Interference & Diffraction Questions

[brad sue]

Hi, please can someone explain me the following questions about interference:

Radio waves are readingly difracted around buildings whereas light waves are negligibly diffracted around building. this is becauseraio waves :
1- are polarized
2-have much longer wavelength (WL) than ligth waves*
3-have much shorter WL that light waves
4- are nearly monochromatic

Monochromatic ligth at normal incidence strikes a thinest film in which the reflected light will be maximum?
1- much less that lambda(Y)
2-Y/4
3-Y/2
4- 3Y/4
5-Y

the underlined answer are suppose to be the good ones. I have plenty question like this but please can someone explain me those two.
Also can someone suggest me a good website where I can read material about interference and diffraction ?
thank you
B

 

[lightgrav]

Waves are noticeably "diffracted" at an edge (shadow boundary);
as the wave propagates past the edge, some of the wave energy
diffuses into the shadow region ... roughly, as cos^2(theta).

See some diagrams (with good captions) about "Huygens Construction"

Now, if this is happening on the other side of the obstacle (building) as well,
radio waves will "fill in" the building's shadow within 10-20 wavelengths.
Objects smaller than (about) a wavelength are almost "transparent".

Interference has to do with the difference between two paths ...
If light that takes path A ends up at the detector IN phase with
the light that took path B , the waves there add up (bright).
If light that takes path A ends up at the detector OUT of phase with
the light that took path B , the waves there cancel (darkness).