Screen Specs

• Made from the same glass that is used in the windshields of high-speed trains and helicopters, the iPhone's screen is exceptionally durable and scratch-resistant. At 3.5 inches on the diagonal, it is also oleophobic, meaning it is resistant to oily grime such as fingerprints. The screen boasts an impressive resolution of 326 pixels per inch, which gives pictures a crisp, clear quality, even when you zoom in. A special technology known as in-plane switching keeps the picture looking clear from almost any angle.
  
  

Touch Sensitivity

• Unlike many displays, the iPhone's screen does not simply show images -- it also provides navigation and controls. Instead of physical buttons, the iPhone's screen displays virtual buttons, which users can tap to open a program or change a setting. Users can also swipe the screen to navigate to new material, or they can make pinching motions to zoom in and out. The screen works by transmitting changes in electrical conductivity; it detects the electrical charge given off by your fingertips. This is why the screen won't work when you wear gloves or try to use a stylus.
  
  

Layers

• The iPhone's screen consists of three layers: the protective glass, the digitizer and the LCD, or Liquid Crystal Display. The digitizer is what detects the electrical current; the LCD is what displays the images, such as the apps on your home screen. Thus, if you just crack the top layer of glass, the screen should still work relatively well. However, if the crack is severe enough that it damages the digitizer or the LCD, then the touch screen will no longer work.
  
  

Solutions

• Before replacing the screen, you should test alternative problems. The screen may be dirty, or a protective shield may be interfering with the detector. You can also try resetting the phone by pressing and holding the "Sleep/Wake" and "Home" buttons simultaneously until the Apple logo appears. If the problem is software-related, this should resolve it. However, if the screen remains blank, you will likely need to replace it. A local Apple Store can quickly replace a broken screen, as can a third-party iPhone specialist. You can also purchase do-it-yourself screen replacement kits, though you should ensure that you purchase the correct size and that the kit contains all necessary parts.