1. Create the circle paths

First create a circle path and fill it with orange color. Then Copy (Ctrl+C) it and Paste in Front (Ctrl+F). Scale down about 40% and drag it slight below the center point of the orange circle.

2. Blend Tool

Select the Blend Tool , click on the top edge of the yellow circle and then click again on the top edge of the orange circle. Or you can blend them by selecting both circles, go to Object > Blend > Make (Ctrl+Alt+B).

3. Add final light reflection

Make an oval path and fill it with black & white Gradient and place it on top of the circles. Note if your color setting is CMYK, make sure the black for the gradient is full black(C=100, M=100, Y=100, K=100). With the oval path selected, go to Transparency pallate, select Screen Blend Mode. If your Transparency is not showing, go to Window > Transparency or press Ctrl+Shift+F10.

Advantages

There are serveral advantages to create this glassy button with Illustrator than Photoshop:

1. It is vector, which mean it is scalable.
2. It is easier and faster to make this with Illustrator (less steps).
3. Easier to change colors, just need to fill the circles with different colors.