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

Which of the following is not true for electromagnetic waves?

  1. They transport energy.

  2. They have momentum.

  3. They travel at different speeds in air depending on their frequency.

  4. They travel at different speeds in medium depending on their frequency.
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Electromagnetic wave have constant velocity in a particular medium and it is independant of frequency and it is equal to $V=\sqrt{\dfrac{1}{\mu _0 \epsilon _0}}=3 \times 10^8 \ m/s$ 

Option C

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

An electromagnetic wave of frequency $\upsilon = 3$ MHz passes from a vacuum into a dielectric medium with permittivity $ = 4$, Then

  1. Wavelength and frequency both become half.

  2. Wavelength is doubled and frequency remains unchanged.

  3. Wavelength and frequency both remains unchanged.

  4. Wavelength is halfved and frequency remains unchanged.
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Frequency remains constant during refraction.

$v _{med}=\dfrac{1}{\sqrt{\mu _0 \epsilon _0 *4}}$=$\dfrac{c}{2}$

$\dfrac {\lambda _{med}}{\lambda _{air}} = \dfrac{v _{med}}{v _{air}}$

= $\dfrac{\dfrac{c}{2}}{c}$ =$\dfrac{1}{2}$
​$\therefore$ wavelength is halved and frequency remains unchanged.

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

Select wrong statement from the following:
Electromagnetic waves

  1. are transverse

  2. travel with same speed in all media

  3. travel with the speed of light

  4. are produced by accelerating charge
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Electromagnetic waves are transverse and do not need a material medium to travel. 
The speed of all electromagnetic waves is equal to the speed of light. In fact this let to the conclusion that light is an electromagnetic wave.

Accelerating charge particles produce electromagnetic waves.
However the speed of electromagnetic waves changes with media as 
$v=\dfrac{c}{n}$
where n is the refractive index of the given medium.

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

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?

  1. Red light

  2. Violet light

  3. Yellow light

  4. All colors have same speed
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

When travelling through a medium, the frequency of light doesn't change. Hence, red light having the maximum wavelength will travel fastest through glass as speed of light is given by $v=f\lambda$ where; $f$ is the frequency and $\lambda$ is the wavelength of light travelling at speed $v$.

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

What is the speed of Light $c$?

  1. $c=3\times 10^{-8} m s^{-1}$
  2. $c=3\times 10^8 km s^{-1}$
  3. $c=3\times 10^8 mm s^{-1}$
  4. $c=3\times 10^8 m s^{-1}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its value is exactly $299792458$ meters per second.
That is, $c=3\times { 10 }^{ 8 }m/s$.

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.