Question

The parallel rays of white light are made an incident normally on an air film of uniform thickness. $250$ fringes are seen in the transmitted light between $4000A^o$ and $6500A^o$. Thickness of air film is

• A. $1.3mm$
• B. $1.5mm$
• C. $0.13mm$
• D. $0.11mm$

Answer 1

The correct option is A $1.3mm$

For a fringe to appear, the two wavelengths must interfere to give a maxima, which appears at a distance where the phase of both the rays are same, that is, at the least count multiple of the waves.

Therefore the first fringe appears at $5200A^o$.

To accommodate 250 fringes, the thickness of the film is 250 times the distance where the first fringe occurs.

Thus the thickness of the air film is 1.3mm