any light source (bulb, LED, etc...) also produces heat. Some more than others.
If you are providing a light source, you want one that mimics daylight (has the spectrum of daylight and not just an ordinary household light bulb). You'll want to put it on a timer so as to turn on and off providing about 12 hours simulated day and night. If it's not on a timer, you'll have to do this yourself. And since all light sources produce heat, you'll have to mount the light far enough away from the Cage so that you light it up but do not add excessive heat to the cage.
The heat pad goes under the glass bottom of the cage. Most of these heat pads do not come with thermostats. If you simply plug it in, it heats up to about 115 degrees which is too hot for a corn snake. So you will also need a thermostat to regulate the heat. You want to measure the heat directly over the pad on the bottom glass of the cage. The heat should be somewhere in the 82 to 88 degree range. Most of us shoot for about 85 degrees. The light source you provide may add additional heat so I would recommend you measure the heat pad without the light and with the light. It might take a while to get everything regulated. (I managed to get mine set right in about 3 hours).
With the light off, I maintain a Hot side temp of 84 degrees and a cool side temp of about 74 degrees. With the light on my temps are 86 and 77.
If you want to, you can go to my Profile Page and there are a few pictures of my setup. I think you can see the light hanging above the cage. It's nothing fancy but it is on a 12 hour timer and it's a "Full Spectrum" type light that I got from a local pet shop for Reptile Lighting.