Interference pattern for thin films

In summary, the question asks for the thickness of a thin polymer film with air on both sides for which second order bright fringes can be observed in reflection when a parallel beam of light with wavelength ##\lambda## is incident normally. The condition for constructive interference for normal incidence is given by the equation $$2nd=(m+1/2)\lambda$$ for integer m. If the thickness satisfies this equation, bright fringes will be observed on the screen. The equation used depends on whether the desired fringes are bright or dark, and the value of m can be substituted to find the corresponding thickness. However, the questioner raises a valid point that there should not be any fringe pattern on the screen at all, and it remains unclear
  • #1
devd
47
1

Homework Statement


A parallel beam of light of wavelength ##\lambda## is incident normally on a thin polymer film with air on both sides. If the film has a refractive index ##n>1##, then, for what value of the thickness, can second order bright fringes be observed in reflection?

Homework Equations


For normal incidence, condition for constructive interference is $$ 2nd=(m+1/2)\lambda$$ for integer m.

The Attempt at a Solution


Frankly, i don't understand the question. For parallel plane waves, the screen should have uniform illumination, light or dark according to whether ##2nd=m\lambda## or ##2nd=(m+1/2)\lambda##, right?

What does one mean by 2nd order fringes in this context? Does it mean, we simply put ##m=2## and find the corresponding thickness?
 
Physics news on Phys.org
  • #2
Yes, the equation you must use depends on whether you're computing for the bright or dark fringes.

You just need to substitute ##m## for the fringe order you need.
 
Last edited:
  • #3
ecastro said:
Yes, the equation you must use depends on whether you're computing for the bright or dark fringes.

You just need to substitute ##m## for the fringe order you need.

My point is, there shouldn't be any fringe pattern on the screen at all! It should either be uniformly dark or uniformly bright according to which eqn the thickness satisfies. What, then, is meant by a fringe of 2nd order, in this context?
 
  • #4
devd said:
It should either be uniformly dark or uniformly bright according to which eqn the thickness satisfies.

There is an interference pattern on the screen. That is why you see a "rainbow" of colors on soap bubbles.
 

1. What is an interference pattern for thin films?

An interference pattern for thin films is a phenomenon where light waves interact with each other and create a visible pattern of light and dark areas. This pattern is caused by the interference of light waves reflecting off two different surfaces of a thin film.

2. How is the interference pattern for thin films created?

The interference pattern for thin films is created when light waves reflect off two surfaces of a thin film that have different refractive indices. When the light waves travel through the film and reflect off the second surface, they interfere with each other, creating the visible pattern.

3. What factors affect the interference pattern for thin films?

The interference pattern for thin films is affected by the thickness of the film, the refractive indices of the two surfaces, and the wavelength of the incident light. Changes in these factors can alter the pattern and even cause it to disappear.

4. What applications does the interference pattern for thin films have?

The interference pattern for thin films has various applications in optics, such as in anti-reflective coatings, color filters, and optical coatings for lenses and mirrors. It is also used in the production of thin film devices, such as solar cells and LCD screens.

5. How is the interference pattern for thin films used in scientific research?

The interference pattern for thin films is used in scientific research to study the properties of light and the behavior of waves. It also helps in understanding the properties of materials, such as their refractive indices and thicknesses, which can be determined by analyzing the interference pattern.

Similar threads

  • Introductory Physics Homework Help
Replies
5
Views
913
  • Introductory Physics Homework Help
Replies
2
Views
759
  • Introductory Physics Homework Help
Replies
8
Views
1K
  • Introductory Physics Homework Help
Replies
3
Views
506
  • Introductory Physics Homework Help
Replies
1
Views
946
  • Introductory Physics Homework Help
Replies
2
Views
1K
  • Introductory Physics Homework Help
Replies
1
Views
1K
  • Introductory Physics Homework Help
Replies
3
Views
1K
Replies
3
Views
843
  • Introductory Physics Homework Help
Replies
7
Views
5K
Back
Top