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Fundamentals Of Physics
Found in: Page 1073

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Short Answer

Figure 35-28 shows four situations in which light reflects perpendicularly from a thin film of thickness L sandwiched between much thicker materials. The indexes of refraction are given. In which situations does Eq. 35-36 correspond to the reflections yielding maxima (that is, a bright film).

The situation corresponding to the reflections yielding maxima is (c).

See the step by step solution

Step by Step Solution

Step 1: Given information:

The thickness of the thin film for each situation is L.

The index of refraction of the thin film for situation (a) is 1.6.

The index of refraction of the thin film for situation (b) is 1.6.

The index of refraction of the thin film for situation (c) is 1.3.

The index of refraction of the thin film for situation (d) is 1.6.

Step 2: Path length difference: 

If two different waves exactly in phase meet each other then they produce fully constructive interference having a bright film.

The formula for the path length difference (2L) of the light in the medium is given by,

2L=m+12λn2

Here, λ is the wavelength of the incident light, n2 is the index of refraction of the medium, and m=0,1,2...... for maxima-bright film in air.

Step 3: Situation yielding maxima: 

According to the equation 35-36,

2L=m+12λn2

For a particular order, the maximum intensity will be possible when,

2L1n2

From the above relation, the reflections yield maxima for a medium having the least value of the index of refraction.

Comparing the given four situations,

1.3<1.6

So, the value of the index of refraction is the least for situation (c).

Hence, the situation corresponding to the reflections yielding maxima (that is, a bright film) is (c).

Hence, the situation corresponding to the reflections yielding maxima (that is, a bright film) is (c).

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