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Principal rays are the rays of light that pass through an object and lens, helping to locate the position, orientation, and size of the image. The three principal rays include: 1. Principal Ray 1: This ray starts from the top of the object, moves parallel to the optical axis until it reaches the lens, and is then refracted through the focal point on the other side of the lens. 2. Principal Ray 2: This ray starts from the top of the object, passes through the center of the lens, and continues straight without bending. It is also known as the "No bending ray." 3. Principal Ray 3: This ray starts from the top of the object, passes through the focal point on the same side of the lens as the object, and is then refracted parallel to the optical axis after passing through the lens. The intersection point of these three principal rays on the other side of the lens determines the position, orientation, and size of the image formed by the optical system.