NTNUJAVA Virtual Physics Laboratory
Enjoy the fun of physics with simulations!
Backup site http://enjoy.phy.ntnu.edu.tw/ntnujava/
October 23, 2017, 11:16:17 am *
Welcome, Guest. Please login or register.
Did you miss your activation email?

Login with username, password and session length
 
   Home   Help Search Login Register  
Youe can not help men permanently by doing for them what they could and should do for themselves. ..."Abraham Lincoln(1809-1865, US President 1861-1865"
Google Bookmarks Yahoo My Web MSN Live Netscape Del.icio.us FURL Stumble Upon Delirious Ask FaceBook

Pages: [1]   Go Down
  Print  
Author Topic: wave concept question  (Read 4773 times)
0 Members and 1 Guest are viewing this topic. Click to toggle author information(expand message area).
leeyiren
Jr. Member
**
Offline Offline

Posts: 39


«
Embed this message
on: September 19, 2010, 01:26:52 pm »

hello, everyone.

We all know that electromagnetic waves travel more slowly in a transparent medium than in a vacuum.

Their frequency stays the same but their wavelength gets less.

v=f X lambda

My question is why "their frequency stays the same but their wavelength gets less." Huh

Can it be that the wavelength stays the same but their frequency gets less?

In that case, the velocity still decreases.

Are there any simulations to show that the frequency stays the same and the wavelength decreases when light enters glass from air?

Remember frequency has the element of time.

Thanks.
« Last Edit: September 19, 2010, 01:29:47 pm by leeyiren » Logged
Fu-Kwun Hwang
Administrator
Hero Member
*****
Offline Offline

Posts: 3080



WWW
«
Embed this message
Reply #1 on: September 19, 2010, 04:52:49 pm » posted from:,,Taiwan

You would not asked the above question if you have understood the meaning of wave.

Wave is the same change of some physics quantity, however, there is a time delay due to distance in space.

You are welcomed to check out  formation of wave

The source determine the frequency of the wave. The wave propagate with the same frequency.
The same phase will produce the same amplitude. \phi=\vec{k}\cdot\vec{r}-\omega t
 when wave move from \vec{r} to \vec{r}+d\vec{r}, there is a time delay dt=\frac{\vec{k}\cdot d\vec{r}}{\omega}.
Since it is always the same change in time (however, different time delay due to distance from the source),
 wave always keep the same frequency when propagate. (otherwise, it is not a wave).

You might want to check out refraction of wave from one media to another media,too!
Logged
leeyiren
Jr. Member
**
Offline Offline

Posts: 39


«
Embed this message
Reply #2 on: September 19, 2010, 05:14:03 pm »

hi prof. thx for ur prompt reply. I hope u can help me clear this doubt.

The textbook says "the shorter the wavelength, the higher the frequency. As the frequency gets higher, the energy increases".

I read about Planck–Einstein equation from wikipedia which is not taught in the textbook or during lessons.
The following equations show how energy, wavelength and frequency is related.
λν = c
E=hv
E=(hc)/λ
h=Planck's constant = 6.62606896(33)×10−34 J•s
c=3x10^8 m/s.

so I can conclude that
increase frequency or decrease wavelength -> E increases.
decrease frequency or increase wavelength -> E decreases.

Am i right?

On the other hand
E=kA^2
The amount of energy carried by a wave is related to the amplitude of the wave. A high energy wave is characterized by a high amplitude; a low energy wave is characterized by a low amplitude.

It is taught to us that

"Putting a lot of energy into a transverse pulse will not effect the wavelength, the frequency or the speed of the pulse. The energy imparted to a pulse will only effect the amplitude of that pulse."

Doesn't this contradict the Planck–Einstein equation?
« Last Edit: September 19, 2010, 05:17:38 pm by leeyiren » Logged
Fu-Kwun Hwang
Administrator
Hero Member
*****
Offline Offline

Posts: 3080



WWW
«
Embed this message
Reply #3 on: September 19, 2010, 09:33:27 pm » posted from:,,Taiwan

For mechanical wave : e.g. wave on a string propagate in x direction (displacement in y direction)
 y(t)=A \sin (kx-\omega t)
So vy(t)=\frac{dy(t)}{dt}=-A\omega \cos(kx-\omega t)
The kinetic energy per unit length K=\frac{1}{2}m v^2=\frac{1}{2} \rho dx A^2\omega^2 \cos^2(kx-\omega t) where \rho is the mass per unit length
So the energy of the wave proportional to amplitude A^2 and frequency f^2, where \omega=2\pi f.

However, the wave speed is determined by the wave media (string) v=\sqrt{\frac{T}{\rho}}
The frequency of the wave is determined by source. And the wavelength \lambda =v/f
If you did not change the frequency of the wave source, putting more energy means increase the amplitude of the wave.
Actually, it is also possible to change the frequency of the wave source, however, more power is required to generate wave with the same amplitude.


E=hc/\lambda is the relation for light. It means that the power of light is proportional to the frequency (assume electric field is the same).
Light source determined the frequency of the generated light.
Logged
leeyiren
Jr. Member
**
Offline Offline

Posts: 39


«
Embed this message
Reply #4 on: September 19, 2010, 09:49:57 pm »

I am grateful for your time. However I dun get your equations and explanations. Is there any other way you can explain?
« Last Edit: September 19, 2010, 09:51:39 pm by leeyiren » Logged
Fu-Kwun Hwang
Administrator
Hero Member
*****
Offline Offline

Posts: 3080



WWW
«
Embed this message
Reply #5 on: September 19, 2010, 10:17:57 pm » posted from:,,Taiwan

Quote
"Putting a lot of energy into a transverse pulse will not effect the wavelength, the frequency or the speed of the pulse. The energy imparted to a pulse will only effect the amplitude of that pulse."
The above statement is under the assumption that frequency of the wave source is not changed!
Logged
Pages: [1]   Go Up
  Print  
Youe can not help men permanently by doing for them what they could and should do for themselves. ..."Abraham Lincoln(1809-1865, US President 1861-1865"
 
Jump to:  


Related Topics
Subject Started by Replies Views Last post
Pulley Question
Dynamics
Bitupon 3 21790 Last post August 24, 2011, 12:19:44 pm
by melody
Intensity of reflected s-wave/p-wave
Optics
Fu-Kwun Hwang 2 34775 Last post October 16, 2007, 04:37:51 pm
by Fu-Kwun Hwang
pressure question
kinematics
m_noha 1 7213 Last post July 20, 2015, 04:47:26 pm
by RandallJasper
Question regarding waves.
Question related to Physics or physics related simulation
leeyiren 3 4855 Last post January 11, 2010, 08:20:10 pm
by Fu-Kwun Hwang
How to use concept mapping tool for physics learning?
Question related to Physics or physics related simulation
lookang 5 7568 Last post September 22, 2010, 07:56:07 pm
by ahmedelshfie
Powered by MySQL Powered by PHP Powered by SMF 1.1.13 | SMF © 2006-2011, Simple Machines LLC Valid XHTML 1.0! Valid CSS!
Page created in 1.079 seconds with 22 queries.since 2011/06/15