Help with Physics Problems

[Blackstars]

Fellow mcaters, I need help solving the folowing physics problems. Please help this poor rookie.
1. Waves from a radio station with a wavelength of 400 m arrive at a home receiver a distance 30 km away from the transmitter by two paths. One is a direct line path and the second by reflection from a mountain directly behind the receiver. What is the minimum distance between the mountain and the receiver such that the destructive interference occurs at the location of the listener?

2.A thin film of magnesium fluoride (n=1.38) is used to coat a camera lens (n=1.56). Which of the following thickness of coating will not allow any strong reflection in the visible spectrum ( wavelength=390 nm to wavelength=700 nm)
(a) 1*10^-5 cm
(b) 2*10^-5 cm
(c) 5*10^-5 cm
(d) 1*10^-4 cm

3. 12. A possible means for making an airplane radar-invincible is to coat the plane with an antireflective polymer. If radar waves have a wavelength of 3 cm and the index of refraction of the polymer is n=1.5, how thick would the coating be?

Thanks

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[Gbemi24]

I am curious to know what book you are using to review MCAT physics because questions 2 and 3 will NEVER be on a MCAT. At any rate this is my solutions to your questions.

1) 400m. For the radio waves to interfere destructively, they must be out of phase 180 degrees. When the radio waves reflect of the mountain they change phase 180 degrees, meaning that they will interfere with the original wave every multiple of the wavelength (i.e. 400, 800, 1200 etc). The minimum distance for destructive interference must therefore be 400m.

2) This is a poor question since it does not indicate the intended wavelength of peak transmission. Usually, for vague questions like this, it is assumed that the intended wavelength of peak transmission is the average wavelength (550nm in this question). If that assumption is made then the answer should be 1.4*10^-5cm. Since this is not one of the answer choices, my best guess would be (a). To arrive at anwers for questions like this, just divide the intended wavelenght by 4.

3) This is a better version of question 2. The answer is 3/4=.75cm

 

[Blackstars]

Gbemi24, please check your pm box.