A soap bubble is a thin spherical film of soapy water which reflects light in a unique and beautiful way. Although the soap bubble reflects all visible light, certain wavelengths of light are more strongly reflected than others. In this article, we will discuss what visible wavelengths of light are strongly reflected from a 390-nm-thick soap bubble.
Essentially, the way in which a soap bubble reflects visible light is dependent on its thickness. The thicker the bubble is, the stronger it reflects the visible light. A 390-nm-thick soap bubble will display different reflections than a thinner or thicker bubble. Generally, a 390-nm-thick soap bubble will strongly reflect a narrow range of visible light with wavelengths between 380nm and 430nm. This is due to the fact that this range of light is what is known as the critical angle of incidence. In other words, when a light wave passes through the soap bubble, it will be most strongly reflected when it is 390nm away from the surface.
Light in this bandwidth of 380-430nm is known as visible light, because humans can see it. Within the visible spectrum, it is the range of the violet-blue hue that is most strongly reflected. Because violet-blue light has a higher frequency than other colors, it is the only visible light able to propagate at a 390-nm depth. This phenomenon is also known as total internal reflection, where the light waves are almost completely reflected back at the same angle as they were received by the bubble.
Another way to look at the reflection of light from a soap bubble is to think about its refractive index. As light passes through the soap bubble, the refractive index determines how much of it gets reflected and how much passes through. When the bubble is 390nm thick, the refractive index will be highest for light with a wavelength of 390nm. This means that those wavelengths of light are the most easily reflected, and therefore the most strongly reflected, from the bubble.
While a 390-nm-thick soap bubble is most strongly reflecting light in the violet-blue range, it also reflects red and green light, albeit less strongly. This causes a beautiful visual effect as the bubble reflects multiple hues and creates a rainbow of color.
Overall, a 390-nm-thick soap bubble will strongly reflect a narrow range of visible light with wavelengths between 380nm and 430nm. This range of light is where the refractive index is highest, creating an effect of total internal reflection. These visible light wavelengths create an beautiful visual effect, displaying multiple hues and producing a rainbow of light.