TBR destructive interference

[nickelbackfan]

---

Members do not see this ad.

A think layer of plastic (n=1.46) coats the surface of a glass plate (n=1.52). When monochromatic light is shinned normally onto plastic from the air above, constructive interference occurs. Destructive interference will occur when the coated glass is immersed in which of the following liquids?

I water (n=1.33)
II carbon tetrachloride (n=1.46)
III Benzene (n=1.5)





Answer is III only

I'm not sure how the difference in index of refractions affects constructive interference. TBR talks about this and how there is a "flip" but I'm pretty lost when it comes to this.

Any help is appreciated, thanks

 

[osprey099]

A think layer of plastic (n=1.46) coats the surface of a glass plate (n=1.52). When monochromatic light is shinned normally onto plastic from the air above, constructive interference occurs. Destructive interference will occur when the coated glass is immersed in which of the following liquids?

I water (n=1.33)
II carbon tetrachloride (n=1.46)
III Benzene (n=1.5)





Answer is III only

I'm not sure how the difference in index of refractions affects constructive interference. TBR talks about this and how there is a "flip" but I'm pretty lost when it comes to this.

Any help is appreciated, thanks

I am not sure about this but here's my reasoning: Water and CCl4 have index of refractions equal or lower that of the plastic so when placed in water, the ray will bend more towards normal since water's n (index) is < plastic (index). This will then bend even more towards normal when it strikes the plate because the plate index of refraction is even higher. When placed in CCl4, the first ray does not bend at all but it then bends when it strikes the plate. Both of these cases can be described as constructive interference (water) or no interference (CCl4). However, since Benzene has a higher index than the plastic, the ray will bend less towards normal and then bend more towards normal as it strikes the plate because the index of Benzene is less than the plate index. Therefore, this is destructive interference and is probably what TBR termed "flip flopping."