Yellow light emitted by sodium lamp in Young's double slit experiment is replaced by monochromatic blue light of the same intensity:

Options

A. fringe width will decrease
B. fringe width will increase
C. fringe width will remain unchanged
D. fringe width will become less intense

Answer

To solve the problem regarding the effect of replacing yellow light with blue light in Young's double slit experiment, we will follow these steps:

### Step-by-Step Solution

1. **Understand the Concept of Fringe Width**:
The fringe width (β) in Young's double slit experiment is given by the formula:
\[ \beta = \frac{D \lambda}{d} \]
where:
- \(D\) = distance from the slits to the screen,
- \(d\) = distance between the slits,
- \(\lambda\) = wavelength of the light used.

2. **Identify the Wavelengths**:
- Yellow light emitted by a sodium lamp has a certain wavelength (\(\lambda_{yellow}\)).
- Blue light has a shorter wavelength (\(\lambda_{blue}\)).
- Generally, the order of wavelengths from longest to shortest is: Red > Orange > Yellow > Green > Blue > Indigo > Violet.

3. **Compare the Wavelengths**:
Since blue light has a shorter wavelength than yellow light, we can express this as:
\[ \lambda_{blue} < \lambda_{yellow} \]

4. **Analyze the Effect on Fringe Width**:
Since fringe width (β) is directly proportional to the wavelength (\(\lambda\)), we can conclude:
\[ \beta_{blue} < \beta_{yellow} \]
This means that when we switch from yellow light to blue light, the fringe width will decrease.

5. **Conclusion**:
Therefore, when yellow light is replaced by blue light of the same intensity in Young's double slit experiment, the fringe width will decrease.

### Final Answer:
The fringe width will decrease when yellow light is replaced by blue light.

---