**Approximate Formula for Wavelength**(

**λ**

**= ax/D**) from

**Interference Pattern**:

- Where,
**λ**= wavelength

**a**= distance between the 2 coherent wave sources

**x**= distance between 2 adjacent antinodal lines

(radiating ripples or interference fringes or points of loudness)

**D**= perpendicular distance between the parallel lines

where a and x are measured

- Empirical evidence from experiment shows:

- that
**x**is directly proportional to**λ**

- that
**x**is inversely proportional to**a**

- therefore, mathematically,
**x**= k**λ**/**a**, where k is found to be = D

- Thus,
**x = Dλ/a**

- And, changing the subject of formula, we have:
**λ****= ax/D**

- Application 1: The "
**Approximate Wavelength Formula,****λ****= ax/D**" can be used to find the wavelength of waves from their interference patterns as**a**,**x**and**D**can be determined from the interference pattern.

- Application 2: From the same formula
**λ = ax/D**or**x = Dλ/a (same formula with different subjects)** - The i
**nterference pattern**of**red, green or blue light**can all be explained by the "**Approximate Wavelength Formula,****x = Dλ/a**"

**x**

_{ red}> x_{blue}_{}

- The distance between adjacent fringes
**x**of red light is bigger than that of green or blue light because the wavelength of red light (λ_{ red}) is longer as compared to that of green light (λ_{green}) or blue light.

- Since λ
_{ red}> λ_{green}> λ_{blue, }from**x = D**λ**/a,**therefore, x_{ red}> x_{green}> x_{blue.}

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