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 physics oscillations and waves huygen's wave theory refraction of water waves reflection and refraction at plane surfaces theories on light

The higher the frequency of light?

  1. The longer (larger) its wavelength.

  2. The shorter (smaller) its wavelength.

  3. The greater its velocity in a vacuum.

  4. The redder it will be
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The relationship between frequency (f) and wavelength (lambda) is given by c = f * lambda, where c is the speed of light. Therefore, frequency and wavelength are inversely proportional. As frequency increases, wavelength must decrease.

Multiple choice physics oscillations and waves huygen's wave theory refraction of water waves reflection and refraction at plane surfaces theories on light

An observer is moving with half the speed of light towards stationary microwave source emitting waves at frequency 10 GHz. What is the frequency of the microwave measured by the observer ?(speed of light = $3 \times10^{8} \ ms^{-1}$)

  1. 15.3 GHz

  2. 10.1 GHz

  3. 12.1 GHz

  4. 17.3 GHz
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation
Doppler effect in light ( speed of observer is not very small compared to speed of light )
$f^{1} = \sqrt{\dfrac{1+V/C}{1-V/C}} f _{source} = \sqrt{\dfrac{1+1/2}{1-1/2}} (10\ GHz)$
$=17.3\ GHz$


Multiple choice physics oscillations and waves huygen's wave theory refraction of water waves reflection and refraction at plane surfaces theories on light

Two monochromatic light source, A and B , emit the same number of photons wavelength of A is $ \lambda _A = 400 nm $, and that of $ \lambda _B =600 nm $. the power of radiated

  1. equal to that of source A

  2. less than that source A

  3. greater than that of source A

  4. can not be compared to that source A using the available data
Reveal answer Fill a bubble to check yourself
C Correct answer
Multiple choice physics oscillations and waves huygen's wave theory refraction of water waves reflection and refraction at plane surfaces theories on light

The wavelength of a monochromatic light in vacuum is $\lambda$. If travels from vacuum to a medium of absolute refractive index $\mu$. The ratio of wavelength of the incident and refracted wave is

  1. $\mu^2 : 1$

  2. 1 : 1
  3. $\mu$ : 1
  4. 1:$\mu$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

${ \left{ \mu \lambda  \right}  } _{ vacuum }={ \left{ \mu \lambda  \right}  } _{ medium }\ \Rightarrow \dfrac { { \lambda  } _{ i } }{ { \lambda  } _{ r } } =\dfrac { \mu  }{ 1 } \ \therefore \ Ratio\ is\ \mu :1$

Multiple choice physics oscillations and waves huygen's wave theory refraction of water waves reflection and refraction at plane surfaces theories on light

The frequency of light of wave length 5000 $\mathring {A}$ is

  1. $1.5 \times 10^5 Hz$
  2. $6 \times 10 Hz$
  3. $6 \times 10^{14} Hz$
  4. $7.5 \times 10^{15} Hz$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Answer is B.

The relation between the velocity of light, frequency and wavelength is given as v = f$\lambda $.
The velocity of light v = $3 \times 10^{8}m/s$.
So, f = $v/\lambda $ = $3 \times 10^{8}m/s / 5000\mathring{A}$ = $6 \times 10^{14} Hz$.
Hence, the frequency of the light is $6 \times 10^{14} Hz$.

Multiple choice physics oscillations and waves huygen's wave theory refraction of water waves reflection and refraction at plane surfaces theories on light

What is the wavelength of light for the least energetic photon emitted in the Lyman series of the hydrogen spectrum. (take hc = 1240 eV nm)

  1. 102 nm

  2. 150 nm

  3. 82 nm

  4. 122 nm
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Find out $\lambda$ of light

$hc=1240eVmm$
Lymen series of the hydrogen spectrum
Solution
For any series the transistion that produces the least energetic photon is the transition between the home base level that defines the series and the level immediately above it. For the hymen series, the home-base level is at $n=1$
So the transition that produces the least energetic photon is the transition from the $n=2$ level to then $n=1$ level.
$\therefore$ The wavelength for the least energetic photon is
$\lambda=\cfrac{hc}{E _2-E _1}$
Here $hc=1240eVnm$
$E _1=-\cfrac{13.6}{1^2}eV\ \quad=-13.6eV$
(as $E _n=-\cfrac{13.6}{n^2}eV)$
$E _2=-\cfrac{13.6}{2^2}eV\ \quad=-3.4eV\ \therefore\lambda=\cfrac{1240eVnm}{-3.4eV-(-13.6eV)}\ \quad=\cfrac{1240eVnm}{102eV}\ \quad=122nm$

Multiple choice motional emf physics

An electric charge $+ q$ moves with velocity $\bar{V}=3\hat{i}+4\hat{j}+\hat{k}$, in an electromagnetic field given by $\bar{E}=3\hat{i}+\hat{j}+\hat2{k}$ $\bar{B}=\hat{i}+\hat{j}+\hat3{k}$.The y-component of the force experienced by +q is:

  1. $-7 q$
  2. $11 q$
  3. $5 q$
  4. $3 q$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Given that,

Velocity, $v=\left( 3\hat{i}+4\hat{j}+\hat{k} \right)$

Electric field, $E=3\hat{i}+\hat{j}+2\hat{k}$

Magnetic field, $B=\hat{i}+\hat{j}+3\hat{k}$

Force,

$ F=q\left( \text{E+v}\times \text{B} \right) $

$ F=q\left( 3\hat{i}+\hat{j}+2\hat{k}+\left( (3\hat{i}+4\hat{j}+\hat{k})\times (\hat{i}+\hat{j}+3\hat{k} \right) \right) $

$ F=q\left( 3\hat{i}+\hat{j}+2\hat{k}+11\hat{i}-8\hat{j}-\hat{k} \right) $

$ F=q\left( 14\hat{i}-7\hat{j}+\hat{k} \right) $

So, y-component of force experienced by +q is

${{F} _{y}}=-7q$

Multiple choice physics energy production

Ozone layer blocks the radiations of wave length

  1. less than $3 \times 10^{-7}$ m
  2. equal to $3 \times 10^{-7}$ m
  3. more than $3 \times 10^{-7}$ m

  4. All of the above
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The ozone layer absorbs harmful ultraviolet radiation, specifically UV-C and some UV-B, which have wavelengths shorter than approximately 300 nm (3 * 10^-7 m).

Multiple choice

How do scientists use the electromagnetic spectrum?

  1. by sending radio waves and reading their response

  2. they use to communicate with aliens

  3. by examining visible light rays

  4. they do not... they use light years
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Scientists use the electromagnetic spectrum to study objects in space by detecting the various wavelengths of radiation they emit, such as radio waves, which can penetrate dust clouds.

Multiple choice

Fluence refers to the . . .

  1. Power density

  2. irradiance multiplied by exposure time. (j/cm2)

  3. Microwave amplification by stimulated emission of radiation

  4. Level of laser radiation to which a person may be exposed
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Fluence is defined as the total energy delivered per unit area, calculated as irradiance (power density) multiplied by the exposure time.

Multiple choice evs - i communication and mass media understanding communication and impact of mass media satellites in communication importance of transport system artificial satellite

A ground receiver station is receiving a signal at (a) 5 MHz and (b) 100MHz, transmitted from a ground transmitter at a height of 400m located at a distance of 125km. Identify whether it is coming via space wave or sky wave propagation or satellite transponder. Radius of earth = $6.4\times { 10 }^{ 6 }m$; maximum number density of electrons in ionosphere = ${ 10 }^{ 12 }{ m }^{ -3 }$ 


  1. $9MHz$
  2. $5MHz$
  3. $4MHz$

  4. All of these.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
Height of the ground transmitter $=30\ cm$
Range of transmission $=100\ km$
maximum distance covered by space wave propagation is
$d=\sqrt{2\ Rh}$
$=\sqrt{2\times 64\times 10^{6}\times 300}$
$=62\ km$
critical frequency for ionospheric propagation is, 
$F _{C}=9(N _{max})^{112}$
$=9(10^{12})^{112}$
$=9\times 10^{6}=9\ MHZ$
Multiple choice modelling gases - the kinetic model ideal gases kinetic theory of gases physics

Solar radiation reaches the earths atmosphere at a rate of $1353 Wm^{-2}$. If 36% of this
radiation is reflected back into space and 18% is absorbed by the earths atmosphere. The
radiant emittance is given by $\sigma T^{4}$
 where $\sigma$ is the Stefan-Boltzmanns constant and T is the
absolute temperature. What maximum temperature would an isolated black body on the
earths surface be expected to attain?
$(\sigma  = 5.67 x10^{-8} Wm^{-2}K^{-4})$.

  1. $120^{o}C$
  2. $63.9^{o}C$
  3. $50.7^{o}C$
  4. $31.4^{o}C$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Of the solar radiation reaching the earths atmosphere
36% is reflected back into space
18% is absorbed by the earths atmosphere
This implies that only 46% reaches the earths surface
If absorbed by a black body, expect that the maximum temperature of this body is given by
where $I _{sc} = 1353 Wm^{-2}$