  A modulation of refractive index can move at the speed of light. How it
interacts with an electromagnetic wave? Does it reflect? We show that an
incident electromagnetic wave, depending on its frequency either is totally
transmitted with a phase shift, or forms a standing wave, or is totally
reflected with the frequency upshift. A short incident pulse is converted into
a wavepacket that has all three parts (transmitted, standing and reflected
waves). The reflected part near the interface exhibits an infinitely growing in
time local frequency. The wavepacket's energy spectral density asymptotically
is the inverse square of frequency. If the refractive index modulation
disappears, the high frequency radiation is released.
