Glowing red, the Copper Oxide reacts with the Carbon. On cooling, you will be able to observe elementary Copper in the tube.
Reduction: 2 CuO + 4e- -> 2 Cu + 2
Oxidization: C -> C4+ + 4e-
2 CuO + C -> 2 Cu + CO2
The product of this reaction, Carbon Dioxide, is evidenced by the lime water in the Erlenmeyer flask. The lime water becomes cloudy, because Calcium Carbonate forms which is very difficult to dissolve.