The iMac, a desktop computer made by Apple, integrates the LED flat-panel display, CPU and hard drive into one unit. The iMac has built-in speakers and an iSight camera, but it does not have a physical control to turn off the screen. While you can set the System Preferences to make the iMac's screen "sleep" or turn off after a certain period of inactivity, if you don't want to wait, you can set the iMac to turn off the screen immediately with a mouse gesture or by pressing a combination of keys on the keyboard.
Click the "Apple" logo from the Finder menu on your iMac.
Click "System Preferences."
Click "Expose and Spaces" Four pull-down menus are in the "Active Screen Corners" section. Each pull-down menu corresponds to one of the four corners of the iMac's screen.
Click a pull-down menu in the "Active Screen Corners," and then click "Put Display to Sleep." This sets the iMac to turn off the screen whenever you use the mouse to move the cursor into a corner. For example, if you set the upper-left pull-down menu to "Put Display to Sleep," every time you move the mouse so the cursor hits the upper-left corner of the iMac screen, the screen turns off.
Press the "Control," "Shift" and "Eject" keys simultaneously on a keyboard that has an eject key to turn off the iMac screen.
Move the iMac's mouse or press a key on the keyboard to turn the screen back on.