Are NXT Light sensors compatible with EV3?
NXT Color Sensors are supported directly by EV3-G, the EV3 software running on the PC or MAC.
NXT Light Sensors are not.

To use Light sensors requires a bit of trickery.  EV3-G has a Raw Sensor block that provides access to the Light sensor values.  The raw sensor returns values from 0-1023, which is very different than the 0-100 of the NXT-G Light Sensor block.

Setting up the Light sensor to use ambient or reflected light is also solved with a bit of trickery.  Use the Sound Sensor Block and toggle the dB setting.

Once that much is tested and working, create a set of MyBlocks to mimic existing Color Sensor blocks.