When light enters water, it undergoes refraction, which is the bending of light as it passes from one medium to another. This occurs because light travels at different speeds in different materials.
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Change in Speed: Light travels faster in air (~299,792 km/s) than in water (~224,900 km/s). As light enters water, its speed decreases, causing the light to change direction.
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Angle of Incidence and Refraction: The amount of bending is described by Snell's Law, which states that the ratio of the sine of the angle of incidence (the angle of the incoming light) to the sine of the angle of refraction (the angle of the light as it enters the new medium) is constant and is related to the indices of refraction of the two media: \[ \frac{\sin(\theta_1)}{\sin(\theta_2)} = \frac{n_2}{n_1} \] where \(n_1\) is the index of refraction of air (approximately 1.0) and \(n_2\) is the index of refraction of water (approximately 1.33).
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Visual Effects: The result of refraction can create visual effects such as the apparent bending of a straw in a glass of water, or cause objects submerged in water to appear shallower than they truly are.
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Total Internal Reflection: If light moves from water to air and strikes the surface at a steep enough angle, it can undergo total internal reflection instead of refracting out of the water. This effect is the basis for phenomena such as fiber optics.
In summary, when light enters water, it slows down and bends, creating various optical effects and changing its perceived position and appearance.