解释此题---**Extracted Content:** **Question Number:** 6 **Question Stem:** 如图所示，红绿两束单色光，同时从空气中沿同一路径以 θ 角从 MN 面射入某长方体透明均匀介质。折射光束在 NP 面发生全反射。反射光射向 PQ 面。若 θ 逐渐增大。两束光在 NP 面上的全反射现象会先后消失。已知在该介质中红光的折射率小于绿光的折射率。下列说法正确的是 ( ) **Chart/Diagram Description:** * **Type:** Geometric figure illustrating the path of light rays through a rectangular transparent block, involving refraction and total internal reflection. * **Main Elements:** * **Shapes:** A rectangle labeled MNPQ, representing a transparent, homogeneous medium. M is the bottom-left corner, N is the top-left, P is the top-right, and Q is the bottom-right. * **Surfaces:** MN (left face), NP (top face), PQ (right face). * **Light Rays:** * An incident ray enters the medium from air through the MN face. * A dashed line represents the normal to the MN surface at the point of incidence. * An angle θ is marked between the incident ray and the MN surface. * The incident ray is shown refracting upon entering the medium at MN. * The refracted ray travels inside the medium towards the NP face. * At the NP face, the ray undergoes total internal reflection, reflecting off the NP surface. * The reflected ray then travels towards the PQ face. * At the PQ face, two distinct emergent rays are shown exiting the medium back into the air. These two rays are slightly separated, indicating dispersion (likely representing the red and green light mentioned in the text). * **Implied Processes:** Refraction at MN, Total Internal Reflection at NP, Refraction at PQ, and Dispersion. **Options:** A. 在 PQ 面上，红光比绿光更靠近 P 点 B. θ 逐渐增大时，红光的**全反射现象先消失** C. θ 逐渐增大时，入射光可能在 MN 面发生全反射 D. θ 逐渐减小时，两束光在 MN 面折射的折射角逐渐增大