The speed of light in water is 2.25 × 10⁸ m/s.
In physics, particularly at the Class 10 level, understanding how light behaves in different materials is fundamental. While light travels fastest in a vacuum, its speed changes when it passes through other media like air, glass, or water.
Speed of Light Explained
Light travels at an astonishing speed, but this speed is maximum only in a vacuum, where there are no particles to impede its motion. This maximum speed is denoted by the letter 'c' and is approximately 3 × 10⁸ m/s.
When light enters a denser medium like water, it interacts with the atoms and molecules of the substance. These interactions cause the light to slow down. The speed of light in any medium is always less than its speed in a vacuum.
How is the Speed of Light in Water Determined?
The change in speed of light when it moves from one medium to another is quantified by a property called the refractive index. The refractive index (n) of a medium is defined as the ratio of the speed of light in a vacuum (c) to the speed of light in that medium (v):
n = c / v
Therefore, the speed of light in a medium can be calculated if we know the speed of light in a vacuum and the refractive index of the medium:
v = c / n
For water, the refractive index is approximately 1.33. Using the speed of light in a vacuum (c ≈ 3 × 10⁸ m/s):
v (water) = (3 × 10⁸ m/s) / 1.33 ≈ 2.25 × 10⁸ m/s
Based on the provided information: The speed of light in water is 2.25 × 10⁸ m/s. This value is commonly used in Class 10 physics to demonstrate the concept of refractive index and how light slows down in a denser medium.
Comparing Speeds
Here's a quick comparison of the speed of light in different media:
| Medium | Approximate Speed of Light (m/s) |
|---|---|
| Vacuum | 3 × 10⁸ |
| Air (approx.) | Slightly less than 3 × 10⁸ |
| Water | 2.25 × 10⁸ |
| Glass | ~2 × 10⁸ |
Note: The speed in air is very close to the speed in a vacuum, so for many calculations, especially at the Class 10 level, it is often approximated as equal to 'c'.
Practical Implications and Phenomena
The change in the speed of light when it enters water leads to several observable phenomena:
- Refraction: This is the bending of light as it passes from one medium to another. When light goes from air to water, it slows down and bends towards the normal (an imaginary line perpendicular to the surface). This is why objects submerged in water appear distorted or at a different depth than they actually are.
- Dispersion: While not as pronounced as in glass, water can slightly separate different colours (wavelengths) of light because the refractive index varies slightly with wavelength. This contributes to phenomena like rainbows.
- Underwater Vision: Our eyes are adapted to see light travelling at the speed of light in air. When underwater, the refractive index difference between water and our cornea changes, affecting how light focuses on the retina. This is why things look blurry underwater without goggles or a mask.
Understanding the speed of light in water is crucial for studying topics like:
- Refraction and Snell's Law
- The concept of optical density
- Applications like fibre optics (where light travels through glass or plastic)
In summary, for Class 10 physics, the key takeaway is that light slows down in water compared to a vacuum, and its speed in water is a specific value, which is 2.25 × 10⁸ m/s.
