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

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 

  1. Will have frequency of $10^9\, Hz.$
  2. Will have frequency of $2\times 10^9\, Hz.$
  3. Will have wavelength of $0.3\,m.$

  4. Fall in the region of radio waves
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

An oscillating charge produces electromagnetic waves of the same frequency as the oscillation. Wavelength lambda = c / f. With f = 10^9 Hz and c = 3 * 10^8 m/s, lambda = 0.3 m. Thus, option C is correct, while A and B are incorrect.

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

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.

  1. (i) and (ii)

  2. (i) and (iv)

  3. (ii) and (iii)

  4. (iii) and (iv)
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Electric field has zero average value in electromagnetic waves

Multiple choice physics superposition of waves-2: stationary (standing) waves: vibrations of air columns from moving to stationary stationary (or standing) waves formation of stationary waves

Energy is not propogated by:

  1. Stationary waves

  2. Electromagnetic waves

  3. Longitudinal progressive waves

  4. transverse progressive waves
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Stationary wave is also known as standing wave. It remains in a constant position. Two opposing waves combine to form a standing wave. Hence energy is not propagated in stationary wave.

Multiple choice physics energy management electricity from nuclear energy nuclear power nuclear energy

The essential distinction between X-rays and $\gamma$-rays is that the $\gamma$-rays :

  1. have greater ionizing power than X-rays

  2. emanate from nucleus, white X-rays emanate from outer part of the atom

  3. have smaller wave-length than X-rays

  4. none of the above

Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A 

Gamma-rays have the highest energy in the EMR spectrum and their waves have the shortest Wavelength. Gamma rays are lot more dangerous and hazardous to human health than X-rays. Moreover gamma rays are the highly penertating and highly energetic ionizing radiation. 

Multiple choice physics superposition of waves-1: interference and beats interference of sound waves superposition and interference of sound waves properties of sound waves

A laser beam can be focussed on an area equal to the square of its wavelength. A He-Ne laser radiates energy at the rate of $1\,nW$ and its wavelength is $632.8\,nm$.The intensity of foucussed beam will be   

  1. $1.5 \times {10^{13}}\,W/{m^2}$
  2. $0.25 \times {10^{4}}\,W/{m^2}$
  3. $3.5 \times {10^{17}}\,W/{m^2}$

  4. None of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

A laser beam can be focused on an area equal to the square of its wavelength.

A He-Ne laser radial co energy 
at the rate$=1nW$
wavelength$=6.32.8nm$
The intensity of focused beam willbe
Area through which energy of beam passes
$=(6.328\times10^{-7})=4\times10^{-13}m^2\I=\cfrac{P}{A}=\cfrac{10^{-9}}{4\times10^{-13}}\ \quad=0.25\times10^4W/m^2$

Multiple choice physics life cycle of stars evolution and end stages of stars the stars stars

The observed wavelength of light coming from a distant galaxy is found to be increased by $0.5\%$ as compared with that coming from a terrestrial source. The galaxy is.

  1. Stationary with respect t to the earth

  2. Approching the earth with velocity of light

  3. Receding from the earth with velocity of light
  4. Receding from the earth with a velocity equal to $1.5\times 10^{6}m/s$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

$\displaystyle\frac{\Delta \lambda}{\lambda}=\frac{v}{c}$
Now, $\Delta\lambda=\displaystyle\frac{0.5}{100} \lambda$
$\Rightarrow \displaystyle\frac{\Delta\lambda}{\lambda}=\frac{0.5}{100}$
$\therefore v=\displaystyle\frac{0.5}{100}\times c=\frac{0.5}{100}\times 3\times 10^8$
$=1.5\times 10^6m/s$
increase in $\lambda$ indicates that the star is receding.

Multiple choice physics household circuits precaution with the electrical circuit precautions to be taken while dealing with current devices used to protect from shock of electric current

A bulb of 40W is producing a light of wavelength 620 nm with 80% of eficiency then no of photons emitted by the bulb in 20 sec are 

  1. $2 \times (10) ^ {18}$
  2. $10 \times (10) ^ {18}$
  3. $10 \times (10) ^ {21}$
  4. $2 \times (10) ^ {20}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Energy of photon emitted by a bulb,

$E=\dfrac{80}{100}\times 40\times 20=640$
Wavelength of light, $\lambda=620nm$
Energy of photons $E'=\dfrac{hc}{\lambda}$
$=3.2\times 10^{-19}J$
$\therefore$ No of photons emitted , $n=\dfrac E{E'}$
$=\dfrac{640}{32\times 10^{-19}}$
$=2\times 10^{20}$

Multiple choice physics motion and measurement of distances motion around us motion and rest moving things around us

A laser signal sent towards the moon returns after t seconds.If c is speed of light, then the distance of the moon from the observer is

  1. ct
  2. $ct^{-1}$
  3. $ct^{-2}$
  4. $\displaystyle \frac{ct}{2}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

A laser signal sent towards the moon returns after t seconds.

Time taken to reach moon is $\dfrac{time}{2}=\dfrac{t}{2}$
Therefore $distance=speed*time=c*\dfrac{t}{2}$