What is an Opacity Mask?
An Opacity Mask allows you to add transparency above other objects.
Opacity Masks facts:
  • Allows an object to fade in transparency.
  • You can edit a masking object.
  • A white mask will give you a visible object.
  • A black mask will hide your object.
  • To obtain a varied degree of transparency use shades of gray.
  • You can add, edit, invert and remove masks.
  • Opacity Mask is displayed inside the Transparency palette.
  • Useful to create realistic light effects.
Definitely, opacity mask is a great and essential tool. We can use it to control the appearance of any shape, composition, text, etc… I assure you this tool will expand your creativity!
Let’s get started:
STEP ONE:
Open a New Document in Illustrator (Mac: Command + N, PC: Control + N). In the New Document dialog box name the document “OpacityMask”, enter 400px for its width and height and click OK.
Creating a new document
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STEP TWO:Let’s learn how to use the Opacity Mask by creating a mirror effect.
To make things simple we are going to apply the mask to one of our default symbols.
Go to the Symbols palette (Mac: Shift + Command + F11, PC: Shift + Control + F11), open theSymbol Library, and choose one symbol. I chose “Mask” from the CELEBRATIONS palette.
Selecting a symbol
On a side note, remember to be creative. Apply the opacity Mask to your own graphics and don’t forget to experiment.
STEP THREE:Now we are going to create a copy of our symbol, but inverted. Select the symbol and go toObject Transform > Reflect. In the Reflect dialog box choose Horizontal and click Copy. As you can see, we create a copy of our symbol, but inverted. Select the inverted symbol and move it to the bottom of the original symbol.
Reflecting our symbol
STEP FOUR:Select the Rectangle Tool (M) and draw a box over the inverted symbol. Now, let’s fill this box with a gradient. Select the box, open the GRADIENT palette (Mac: Command + F9, PC: Control + F9), and add the following settings:
(Type: Linearangle: -90°Location: 15%)
Applying the gradient
You should get a gradient like this one:
You should get something like this
STEP FIVE:
It’s time to add the Layer Mask! Open the TRANSPARENCY palette (Mac: Shift + Command + F10, PC: PC: Shift + Control + F10). Select the box with the gradient and your inverted symbol, from the menu of the TRANSPARENCY palette select Make Opacity Mask.
Making the opacity mask
(Clip gives a balck background to the mask in order to crop the masked graphic to the borders of the masking box).
This is what I get:
That’s all! Easy, huh? 
If you are not satisfied with the final result go back to the TRANSPARENCY palette, click the thumbnail and the mask will become editable.
Final Product