We know that visible light is the type of electromagnetic wave to which our eyes respond. Like all other electromagnetic waves, it obeys the equation

where c=3×108m/sc=3×108m/s size 12{c=3 times “10” rSup { size 8{8} } `”m/s”} {} is the speed of light in vacuum, ff size 12{f} {} is the frequency of the electromagnetic waves, and λλ size 12{λ} {} is its wavelength. The range of visible wavelengths is approximately 380 to 760 nm. As is true for all waves, light travels in straight lines and acts like a ray when it interacts with objects several times as large as its wavelength. However, when it interacts with smaller objects, it displays its wave characteristics prominently. Interference is the hallmark of a wave, and in [link] both the ray and wave characteristics of light can be seen. The laser beam emitted by the observatory epitomizes a ray, traveling in a straight line. However, passing a pure-wavelength beam through vertical slits with a size close to the wavelength of the beam reveals the wave character of light, as the beam spreads out horizontally into a pattern of bright and dark regions caused by systematic constructive and destructive interference. Rather than spreading out, a ray would continue traveling straight ahead after passing through slits.

Light has wave characteristics in various media as well as in a vacuum. When light goes from a vacuum to some medium, like water, its speed and wavelength change, but its frequency ff size 12{f} {} remains the same. (We can think of light as a forced oscillation that must have the frequency of the original source.) The speed of light in a medium is v=c/nv=c/n size 12{v=c/n} {}, where nn is its index of refraction. If we divide both sides of equation

c=fλc=fλ size 12{c=f`λ} {} by
nn size 12{n} {}, we get c/n=v=fλ/nc/n=v=fλ/n size 12{c/n=v=f`λ/n} {}. This implies that
v=fλnv=fλn size 12{v=f`λ rSub { size 8{n} } } {}, where λnλn size 12{λ rSub { size 8{n} } } {} is the wavelength in a medium and that

where λλ size 12{λ} {} is the wavelength in vacuum and nn size 12{n} {} is the medium’s index of refraction. Therefore, the wavelength of light is smaller in any medium than it is in vacuum. In water, for example, which has n=1.333n=1.333 size 12{n=1 “.” “333”} {}, the range of visible wavelengths is (380nm)/1.333(380nm)/1.333 size 12{ ( “380”`”nm” ) “/1” “.” “333”} {} to (760nm)/1.333(760nm)/1.333 size 12{ ( “760”`”nm” ) “/1” “.” “333”} {}, or λn=285to570nmλn=285to570nm size 12{λ rSub { size 8{n} } =”285″`”to”`”570″`”nm”} {}. Although wavelengths change while traveling from one medium to another, colors do not, since colors are associated with frequency.