Passage 6: Soap

Soap bubbles, despite their delicate and transient nature, have fascinated scientists

and laypeople alike for hundreds of years. When passing a white light source

through a soap bubble, a series of colorful bands can be observed along the surface

which changes with thickness of the soap film.

Scientists studying this phenomenon came up with an experiment to investigate

this behavior of light and physical properties of the soap bubbles themselves.

Materials:

• Soap solution

• White light source

• Camera with infrared, visible, and UV filters

• Spectrometer

• Polarizing filters

Procedure:

First, a soap solution was prepared with water, a small amount of soap, and glycerin.

The presence of glycerin was helpful to counter a primary limiting factor for the

stability of soap bubbles: surface tension. The bubbles were then illuminated with

the white light source, and the infrared, visible, and UV camera filters were used to

capture images of the three spectra.

Interference patterns were recorded as the bubbles expanded. Spectrometry was

utilized to analyze the light that was both reflected and transmitted through the film

at different angles. Results are tabulated below.

By the end of the experiment, the researchers concluded that the interference

patterns had the most observable effect on the vibrance of colors in the visible

spectrum. Infrared radiation was largely absorbed, but UV light experienced

significant scattering. When viewed through the polarizing filters, intensity of light

varied consistently with the orientation of the polarizer.

Imagine that a soap solution containing highly conjugated compounds exhibits

fluorescence after being illuminated with certain wavelengths of light. Which

wavelength of light would you expect would induce maximum fluorescence,

and what color would the emitted light be after removing the light source?

A) 250 nm; blue

B) 350 nm; green

C) 450 nm; yellow

D) 650nm; red

Click to reveal answer

Correct answer: D. Conjugated aromatic systems tend to have

shorter gaps between their pi electron orbitals, so it requires less energy to excite

these electrons. This results in conjugated systems absorbing longer wavelengths of

light. 650nm is the longest wavelength of light out of the answer choice provided,

and is therefore the correct answer given this trend. Fluorescence is a phenomenon

in which excited electrons return to ground state, releasing a photon proportional

to the decrease in energy it experiences by returning to a lower energy orbital. The

excitation event that induces its jump to a higher energy orbital is the direct result

of a form of absorbed energy (in this case, 650 nm light), so the released photon

should be close in energy to the incident photon. Thus, red light (lower in energy

than 650nm) is likely the color of the emitted light.

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Day 35 MCAT Practice Question