LIGHT AS A WAVES

for a long time in the 17th and 18th centuries, scientists debated the nature of light. did light is a particle or a waves ? newton favored light as a particle, but by the end of 18th century thomas young makes an experiment that showed that light is also form as a waves too. the experiment that thomas young set up is a double slit experiment

the complete interference pattern can be explained in this way. where there is a bright fringe on the screen, the two rays are arriving in step. their path lengths from the two slits differ by a whole number of wavelengths plus half a wavelength. at positions in between, the path difference between the two rays involves a fraction of a wavelength, and there is partial canceling out of the light.

we can explain constructive interference in terms of phase or path difference:

• constructive interference (waves in phase) - path difference = whole number of wavelength
• destructive interference (waves out of phase) - path difference = whole number of wavelengths plus half a wavelength

determining λ

the double slit experiment can be used to determine the wavelength of light, λ. the following quantities have to be measured :

1. slit separation (a) : the distance between the centers of the slits, can be measured with a traveling microscope
2. fringe separation (x) : the distance between the centers of adjacent bright or dark fringes, can be measured with meter rule or a traveling microscope
3. slit to screen distance (D) : the distance from the mid point of the slits to the central fringe on the screen, can be measured using a meter rule

once these three quantities have been measured, the wavelength λ of the light can be found using λ=ax/D

the derivation of λ=ax/D

using phytagoras' theorem we can write an expression for S1P and S2P

S1P²=S1E²+PE²=D²+(x+a/2)²

S2P²=S2F²+PF²=D²+(x-a/2)²

subtracting this equation give

S1P²-S2P²=(x+a/2)²-(x-a/2)²

each side of this equation is the difference of two squares, so we can write

(S1P+S2P)(S1P-S2P)= (x+a/2 +x-a/2)(x+a/2 +x-a/2)=2ax

now, both S1P and S2P are very similar in length to the slit to screen distance D, we can write

S1P+S2P=2D

the difference between S1P and S2P is the path difference between the two rays, and this equal to one wavelength, λ gives

S1P-S2P=λ

substituting gives

2Dλ=2ax

and hence

λ=ax/D

WAVES

for the article please open this link

http://cnx.org/content/m12378/latest/

for the power point

http://www.4shared.com/file/205618246/82df12c/lecture161.html

for the video

http://www.4shared.com/file/205635336/ce048661/video-1.html

HOW AIRPLANES WORK

for power point about HOW AIRPLANES WORKS please download on this link

http://www.4shared.com/file/205605832/829cd749/How_airplanes_work3.html

for more info please read this following article

http://www.howstuffworks.com/airplane.htm

http://www.grc.nasa.gov/WWW/K-12/airplane/guided.htm