class 12 physics chapter 8 electromagnetic waves important questions

Dear students class 12 physics chapter 8 electromagnetic waves important questions pdf have been collected at one place to get you all prepared for your 12th Bihar board exam, as well as for UP board exams ,Jharkhand board exam ,MP board exams ,Rajsthan board exams ,cbse board exams and other board exams. Visit my nawendu classes youtube channel for more help.

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Q.(1)– What is electromagnetic wave ?

Bihar Board-2020 ,2022

Ans. – Electromagnetic waves are those waves in which there are sinusoidal variation of electric and magnetic field at right angles to each other as well as at right angles to the direction of wave propagation .e.g.- x ray ,gamma ray ,ultraviolet visible light ,infrared , ,micro wave ,radio wave.

Q.(2)– Write the expression for the electrical energy density and magnetic energy density associated with electromagnetic wave and show that their ratio is 1.

Bihar Board-2011

Answer- Electrical energy density =\[{{U}_{E}}=\frac{1}{2}{{\varepsilon }_{0}}{{E}^{2}}\]

Magnetic energy density =${{U}_{B}}=\frac{1}{2{{\mu }_{0}}}\times {{B}^{2}}$

$\frac{{{U}_{E}}}{{{U}_{B}}}=\frac{\frac{1}{2}{{\varepsilon }_{0}}{{E}^{2}}}{\frac{1}{2{{\mu }_{0}}}{{B}^{2}}}$

$={{\varepsilon }_{0}}{{\mu }_{0}}\frac{{{E}^{2}}}{{{B}^{2}}}$

\[\frac{1}{{{\left( \frac{1}{\sqrt{{{\varepsilon }_{0}}{{\mu }_{_{0}}}}} \right)}^{2}}}\times {{\left( \frac{E}{B} \right)}^{2}}\]

=$\frac{1}{{{\left( c \right)}^{2}}}\times {{\left( c \right)}^{2}}$=1

Where c= $\frac{1}{\sqrt{{{\varepsilon }_{0}}{{\mu }_{0}}}}$= $\frac{E}{B}$

Q.(3)– Out of electric field vector \[\vec{E}\] and magnetic field vector \[\vec{B}\] in an electromagnetic wave, which is more effective and why?

Bihar Board-2014

Answer- For electromagnetic waves

$\frac{E}{B}=c$

since $c=3\times {{10}^{8}}m/s$

$c\gg 1$

$\frac{E}{B}\gg 1$

$E\gg B$

So at any instant the value electric field is far greater than the value of magnetic field of in an electromagnetic wave.

Hence electric field vector \[\vec{E}\] is more effective than magnetic field vector \[\vec{B}\] in an electromagnetic wave.

Q.(4)– Write any two properties of x rays.

Bihar Board- 2013, 2016

Answer-The two properties of electromagnetic waves are as follows

(i) x rays are an electromagnetic waves of wavelength 0.01 angstrom to 10 angstrom.

(ii) x rays have a high penetrating power.

Q.(5)– Do x rays and $\gamma $ rays have same nature and origin?

Answer- Both are electromagnetic waves.

When a beam of fast moving electrons strike on metal plate then x rays are produced.

When a nucleus of a radioactive element disintegrates then $\gamma $ rays are produced.

Q.(6)– What is Green House Effect?

Answer- Our atmosphere is transparent for visible light. So the thermal energy coming from sun is absorbed by earth and objects on it. This energy is re-radiated (emitted). But energy of emitted radiation is low. So emitted radiation is mostly infrared according to plank’s law ($E=\frac{hc}{\lambda }$).

Infrared radiations are reflected by atmosphere (co₂ has main role in this reflection) due to which there is a abundance of infrared radiation on earth surface. Which causes rise in temperature of earth-surface.

This phenomena is known as green house effect.