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Electromagnetic Waves :: Doubts continue …

Siva Sai asked:

why are oscillating electric and magnetic feilds created when an elctromagnetic wave is travelling?


[wiki]Electromagnetic waves[/wiki] are produced by mutually perpendicular time varying electric and magnetic field mutually reinforcing each other.

EM Wave

A numerical Problem from optics

AK asks:

When an object is kept at a distance of 60cm from a concave mirror, the magnification is 1/2. Where should the object be placed to get magnification of 1/3??

please answer this question….


In the first part, u=-60, m=(-)1/2 implies v=-30 cm (Since the image must be real as concave lens form virtual image of bigger size only)

Substituting in eqn mirror_formula

or  mirrorformula

we get f = –20 cm

In II case




substituting in mirrorformula

we get u=4f=4 x (-20)=-80 cm

(If this is not the expected answer please respond)

Origin of colours and Atomic Spectra

Ramu asked:

“we know about absorption & emission spectra. We get colours; so what is orgin of colurs? why the atoms emit colour spectra”


Light is emitted when an excited atom returns to its ground state. When excited, the electrons jump from lower level to higher level absorbing the energy equal to the difference in energy levels corresponding to the orbits. When these electrons come back to the original states the energy is emitted.

As we know E = hν=hc/λ

Accordingly light of different wavelengths (and hence different colours) are emitted depending on the energy difference between the levels of transition.

For more details, please refer to these links.

  1. http://csep10.phys.utk.edu/astr162/lect/light/absorption.html
  2. http://en.wikipedia.org/wiki/Spectrum
  3. http://www.archive.org/stream/originofspectra00footuoft#page/n5/mode/2up
  4. http://en.wikipedia.org/wiki/Color
  5. http://www.newton.dep.anl.gov/askasci/phy05/phy05192.htm

Photoelectric Effect

Puneet Asked:

“In [wiki]photoelectric effect[/wiki] electrons emit out leaving behind positive charge then why [wiki]electron emission[/wiki] not stop after sometime.”


The photoelectric current can continue for long only if the lost electrons are replenished by some means. Unless, the positive potential will go on increasing until a stage is reached when no more electrons can be released with the the particular frequency of incident radiation.

Photoelectric effect takes place in the case of metals which have plenty of free electrons. The electrons lodged off by light could easily be regained as there is not much energy difference from a stray electron and a free electron on the surface of a metal.

Further, in a metal, the free electrons does  not belong to a single atom, each atom is surrounded by a sea of electrons. SO it is not right to say that each electron released creates an ion each – but creates a net positive charge for each electron lost.

(Comments and further explanations from visitors solicited)

In a pn junction, if diffusion current equals drift current, then why is there a potential barrier?

“In a [wiki]pn junction[/wiki], if diffusion current equals drift current, then why is there a potential barrier?”

Ans: When a [wiki]diode [/wiki]conducts, there is no [wiki]potential barrier[/wiki]

Magnetic Field of Earth

RS asks:

“could u please explain me what is earth’s [wiki]magnetic field[/wiki]? in detail and easily in brief?”


Please refer to http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magearth.html for details. If you have any specific doubts related to it please ask as comment here.

Also see http://en.wikipedia.org/wiki/Earth’s_magnetic_field


Why Water is transparent?

Sahurupika asks

“why [wiki]water [/wiki]has no colour?”


Simple questions are always difficult to answer.

An object appear coloured or colourless depending on the colours of the spectrum absorbed, transmitted or reflected by it. We see an object in the colour we receive from it. Water allows all the colours to pass through them and therefore we see it as [wiki]transparent[/wiki].

(From another point of view “Water is one of the most essential substance for the existence of water. [wiki]Transparent [/wiki]and colourless appearance of water tels about its [wiki]purity[/wiki].”)

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March 2021

Schrodinger’s Cat in Daily Life

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