In which of the following media does light travel fastest, if we ignore vacuum and consider typical conditions?

Introduction / Context:
The speed of light is not the same in all materials. In a vacuum, light has its maximum possible speed, and in any material medium such as air, water or glass, light slows down. The amount by which light slows depends on the optical properties of the medium, expressed through its refractive index. This question checks whether you know in which of the listed everyday media light travels fastest.

Given Data / Assumptions:
- We compare four media: air, water, glass and diamond.
- We ignore vacuum in the options, although light is fastest there.
- We assume common types of glass and typical atmospheric air at standard conditions.
- Refractive index is used to compare the relative speeds of light.

Concept / Approach:
The speed of light in a medium is given by the relation v = c / n, where c is the speed of light in vacuum and n is the refractive index of the medium. A larger refractive index means that light travels more slowly. Air has a refractive index very close to 1, water has a refractive index around 1.33, common glass is around 1.5, and diamond is around 2.4. Since air has the smallest refractive index of the four, light travels fastest in air compared with water, glass and diamond.

Step-by-Step Solution:
Step 1: Recall the formula v = c / n, where v is the speed in the medium and n is the refractive index. Step 2: Compare typical refractive indices: air is about 1.00, water about 1.33, glass about 1.5 and diamond about 2.4. Step 3: For air, v_air is approximately c / 1.00, which is almost equal to c. Step 4: For water, v_water is c / 1.33, which is smaller than v_air, so light is slower in water than in air. Step 5: For glass, v_glass is c / 1.5, which is even smaller, so light is slower in glass. Step 6: For diamond, v_diamond is c / 2.4, the smallest of all, so light is slowest in diamond among the given media.

Verification / Alternative check:
Experiments with refraction support this ordering. Light bends more when entering a medium with a higher refractive index because it slows down more. Glass bends light more than water, and diamond bends light very strongly, which matches the idea that light is slower in these materials. Air causes only a very small bending at the air to vacuum interface, consistent with its refractive index being very close to 1 and therefore its speed being close to that in vacuum.

Why Other Options Are Wrong:
Water: Light is slower in water than in air because water has a higher refractive index than air.
Glass: Common glass has a higher refractive index than water, so light is slower in glass than in water and air.
Diamond: Diamond has a very high refractive index, so light travels slowest in diamond among the listed media.

Common Pitfalls:
Students sometimes assume that denser materials always slow light more, without distinguishing between mass density and optical density. While related, it is the optical property measured by the refractive index that directly determines the speed of light in a medium. Another mistake is forgetting that vacuum is not listed in the options, even though light is fastest there. When only material media like air, water, glass and diamond are provided, always choose the one with the lowest refractive index.

Final Answer:
Among the given media, light travels fastest in air.