Make a Realistic Rainbow in photoshop part 1

Introduction :

1. Create and transform the rainbow

This is the image that we're going to use in this Photoshop tutorial. Save it on your hard drive (right click / save picture as...) and open it in Photoshop:

Add a new layer by clicking on the Create a new layer icon

at the bottom of your layers palette.
Rename this layer to Rainbow by double clicking on the layers name in the layers palette.
Select the Gradient tool

in the tool bar (if it's hidden, look under the Paint Bucket tool

)
Let check the option bar first before we continue.
Make sure that a Linear Gradient

is selected.
Click once on the button marked with A to open the Gradient picker window.
Select the Transparent Rainbow gradient at B and click on the button marked with A a second time to close the Gradient picker window.
Note: If the gradient is not available, then you have to click on the button marked with C and in the window that opens select Reset Gradients... (D) and when a window opens that asks you to replace the current gradients with the default gradients you simply click on OK . You might want to save your current gradients first if they contain a gradient that you made yourself.
Selecting gradient

Continue by making sure that all other settings in the option bar are the same as the ones shown in this screenshot:

Now hold down the shift key (this ensures that the gradient is drawn under a straight angle) and click somewhere in the middle of the screen and drag your cursor down over a distance as far as the diameter of the rainbow in this screenshot.
Continue by releasing the mouse button and shift key:
Drawing the gradient

Go to the menu and select Filter / Distort / Polar Coordinates...
Select Rectangular to Polar and click OK:

The gradient is now transformed into a an elliptical shape. If your document would have been a square, then it would have been a circle. It's not so important that it's an ellipse as you will find out in the next few steps.
Note: I made sure that the outer ring of the rainbow is red. It turns out that this is always the case with rainbows.
Rainbow in the shape of an ellipse

Zoom out to about 30-35% so that the gray area outside the canvas becomes visible.
Press Ctrl + T (Command + T on the Mac) to activate the Free Transformation tool.
Move the rainbow to the right and drag the lower right corner of the transformation box to change the rainbow's proportions and size until you have something like this: