Understanding Light When Changing Medium

When light travels from one medium to another, like from air to water or from water to glass, it undergoes a phenomenon called refraction. Refraction happens because the speed of light changes as it enters a different medium. The extent to which a medium slows down light is related to its refractive index.

Properties of Light and How They Change

Light has several physical properties, including velocity (speed), wavelength, and frequency. Let's look at how these properties behave when light moves from one medium to another:

1. Velocity (Speed): The speed of light is different in different media. For example, light travels fastest in a vacuum, slightly slower in air, and significantly slower in denser media like water or glass. So, velocity changes when light moves from one medium to another.

2. Wavelength: Wavelength ($\lambda$) is the distance between two consecutive crests or troughs of a wave. The relationship between velocity ($v$), frequency ($f$), and wavelength ($\lambda$) is given by the equation:

   $\qquad v = f \lambda$

   Since the velocity ($v$) changes and the frequency ($f$) remains constant (as we will discuss next), the wavelength ($\lambda$) must also change proportionally. If velocity decreases, wavelength decreases, and vice versa. So, wavelength changes.

3. Frequency: Frequency ($f$) is the number of wave cycles that pass a point per second. It represents the source characteristic of the light, determined by the source emitting the light (e.g., a light bulb or a laser). When light travels from one medium to another, the source of the light does not change, and therefore the frequency of the light remains constant. The number of waves arriving at the boundary between the two media per second must equal the number of waves leaving the boundary per second to maintain continuity. Thus, frequency does not change.

4. Refractive Index: The refractive index ($n$) of a medium is a measure of how much the speed of light is reduced in that medium compared to the speed of light in a vacuum ($c$). It is defined as $n = c/v$. Since the velocity ($v$) of light changes from one medium to another, the refractive index ($n$) also changes and is different for different media.

Summary of Changes

Here is a summary of how these properties change when light passes from one medium to another:

Based on this analysis, the physical property which does not change when light passes from one medium into another medium is frequency.