The correct option for the question "Total internal reflection occurs when light moves from?" is **A. a dense medium to a less dense medium**. ### Detailed Explanation **Understanding Total Internal Reflection:** Total internal reflection is a phenomenon that occurs when a light wave traveling in a denser medium hits the boundary of a less dense medium at an angle greater than the critical angle. This results in the light being completely reflected back into the denser medium rather than refracting into the less dense medium. **Key Concepts:** 1. **Medium Density:** - A dense medium has a higher refractive index (n) compared to a less dense medium. For example, glass (n ≈ 1.5) is denser than air (n ≈ 1.0), and water (n ≈ 1.33) is denser than air. 2. **Critical Angle (θc):** - The critical angle is the angle of incidence above which total internal reflection occurs. It can be calculated using Snell's Law: \[ n_1 \sin(\theta_1) = n_2 \sin(\theta_2) \] where \( n_1 \) is the refractive index of the denser medium, \( n_2 \) is the refractive index of the less dense medium, \( \theta_1 \) is the angle of incidence, and \( \theta_2 \) is the angle of refraction. - When light travels from a denser medium (n1) to a less dense medium (n2), if the angle of incidence exceeds the critical angle, no light will pass into the second medium; instead, it will reflect entirely back into the first medium. **Why Option A is Correct:** - Option A states that total internal reflection occurs when light moves from a dense medium to a less dense medium. This is the fundamental condition for total internal reflection to happen. For example, if light travels from water (dense) to air (less dense) and strikes the boundary at an angle greater than the critical angle, it will reflect back into the water. ### Why the Other Options are Incorrect: **Option B: Air to Water** - This option describes light moving from a less dense medium (air) to a denser medium (water). In this case, light will refract into the water rather than reflecting back. Total internal reflection cannot occur in this scenario because the light is not moving from a denser to a less dense medium. **Option C: Water to Glass** - Here, light is moving from water (n ≈ 1.33) to glass (n ≈ 1.5). Since glass is denser than water, this situation does not meet the criteria for total internal reflection. Instead, light will refract into the glass. **Option D: A less dense to a dense medium** - This option is essentially the same as options B and C. It describes light moving from a less dense medium to a denser medium, which cannot result in total internal reflection. Instead, the light will refract into the denser medium. ### Summary of Key Points: - Total internal reflection occurs only when light travels from a denser medium to a less dense medium. - The critical angle is crucial for determining whether total internal reflection will occur. - Light moving from a less dense medium to a denser medium will always refract, not reflect. - Understanding the refractive indices of different materials is essential for predicting the behavior of light at boundaries. ### Revision Summary: - Total internal reflection requires light to move from a dense medium to a less dense medium. - The critical angle determines the conditions for total internal reflection. - Light refracts when moving from a less dense to a denser medium. - Familiarize yourself with the refractive indices of common materials to apply these concepts effectively.