If white light is used in Young's double -slit experiment
a) bright white fringe is formed at the centre of the screen
b) fringes of different colours are observed on both sides of central fringe clearly only in the first order
c) the first order violet fringes are closer to the centre of the screen than the first order red fringes
d) the first order red fringes are closer to the centre of the screen than the first order violet fringes

Options

A. only a and d are true
B. only a and b are true
C. only a, b and c are true
D. all are true

Answer

To solve the problem regarding the Young's double-slit experiment with white light, we need to analyze each of the four statements provided.

### Step 1: Analyze Statement A
**Statement A:** "Bright white fringe is formed at the centre of the screen."
- In the Young's double-slit experiment, the central fringe is formed due to constructive interference of all the colors of light (white light is a combination of all colors). Therefore, this statement is **correct**.

### Step 2: Analyze Statement B
**Statement B:** "Fringes of different colours are observed on both sides of central fringe clearly only in the first order."
- As we move away from the central fringe, the colors start to disperse due to the different wavelengths of light. The first-order fringes will show distinct colors, while higher-order fringes will show less distinct colors due to increasing overlap. Thus, this statement is also **correct**.

### Step 3: Analyze Statement C
**Statement C:** "The first order violet fringes are closer to the centre of the screen than the first order red fringes."
- The position of the fringes is determined by the wavelength of light. Violet light has a shorter wavelength than red light. Since the fringe separation is inversely proportional to the wavelength, the violet fringe will be closer to the center than the red fringe. Therefore, this statement is **correct**.

### Step 4: Analyze Statement D
**Statement D:** "The first order red fringes are closer to the centre of the screen than the first order violet fringes."
- This statement contradicts Statement C. Since we established that the violet fringes are closer to the center due to their shorter wavelength, this statement is **incorrect**.

### Conclusion
Based on the analysis:
- Statements A, B, and C are correct.
- Statement D is incorrect.

Thus, the correct option is A, B, and C.