解这道题---**Textual Information:** * **Question Stem:** 如图，两个平面镜平行放置，光线经过平面镜反射时，∠1=∠2=40°，则∠3的度数为\_\_\_\_\_\_\_． (As shown in the figure, two plane mirrors are placed in parallel. When light reflects off the plane mirrors, ∠1=∠2=40°, then the measure of ∠3 is \_\_\_\_\_\_\_.) * **Given Information:** ∠1 = 40°, ∠2 = 40°. Note that the question states ∠1=∠2=40°, which is consistent with the law of reflection (angle of incidence equals angle of reflection) if ∠1 and ∠2 are defined with respect to the mirror surface. In the diagram, ∠1 and ∠2 are shown between the light ray and the mirror. If we consider the normal to the mirror, the angle of incidence would be 90° - ∠1 and the angle of reflection would be 90° - ∠2. The law of reflection states the angle of incidence equals the angle of reflection. **Chart/Diagram Description:** * **Type:** Ray diagram illustrating reflection of light between two parallel mirrors. * **Main Elements:** * Two horizontal lines representing plane mirrors, indicated by hatching below them. The top mirror is above the bottom mirror. The mirrors are stated to be parallel. * An incoming light ray from the left, striking the top mirror and reflecting off it. * A reflected light ray from the top mirror, traveling downwards and to the right, striking the bottom mirror and reflecting off it. * An outgoing light ray reflecting from the bottom mirror, traveling upwards and to the right. * Angles labeled: * Angle 1 (∠1) is the angle between the incoming ray and the top mirror surface. * Angle 2 (∠2) is the angle between the reflected ray from the top mirror and the top mirror surface. * Angle 3 (∠3) is the angle between the reflected ray from the bottom mirror and the bottom mirror surface. * **Annotations:** The angles are labeled as 1, 2, and 3. Arrows on the lines indicate the direction of the light rays. * **Relative Position and Direction:** Light enters from the top left, reflects downwards to the right, then reflects upwards to the right. The two mirrors are parallel. * **Given Values:** ∠1 = 40°, ∠2 = 40°.