Question

1) A plane wave is projected towards a small opening in a wall and spreads out once it passes the opening. Rank in order the amount of diffraction of each wave below from most spreading to least spreading. A) Most Ocean wave, Sound wave, Blue light, Red light Least. B) Most+Red light, Blue Light, Ocean wave, Sound wavefLeast. C) Most+Blue light, Red Light Sound Wave, Ocean wave Least. D) MostSound wave, Ocean wave, Red Light, Blue Light Least. E) Most+Ocean wave, Sound wave, Red Light, Blue Light Least. 2) A siren creates a sound of 100 decibels at a distance of 10 m. At 100 m, the sound intensity level is A) 90 dB B) 100 dB C) 10 dB D) 80 dB E) 1.0 dB 3) Which of the following is a false statement? A) A yellow star approaching the earth will appear orange (i.e. more red). B) If you bike towards a stationary fire alarm it will sound higher pitched than if you were stationary. C) If you are driving ahead of a moving train at the same speed as the train it will sound the same as if neither you nor the train were moving. D) Distant galaxies appear red-shifted because of the expansion of the universe. E) A cop hiding behind bushes watches you speed past him with a radar gun. As you are driving away from him, the radio waves reflected from your car return to the gun with a lower frequency than the emitted waves. 4) A slide has a rectangular opening with both a width w and a length I, where w, 1-2. When a laser is shined at the opening the interference pattern shown at right is observed on a screen behind the slide. Which of these diagrams most accurately represents the rectangular opening? A) B) D) E)

Answer 1

1) The angle of diffraction at a single slit increases as the wavelength of the wavd increases. Here, ocean wave has largest wavelength, then sound wave, red light and blue light have wavelengths in decreasing order.

So, option E would be the correct answer.

2) The intensity of sound at a distance d from the source,

I = I_{​​​​​​s} / d^{​​​​​​2}

Where, I_{​​​​​​s} is the intensity of sound at the source.

At a distance of 10 m, the intensity of sound is 100 dB.

I = 100 dB = 10¹⁰ × I_{​​​​​​0}

Intensity of sound at the source,

I = 10¹⁰ × I_{​​​​​​0} × 10²

= 10¹² × I_{​​​​​​0}

Intensity of sound at a distance of 100 m,

I = 10¹² × I_{​​​​​​0} / 100^{​​​​2}

= 10⁸ × I_{​​​​​​0}

= 80 dB

Option D would be correct.

3) Option A would be wrong. When a body producing light moves towards the observer, the apparent frequency of light emitted by the body would increase. Here, the frequency of light emitted by the body decreases as the body moves towards the observer. So, the statement would be wrong. Here, the yellow light priduced by the star would have a decrease in wavelength(Blue shift).

4) Option D would be the correct answer.