6. Go mmh Two parallel slits are illuminated by light composed of two wavelengths. One wavelength is AA = 645 nm. The other wavelength is AB and is unknown. On a viewing screen, the light with wavelength λA = 645 nm produces its third-order bright fringe at the same place where the light with wavelength AB produces its fourth dark fringe. The fringes are counted relative to the central or zeroth-order bright fringe. What is the unknown wavelength?

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**Problem 6: Interference from Two Slits**

Two parallel slits are illuminated by light composed of two wavelengths. One wavelength is \(\lambda_A = 645 \, \text{nm}\). The other wavelength is \(\lambda_B\) and is unknown. On a viewing screen, the light with wavelength \(\lambda_A = 645 \, \text{nm}\) produces its third-order bright fringe at the same place where the light with wavelength \(\lambda_B\) produces its fourth dark fringe. The fringes are counted relative to the central or zeroth-order bright fringe. What is the unknown wavelength?

**Explanation:**

To determine the unknown wavelength \(\lambda_B\), we must apply the principles of interference for both bright and dark fringes in a double-slit experiment.

### Bright Fringe (Constructive Interference):
For \(\lambda_A\), the position of the m-th order bright fringe is given by:
\[ d \sin \theta = m \lambda_A \]
where:
- \(d\) is the distance between the two slits,
- \(\theta\) is the angle of the fringe,
- \(m\) is the order of the fringe (here, \(m = 3\) for the third bright fringe).

### Dark Fringe (Destructive Interference):
For \(\lambda_B\), the position of the m-th order dark fringe is given by:
\[ d \sin \theta = \left( m + \frac{1}{2} \right) \lambda_B \]
where:
- \(m\) is the order of the fringe (here, \(m = 4\) for the fourth dark fringe).

Since both the bright fringe of \(\lambda_A\) and the dark fringe of \(\lambda_B\) occur at the same location on the screen (\(\sin \theta\) is the same for both), we can set the two equations equal:

\[ 3 \lambda_A = \left( 4 + \frac{1}{2} \right) \lambda_B \]
\[ 3 \times 645 \, \text{nm} = \frac{9}{2} \lambda_B \]
\[ 1935 \, \text{nm} = \frac{9}{2} \lambda_B \]
\[ \lambda_B = \frac{1935 \, \text{nm} \times
Transcribed Image Text:**Problem 6: Interference from Two Slits** Two parallel slits are illuminated by light composed of two wavelengths. One wavelength is \(\lambda_A = 645 \, \text{nm}\). The other wavelength is \(\lambda_B\) and is unknown. On a viewing screen, the light with wavelength \(\lambda_A = 645 \, \text{nm}\) produces its third-order bright fringe at the same place where the light with wavelength \(\lambda_B\) produces its fourth dark fringe. The fringes are counted relative to the central or zeroth-order bright fringe. What is the unknown wavelength? **Explanation:** To determine the unknown wavelength \(\lambda_B\), we must apply the principles of interference for both bright and dark fringes in a double-slit experiment. ### Bright Fringe (Constructive Interference): For \(\lambda_A\), the position of the m-th order bright fringe is given by: \[ d \sin \theta = m \lambda_A \] where: - \(d\) is the distance between the two slits, - \(\theta\) is the angle of the fringe, - \(m\) is the order of the fringe (here, \(m = 3\) for the third bright fringe). ### Dark Fringe (Destructive Interference): For \(\lambda_B\), the position of the m-th order dark fringe is given by: \[ d \sin \theta = \left( m + \frac{1}{2} \right) \lambda_B \] where: - \(m\) is the order of the fringe (here, \(m = 4\) for the fourth dark fringe). Since both the bright fringe of \(\lambda_A\) and the dark fringe of \(\lambda_B\) occur at the same location on the screen (\(\sin \theta\) is the same for both), we can set the two equations equal: \[ 3 \lambda_A = \left( 4 + \frac{1}{2} \right) \lambda_B \] \[ 3 \times 645 \, \text{nm} = \frac{9}{2} \lambda_B \] \[ 1935 \, \text{nm} = \frac{9}{2} \lambda_B \] \[ \lambda_B = \frac{1935 \, \text{nm} \times
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