Have you seen Avatar？It’s a popular movie this year. It is three-dimensional(3D), so you may have to wear a special pair of glasses to see it. The 3D glasses make the images(图像) from the movie look like they are coming straight at you.
      More and more 3D movies are coming to theaters. 3D TV sets have been produced in Japan now. Three-dimensional technology can make TV and movies more exciting. But people with eye problems may get headaches if they spend too much time watching 3D movies.
    When we look at an object(物体), each eye sees it at a different angle(角度). Our eyes send the two images to our brain(大脑), and the brain puts them together. That's how we see depth(深度). So when we watch a 3D movie, one film projector(投影机) projects a left eye image and the other projects a right eye image. 3D glasses allow us to see a different image in each eye.
    There are a lot of people walking around with very less serious eye problems. Under normal situations, the brain can adjust to their eyes so they can see things normally.
    But when they are watching a 3D movie, the images projected to their eyes can't be put together by their brain. So the brain needs to work harder at reading the images. That makes it easier for these people to get headaches.
    Dr John Hagan, an eye doctor in Kansas City, said some people who do not see depth normally cannot see in 3D at all.