Which one of the following statements is not correct for light rays?
Light speeds down as it leaves a water surface and enters the air
Let's analyze each statement regarding the behavior of light rays to determine which one is not correct.
Understanding Light Speed in Different Media
Light travels at different speeds depending on the medium it is passing through. The speed of light is related to the optical density of the medium. Generally, light travels faster in a rarer medium (like air or vacuum) and slower in a denser medium (like water or glass).
Analysing the Statements on Light Rays
Let's examine each given statement:
- Light travels at different speeds in different media: This statement is correct. The speed of light is maximum in vacuum, approximately \(3 \times 10^8\) metres per second. When light enters a medium like air, water, or glass, its speed decreases. The extent to which the speed decreases depends on the refractive index of the medium.
- Light travels at almost 300 million metres per second in air: The speed of light in vacuum is approximately \(3 \times 10^8\) m/s, which is 300 million m/s. Air is a very rare medium, and the speed of light in air is very close to its speed in vacuum, slightly less. So, stating that light travels at 'almost' 300 million metres per second in air is a correct approximation.
- Light speeds down as it leaves a water surface and enters the air: Water is optically denser than air. When light travels from an optically denser medium (water) to an optically rarer medium (air), its speed *increases*. Therefore, light speeds *up*, not down, when it leaves water and enters air. This statement is incorrect.
- Light speeds up as it leaves a glass surface and enters the air: Glass is optically denser than air. When light travels from an optically denser medium (glass) to an optically rarer medium (air), its speed *increases*. Therefore, light speeds *up* when it leaves glass and enters air. This statement is correct.
Based on the analysis, the statement that is not correct for light rays is "Light speeds down as it leaves a water surface and enters the air".
| Medium | Optical Density Relative to Vacuum | Speed of Light |
|---|---|---|
| Vacuum | Rarer (Reference) | Highest (\(\approx 3 \times 10^8\) m/s) |
| Air | Slightly denser than vacuum, much rarer than water/glass | Very high (slightly less than vacuum) |
| Water | Denser than air | Lower than in air |
| Glass | Denser than water | Lower than in water |
Conclusion on Light Ray Properties
The speed of light changes when it moves from one medium to another. This change in speed causes the light ray to bend (refraction) if it enters at an angle other than normal to the surface. The direction of bending depends on whether the light is going from a rarer to a denser medium or vice-versa, which is directly related to whether it speeds up or slows down.
- When light goes from Rarer to Denser medium, it slows down and bends towards the normal.
- When light goes from Denser to Rarer medium, it speeds up and bends away from the normal.
In the case of light leaving water and entering air, it goes from a denser medium (water) to a rarer medium (air), so it should speed up. Statement 3 incorrectly claims it speeds down.
Revision Table: Properties of Light Rays
| Property | Description | Relation to Speed/Medium |
|---|---|---|
| Propagation | Travels in straight lines in a uniform medium. | Constant speed in a uniform medium. |
| Speed in Media | Speed varies in different media. | Inversely related to the optical density of the medium. Faster in rarer, slower in denser. |
| Refraction | Bending of light as it passes from one medium to another. | Caused by the change in speed; direction of bending depends on whether speed increases or decreases. |
| Speed change (Denser to Rarer) | Speed increases. | Light bends away from the normal. |
| Speed change (Rarer to Denser) | Speed decreases. | Light bends towards the normal. |
Additional Information on Light and 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 its speed in vacuum. It is defined as:
$$n = \frac{\text{Speed of light in vacuum (c)}}{\text{Speed of light in medium (v)}}$$
A higher refractive index means a lower speed of light in that medium. Air has a refractive index close to 1 (slightly > 1), water has a refractive index of about 1.33, and typical glass has a refractive index around 1.5 or higher. Since \(n_{\text{water}} > n_{\text{air}}\) and \(n_{\text{glass}} > n_{\text{air}}\), the speed of light in water and glass is less than in air. When light goes from water to air, it goes from a higher \(n\) to a lower \(n\), so its speed must increase. When light goes from glass to air, it goes from a higher \(n\) to a lower \(n\), so its speed must increase.
This confirms that statement 3 is incorrect because light speeds up, not down, when going from water to air.
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Important Questions from Refraction and Reflection
Which of the following is NOT an example of refraction of light?
If the object distance and the image distance from a concave mirror is -20 cm, what is the focal length of the mirror?
Water drops shine on a lotus leaf due to:
A convex lens 'A' of focal length $10 \text{ cm}$ and another convex lens 'B' of focal length $20 \text{ cm}$ are kept along the same axis with a distance '$d$' between them. If a parallel beam of light falling on 'A' leaves 'B' as a parallel beam, then the distance '$d$' in $cm$ will be :
- A ray is incident at an angle of incidence $i$ on one surface of a small angle prism (with angle of prism $A$ and refractive index $\mu$). The ray emerges normally from the opposite surface, causing a total angle of deviation $\delta$ from its original path. Assuming all angles are small, the angle of incidence $i$ is nearly equal to:
