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 determining which color of light emerges first from a glass slab when a short pulse of white light is incident at normal incidence, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Normal Incidence**:
- When light strikes a surface at normal incidence, the angle of incidence (i) is 0 degrees. This means that the angle of refraction (r) is also 0 degrees. Therefore, there is no bending of the light as it enters the glass slab.

2. **Speed of Light in Different Mediums**:
- The speed of light varies in different mediums. In a vacuum, all colors of light travel at the same speed (approximately 3 x 10^8 m/s). However, when light enters a medium like glass, its speed decreases depending on the wavelength of the light.

3. **Wavelength and Speed Relationship**:
- The speed of light in a medium is inversely proportional to its refractive index (n) and directly proportional to its wavelength (λ). The relationship can be expressed as:
\[ v = \frac{c}{n} \]
- Where \( v \) is the speed of light in the medium, \( c \) is the speed of light in vacuum, and \( n \) is the refractive index of the medium for that specific color.

4. **Dispersion of Light**:
- Different colors of light have different wavelengths. In general, red light has the longest wavelength, while violet light has the shortest. The order of wavelengths from longest to shortest is: red, orange, yellow, green, blue, indigo, violet.

5. **Velocity of Different Colors**:
- Since the speed of light in a medium is higher for longer wavelengths, red light, having the longest wavelength, will travel faster through the glass slab compared to other colors.

6. **Conclusion**:
- As the white light pulse exits the glass slab, the first color to emerge will be the one with the highest speed, which is red light.

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

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