The correct option for the question is **D. The speed of light decreases in water**. ### Detailed Explanation: When light travels from one medium to another, such as from air (a less dense medium) into water (a denser medium), it undergoes a phenomenon known as refraction. Refraction is the bending of light as it passes from one medium to another due to a change in its speed. 1. **Speed of Light in Different Media**: - The speed of light in a vacuum is approximately \(3 \times 10^8\) meters per second. However, when light enters a medium like water, its speed decreases. The speed of light in water is about \(2.25 \times 10^8\) meters per second. - This decrease in speed is crucial because it is the primary reason light bends towards the normal (the imaginary line perpendicular to the surface at the point of incidence). 2. **Understanding Refraction**: - According to Snell's Law, the relationship between the angles of incidence and refraction is given by: \[ n_1 \sin(\theta_1) = n_2 \sin(\theta_2) \] where: - \(n_1\) is the refractive index of the first medium (air), - \(n_2\) is the refractive index of the second medium (water), - \(\theta_1\) is the angle of incidence, - \(\theta_2\) is the angle of refraction. - The refractive index \(n\) of a medium is defined as the ratio of the speed of light in a vacuum to the speed of light in that medium: \[ n = \frac{c}{v} \] where \(c\) is the speed of light in a vacuum and \(v\) is the speed of light in the medium. 3. **Why Light Bends Towards the Normal**: - When light enters water from air, it slows down (decreases in speed). As a result, the angle of refraction (\(\theta_2\)) is smaller than the angle of incidence (\(\theta_1\)). This means that the light ray bends towards the normal line. ### Analysis of Other Options: - **A. The speed of light increases in water**: - This statement is incorrect. The speed of light actually decreases when it enters water from air. This is the opposite of what happens, making this option false. - **B. The wavelength of light decreases in water**: - While it is true that the wavelength of light decreases when it enters a denser medium, this is a consequence of the decrease in speed, not the primary reason for the bending. The bending occurs due to the change in speed, so this option is not the best answer. - **C. The refractive index of water is less than that of air**: - This statement is incorrect. The refractive index of water (approximately 1.33) is greater than that of air (approximately 1.00). Therefore, this option is also false. ### Summary of Key Points: - Light bends towards the normal when it enters a denser medium (like water) because its speed decreases. - Snell's Law describes the relationship between the angles of incidence and refraction based on the refractive indices of the two media. - The refractive index of water is greater than that of air, confirming that light slows down and bends towards the normal when entering water. ### Revision Summary: - Light slows down when entering a denser medium, causing it to bend towards the normal. - Snell's Law relates the angles of incidence and refraction to the refractive indices of the media. - The refractive index of water is greater than that of air, confirming the decrease in speed. - Understanding the relationship between speed, wavelength, and refractive index is crucial for mastering refraction concepts.