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 physics quantum physics photons concept of photon photons and photoelectric effect

When the momentum of a photon is changed by an amount $p'$ then the corresponding change in the de-Broglie wavelength is found to be $0.20$%. Then, the original momentum of the photon was

  1. $300 p'$
  2. $500 p'$
  3. $400 p'$
  4. $100 p'$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
As, we know de-Broglie wavelength,
$\lambda = \dfrac{h}{p}$
$\therefore \lambda \propto \dfrac{1}{p}$
$\Rightarrow \dfrac{\Delta p}{p} = - \dfrac{\Delta \lambda}{\lambda} \therefore \left| \dfrac{\Delta p}{p} \right | = \left| \dfrac{\Delta \lambda}{\lambda} \right |$
$\Rightarrow \dfrac{p'}{p} = \dfrac{0.20}{100} = \dfrac{1}{500}$
or, $p = 500 p'$
Multiple choice physics quantum physics photons concept of photon photons and photoelectric effect

The formula for kinetic mass of photon is:
where $h$ is Planck's constant and $v,\lambda, c$ are frequency, wavelength and speed of photon respectively.

  1. $\cfrac { hv }{ \lambda } $
  2. $\cfrac { h }{ c\lambda } $
  3. $\cfrac { hv }{ c } $
  4. $\cfrac { h\lambda }{ c } $
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Energy of the photon is given by,

$E=hv=\dfrac{hc}{\lambda}$,  ...(1)
also for particle of zero mass,
$E=mc^2$,   ...(2)
form eq(1) and (2) we get,
$mc^2=\dfrac{hc}{\lambda}$
$\therefore$ kinetic mass of photon$= m=\dfrac{h}{c\lambda}$

Multiple choice physics quantum physics photons concept of photon photons and photoelectric effect

If momentum of a photon of an electromagnetic radiation is $3.3\times 10^{-29}$ kg m/sec, then frequency of associated wave is:

  1. $3.0\times 10^3Hz$
  2. $6.0\times 10^3Hz$
  3. $7.5\times 10^5Hz$
  4. $1.5\times 10^{13}Hz$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

$p=\dfrac {hv}{c}$
$\Rightarrow v=\dfrac {cp}{h}=\dfrac {(3\times 20^8)\times (3\cdot 3\times 10{-29})}{6\cdot 6\times 10^{-34}}$


$=1\cdot 5\times 10^{13}Hz.$

Multiple choice physics sound: production of sound oscillation - amplitude, time period and frequency of oscillation time period, frequency and amplitude of sound oscillatory and periodic motion

The frequency of light of wave length $5000{A^\circ}$ is :

  1. $1.5 \times 10^3 Hz$
  2. $6 \times 10^8 Hz$
  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

We know that $ n = \dfrac{v}{\lambda } $
where, $ v = $ velocity of light
$\lambda  =$ wavelenght
So, $ n = \dfrac{ 3 \times  10^{8}}{5000 \times  10^{-10} } = 6 \times 10^{14} Hz$

Multiple choice nuclear reactions nuclear structure nuclei atomic nuclei physics

In pair annihilation the least number of  $\gamma $- ray photons produced is :

  1. 2

  2. 3

  3. 4

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

In a pair annihilation, atleast two gamma rays must be produced. For example, when an electron encounters a positron, they annihilate to produce two gamma rays each having $0.511$ MeV energy.

Multiple choice nuclear reactions nuclear structure nuclei atomic nuclei physics

Consider a hypothetical annihilation of a stationary electron with a stationary positron. What is the wavelength of resulting radiation?

  1. $\dfrac{h}{m _{0}c}$
  2. $\dfrac{h}{2m _{0}c}$
  3. $\dfrac{2h}{m _{0}c}$
  4. $\dfrac{h}{4\pi m _{0}c}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
According to De Broglie eave length
$\lambda =\dfrac { h }{ mc } \quad \quad \quad E=\dfrac { hc }{ \lambda  } $
                             $\lambda =\dfrac { hc }{ E } =hc$
When an electron and a positron collide then a hypothetical annihilation of a stationary electron with a stationary position.
Multiple choice nuclear reactions nuclear structure nuclei atomic nuclei physics

When an electron and a positron are annihilated, then the number of photons produced is

  1. 2

  2. 1

  3. 3

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

Two photons are produced during the annihilation of an electron and a positron along with $1.02$ MeV released energy.

Multiple choice nuclear reactions nuclear structure nuclei atomic nuclei physics

For a pair production, the minimum frequency of the gamma ray must be:

  1. 2.5 x 10$^{14}$ Hz
  2. 2.5 x 10$^{20}$ Hz
  3. 2.5 x 10$^{28}$ Hz
  4. 2.5 x 10$^{34}$ Hz
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

For production to occur, the energy must be greater than the rest mass of proton + electron.
$E > ( 2 \times 0.511\ MeV = 1.022\ MeV)$
$h \nu > 1.022 \times  10^{6} \times 1.6 \times 10^{-19} J$
$\nu > \dfrac{1.6352 \times  10^{-13}}{h} = 2.5 \times 10^{20} Hz$

Multiple choice nuclear reactions nuclear structure nuclei atomic nuclei physics

$\gamma $ -ray photon of following energy undergoes pair production : 

a) $0.85Mev $      b) $1.00Mev $
c)  $1.02Mev $     d) $1.82Mev$

  1. a,b

  2. c,d

  3. b,c,d

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

The pair production becomes possible with gamma energies exceeding $1.02\ MeV$, and becomes important as an absorption mechanism at energies over $5\ MeV$.

Multiple choice nuclear reactions nuclear structure nuclei atomic nuclei physics

Consider a hypothetical annihilation of a stationary electron with a stationary positron. What is the wavelength of the resulting radiation?

  1. $\lambda = \displaystyle\frac{h}{2m _ec}$
  2. $\lambda = \displaystyle\frac{2h}{m _ec^2}$
  3. $\lambda = \displaystyle\frac{h}{2m _ec^2}$

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

Annihilation is the conversion of matter into energy.
Using equation $E={ m } _{ e }{ c }^{ 2 }$
$\dfrac { hc }{ \lambda  } =2{ m } _{ e }{ c }^{ 2 }$
$\Rightarrow \lambda =\dfrac { h }{ { 2m } _{ e }{ c } } $

Multiple choice chemistry nuclear physics nuclear reactor the nuclear power station isotopes and nuclear chemistry

Which one moves with greatest speed :

  1. $\alpha$-rays
  2. $\beta$-rays
  3. $\gamma$-rays

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

A

Alpha particles are released by high mass, proton rich unstable nuclei. The alpha particles is a helium nucleus, it consists of two protons and two neutrons. It contains no electron to balance the two positively charged protons. Alpha particles are therefore positively charged particles moving at high speed. 

Multiple choice commercial applications global environmental change causes of resource depletion causes of greenhouse effect ozone layer depletion

Identify the wrong statement with reference to UV rays.

  1. UV-A is more harmful to animals than UV-B.

  2. UV radiation falls in the range between 100-300 nm wavelength.

  3. UV radiation is classified into three categories.

  4. UV radiation kills microorganisms.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

  •  The UV region covers the wavelength range 100-400 nm and is divided into three bands: UVA (315-400 nm) UVB (280-315 nm)
  • The most common form of UV radiation is sunlight, which produces three main types of UV rays: UVA, UVB, UVC.
  • At certain wavelengths, UV is mutagenic to bacteria, viruses and other microorganisms. Particularly at wavelengths around 260–270 nm, UV breaks molecular bonds within microorganismal DNA, producing thymine dimers that can kill or disable the organisms.
  • UVB, the chief cause of skin reddening and sunburn, tends to damage the skin's more superficial epidermal layers. It plays a key role in the development of skin cancer and a contributory role in tanning and photoaging.

So, the correct answer is 'UV-A is more harmful to animals than UV-B.'.

Multiple choice

Which of the following is NOT true about electromagnetic waves?

  1. they do NOT need a medium to travel

  2. the do NOT lose energy as they travel

  3. the DO need a medium to travel

  4. they travel slower through a medium
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Electromagnetic waves are unique because they do NOT require a medium to propagate; they can travel through vacuum (like light from the Sun). Option C states they DO need a medium, which is false, making it the correct answer to a NOT-true question. Option A describes this true property incorrectly marked as false. Option D is true because EM waves do travel slower through denser media due to interactions.