A beam of light consisting of two wavelengths 6500Å and 5200Å is used to obtain interference fringes in a young's double slit experiment the distanece between the slits is 2mm and the distance between the plane of the slits and screen is 120 cm.
(a). Find the distance of the third bright fringe on the screen from the central maxima for the wavelength 6500Å
(b). What is the least distance from the central maxima where the bright fringes due to both the wave lengths coincide?

Explanation

To solve the given problem, we will break it down into two parts as specified in the question.

### Part (a): Finding the distance of the third bright fringe on the screen from the central maxima for the wavelength 6500Å.

1. **Identify the given values**:
- Wavelength, \( \lambda_1 = 6500 \, \text{Å} = 6500 \times 10^{-10} \, \text{m} \)
- Distance between the slits, \( d = 2 \, \text{mm} = 2 \times 10^{-3} \, \text{m} \)
- Distance from the slits to the screen, \( D = 120 \, \text{cm} = 1.2 \, \text{m} \)
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