On Apr 21, 8:20 am, suresh wrote:
hi
they say that when a radio wave hits a reflective surface and bounces
back it under goes a 180 degrees phase shift .

1,what exactly is the reason for this phase change, is it because of
the change in direction or any thing else.

2,does this phase shift of 180 degrees mean a shift in polarity(ie
from +ve to -ve and vice versa)

Google can be your friend. Please learn to use it. One thing I'd add
to the paragraph below is that the metal creates what's commonly
called a "boundary condition." In this case, it enforces the electric
field in the metal to be (essentially) zero...which means that right
next to the metal surface, whatever electric field there is must be
perpendicular to the metal. That will/can help you understand the
"angle of incidence equals angle of reflection" thing. Rather than
typing in a similar answer, here's something that looks pretty good
from www.physicsforums.com:

"Specular (mirror) reflection by metals of electromagnetic (EM) waves,
including light, is not a quantum process. There are not absorbed and
reemitted photons.
An EM wave has an alternating electric field that oscillates at the
frequency of the wave. The limit condition of electric fields and
perfect conductors is that the electric field in the perfect conductor
must be zero. If not, the current should be infinite.
When free electrons in a metal "feel" an alternating electric field,
they began to oscillate at the same frequency as the field. An
oscillating electron radiates an EM wave polarized in the direction of
its movement. That is, with the electric field parallel to the
movement. This EM wave is radiates in all directions, with a maximum
in the plane perpendicular to movement and a zero in the direction of
movement.
In a perfect conductor, the amplitude of the emitted EM wave is
identical to the incident wave. In a real metal the emitted wave is
slightly smaller than the incident one.
The phase of the emitted EM wave is such that at the metal surface and
in the metal side, the addition of the two waves is zero. In the
incident wave side the two waves travel in opposite directions. Seen
from outside one has the impression that the wave coming from the
metal is the reflection of the incoming wave. It has the same
frequency and amplitude. But, in reality, it is a wave that has been
emitted by the metal electrons.
When the electric field is parallel to the surface of the mirror, the
reemitted wave has the same polarization. When it is not the case the
polarization of the reemitted wave is a little trickier.
There is not need of metals to obtain a "metallic" reflection: free
electrons suffice. There are a lot of them in the ionospheric plasma.
The ionosphere reflects radio waves at low radio frequencies (under a
few MHz). And some metals can be transparent to light. The potassium
is transparent to near UV.
Real metals are not perfect and their conductivity is frequency
dependant. But except copper and gold, they reflect fairly uniformly
in the visible band."