple choice laws of heat transfer heat and thermodynamics physics

Choose the correct answer from the alternatives given.
Radiations of intensity $0.5\ W/m^2$ are striking a metal plate. The pressure on the plate is then

  1. $0.166 \, \times \, 10^{-8} \, N \, m^{-2}$
  2. $0.332 \, \times \, 10^{-8} \, N \, m^{-2}$
  3. $0.111 \, \times \, 10^{-8} \, N \, m^{-2}$
  4. $0.083 \, \times \, 10^{-8} \, N \, m^{-2}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Given: The intensity of the incident radiations is $0.5\ W/m^2$.


To find: The radiation pressure on the plate.

The radiation pressure experienced by the plate is given as;
$P = \dfrac{I}{c}\\Rightarrow \dfrac{0.5}{3 \times 10^8}\\Rightarrow  0.166 \times 10^{-8} N m^{-2}$

Option $(A)$ is correct.

Multiple choice laws of heat transfer heat and thermodynamics physics

Star A emits radiation of maximum intensity at a wavelength of $5000 \mathring{A}$ and it has temperature $ 1227^oC $. If star B has temperature $ 2727^oC $ , then the maximum intensity would be observed at 

  1. $ 4000 \mathring{A} $
  2. $ 2250 \mathring{A} $
  3. $ 3000 \mathring{A} $
  4. $ 2500 \mathring{A} $
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

$\begin{array}{l} \dfrac { { \lambda { m^{ 1 } } } }{ { \lambda m } } =\dfrac { T }{ { { T^{ 1 } } } }  \ \dfrac { { \lambda { m^{ 1 } } } }{ { 5000A } } =\dfrac { { 1227+273k } }{ { 2727+273k } }  \ \lambda { m^{ 1 } }=2500A \end{array}$

Multiple choice electromagnetic spectrum electromagnetic waves physics

If $c$ is the speed, $\nu$ is frequency and $\displaystyle \lambda $ is wavelength of EM waves, then

  1. $\displaystyle c=\nu\lambda $
  2. $\displaystyle \frac { \lambda }{ \nu } =c$
  3. $\displaystyle \frac { \nu }{ \lambda } =c$
  4. $\displaystyle \frac { 1 }{ \lambda } =\frac { c }{ \nu} $
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$\displaystyle c=\nu \lambda $
$c=$speed
$\nu=$Frequency
$\displaystyle \lambda $ = wavelength

Multiple choice electromagnetic spectrum electromagnetic waves physics

The broad wavelength range of visible spectrum is:

  1. $4000-8000A^o$
  2. $2000-4000A^o$
  3. $10000-20000A^o$

  4. None of the above
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The wavelength range of $4000-8000 A^o$ is known as visible spectrum as waves within this wavelength range create a sensation of vision in our eyes.

Multiple choice electromagnetic spectrum electromagnetic waves physics

Identify which of the following light rays has the highest energy?

  1. Violet

  2. Green

  3. Yellow

  4. Orange

  5. Red
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Energy of light ray      $E = h\nu$                $\implies E \propto \nu$

Among all the visible rays, violet ray has the highest frequency, Thus violet ray has the highest energy. 

Multiple choice electromagnetic spectrum electromagnetic waves physics

The portion of the spectrum beyond the red end is called

  1. UV spectrum

  2. Infrard spectrum

  3. Microwave

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

The portion of spectrum just beyond the red end is called infrared spectrum, while the portion of the spectrum just before the voilet end is called the ultravoilet spectrum.

Multiple choice electromagnetic spectrum electromagnetic waves physics

Wavelength of gamma rays are :

  1. ${ 10 }^{ -10 }m$ to less than ${ 10 }^{ -14 }m$
  2. ${ 10 }^{ -14 }m$ to less than ${ 10 }^{ -10 }m$
  3. ${ 10 }^{ -11 }m$ to less than ${ 10 }^{ -14 }m$
  4. ${ 10 }^{ -14 }m$ to less than ${ 10 }^{ -6 }m$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Electromagnetic radiations are present all around us in different forms such as microwaves, radio waves, gamma rays, and X-rays. These radiations can be defined as a form of energy produced by the movement of electrically charged particles that can be found in matter or vacuum or by oscillating magnetic or electric disturbance.


Properties of electromagnetic radiations :

1) They travel through empty space
2) The speed of light always remains constant i.e. 2.99792458 X 10 8 m/s.
3) Wavelength is the measure between the distance of either troughs or crests.  Its symbol is 'Lambda'.

Gamma rays have no mass.  They arise from the high-frequency end of the electromagnetic spectrum.  They have the highest penetration power.   They are at least ionizing.  The Gamma rays carry a large amount of energy and can travel through the thick and thin material. Gamma rays have frequencies greater than about 1018 cycles per second or Hertz.  They have wavelengths of less than 100 picometer.  Gamma rays can kill living cells.  It is used to kill cancerous cells.   They can also kill bacteria.

Multiple choice electromagnetic spectrum electromagnetic waves physics

Electromagnetic wave with frequencies greater than the critical frequency of ionosphere cannot be used for communication using sky wave propagation because

  1. The refractive index of ionosphere becomes very high for $f > f _ { c }$
  2. The refractive index of ionosphere becomes very low for $f > f _ { c }$
  3. The refractive index of ionosphere becomes very high for $f < f _ { c }$
  4. The refractive index of ionosphere becomes very low for $f < f _ { c }$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The refractive index of the ionosphere is given by n = sqrt(1 - (f_c/f)^2). For f > f_c, the refractive index is real and less than 1, but as f increases, the ionosphere becomes less effective at refracting the wave back to Earth, eventually allowing it to pass through.

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.