While watching 3D movies in the theater, we have to wear special glasses because:

3D movies are designed to create an illusion of depth, allowing the viewer to perceive a flat image as three-dimensional, similar to how we see objects in real life using both eyes.

In human vision, each eye sees the same object from slightly different angles. The brain then combines these two images to form a single three-dimensional perception — a phenomenon known as stereopsis.

3D films use a similar principle. Two images are projected simultaneously onto the screen — one meant for the left eye and the other for the right eye. These images are slightly offset to mimic the natural difference in the views each eye would have.

The special 3D glasses (either polarized or with color filters in older anaglyph systems) ensure that each eye sees only its intended image. The brain then merges both images to create the perception of depth, giving a true three-dimensional effect.

Option B is incorrect because 3D movies do not rely on special colors outside the visible range.

Option C is incorrect because brightness levels in 3D movies are not harmful to the eyes.

Option D is incorrect because if both eyes saw the same image, the 3D effect would disappear.

Therefore, we wear 3D glasses because they allow our left and right eyes to see different images, creating the illusion of depth.

Hence, the correct answer is Option A: Glasses allow our left and right eyes to see different images.