A short pulse of white light is incident from air to a glass slab at normal incidence. After travelling through the slab, the first colour to emerge is.

Options

A. blue
B. green
C. violet
D. red

Explanation

To solve the problem of which color of light emerges first from a glass slab when white light is incident at normal incidence, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Normal Incidence**:
- When light hits a surface at normal incidence, the angle of incidence is 0 degrees. This means that the light does not bend as it enters the glass slab. The angle of refraction is also 0 degrees.

2. **Properties of Light in Different Media**:
- Different colors of light (wavelengths) travel at different speeds in a medium. In air, all colors travel at the same speed, but in glass, they do not.

3. **Velocity of Light in Glass**:
- The speed of light in a medium is inversely related to its refractive index. Different colors of light have different refractive indices in glass, which causes them to travel at different velocities.

4. **Wavelength and Velocity Relationship**:
- The velocity of light is directly proportional to its wavelength. This means that the color with the longest wavelength will travel the fastest in the glass slab.

5. **Identifying the Colors**:
- Among the options given (blue, green, red, violet), we need to identify which color has the longest wavelength.
- The order of wavelengths from longest to shortest is: Red > Green > Blue > Violet.

6. **Conclusion**:
- Since red light has the longest wavelength, it will travel the fastest in the glass slab and will be the first color to emerge after passing through the slab.

### Final Answer:
The first color to emerge from the glass slab is **red**.

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