<< Chapter < Page Chapter >> Page >
  • Discuss the wave character of light.
  • Identify the changes when light enters a medium.

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

c = f λ , size 12{c=f`λ,} {}

where c = 3 × 10 8 m/s size 12{c=3 times "10" rSup { size 8{8} } `"m/s"} {} is the speed of light in vacuum, f size 12{f} {} is the frequency of the electromagnetic waves, and λ 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.

Making connections: waves

The most certain indication of a wave is interference. This wave characteristic is most prominent when the wave interacts with an object that is not large compared with the wavelength. Interference is observed for water waves, sound waves, light waves, and (as we will see in Special Relativity ) for matter waves, such as electrons scattered from a crystal.

Part a of the figure shows a thin bright orange laser beam emitted from an observatory traveling in a straight line up into a starry sky. Part b of the figure shows a horizontal pattern of orange red spots produced when a laser beam has passed through a grid of slits. The central spot is the brightest and the spots get dimmer as you move away from the center..
(a) The laser beam emitted by an observatory acts like a ray, traveling in a straight line. This laser beam is from the Paranal Observatory of the European Southern Observatory. (credit: Yuri Beletsky, European Southern Observatory) (b) A laser beam passing through a grid of vertical slits produces an interference pattern—characteristic of a wave. (credit: Shim'on and Slava Rybka, Wikimedia Commons)

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 f 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 / n size 12{v=c/n} {} , where n is its index of refraction. If we divide both sides of equation c = f λ size 12{c=f`λ} {} by n size 12{n} {} , we get c / n = v = f λ / n size 12{c/n=v=f`λ/n} {} . This implies that v = f λ n size 12{v=f`λ rSub { size 8{n} } } {} , where λ n size 12{λ rSub { size 8{n} } } {} is the wavelength in a medium    and that

λ n = λ n , size 12{λ rSub { size 8{n} } = { {λ} over {n} } ,} {}

where λ size 12{λ} {} is the wavelength in vacuum and n 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 . 333 size 12{n=1 "." "333"} {} , the range of visible wavelengths is ( 380 nm ) /1 . 333 size 12{ \( "380"`"nm" \) "/1" "." "333"} {} to ( 760 nm ) /1 . 333 size 12{ \( "760"`"nm" \) "/1" "." "333"} {} , or λ n = 285 to 570 nm 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.

Section summary

  • Wave optics is the branch of optics that must be used when light interacts with small objects or whenever the wave characteristics of light are considered.
  • Wave characteristics are those associated with interference and diffraction.
  • Visible light is the type of electromagnetic wave to which our eyes respond and has a wavelength in the range of 380 to 760 nm.
  • Like all EM waves, the following relationship is valid in vacuum: c = f λ size 12{c=f`λ} {} , where c = 3 × 10 8 m/s size 12{c=3 times "10" rSup { size 8{8} } `"m/s"} {} is the speed of light, f size 12{f} {} is the frequency of the electromagnetic wave, and λ size 12{λ} {} is its wavelength in vacuum.
  • The wavelength λ n size 12{λ rSub { size 8{n} } } {} of light in a medium with index of refraction n size 12{n} {} is λ n = λ / n size 12{λ rSub { size 8{n} } =λ/n} {} . Its frequency is the same as in vacuum.

Conceptual questions

What type of experimental evidence indicates that light is a wave?

Got questions? Get instant answers now!

Give an example of a wave characteristic of light that is easily observed outside the laboratory.

Got questions? Get instant answers now!

Problems&Exercises

Show that when light passes from air to water, its wavelength decreases to 0.750 times its original value.

1 / 1 . 333 = 0 . 750 size 12{1/1 "." "333"=0 "." "750"} {}

Got questions? Get instant answers now!

Find the range of visible wavelengths of light in crown glass.

Got questions? Get instant answers now!

What is the index of refraction of a material for which the wavelength of light is 0.671 times its value in a vacuum? Identify the likely substance.

1.49, Polystyrene

Got questions? Get instant answers now!

Analysis of an interference effect in a clear solid shows that the wavelength of light in the solid is 329 nm. Knowing this light comes from a He-Ne laser and has a wavelength of 633 nm in air, is the substance zircon or diamond?

Got questions? Get instant answers now!

What is the ratio of thicknesses of crown glass and water that would contain the same number of wavelengths of light?

0.877 glass to water

Got questions? Get instant answers now!

Questions & Answers

how to study physic and understand
Ewa Reply
what is work
Fredrick Reply
the transfer of energy by a force that causes an object to be displaced; the product of the component of the force in the direction of the displacement and the magnitude of the displacement
AI-Robot
why is it from light to gravity
Esther Reply
difference between model and theory
Esther
Is the ship moving at a constant velocity?
Kamogelo Reply
The full note of modern physics
aluet Reply
introduction to applications of nuclear physics
aluet Reply
the explanation is not in full details
Moses Reply
I need more explanation or all about kinematics
Moses
yes
zephaniah
I need more explanation or all about nuclear physics
aluet
Show that the equal masses particles emarge from collision at right angle by making explicit used of fact that momentum is a vector quantity
Muhammad Reply
yh
Isaac
A wave is described by the function D(x,t)=(1.6cm) sin[(1.2cm^-1(x+6.8cm/st] what are:a.Amplitude b. wavelength c. wave number d. frequency e. period f. velocity of speed.
Majok Reply
what is frontier of physics
Somto Reply
A body is projected upward at an angle 45° 18minutes with the horizontal with an initial speed of 40km per second. In hoe many seconds will the body reach the ground then how far from the point of projection will it strike. At what angle will the horizontal will strike
Gufraan Reply
Suppose hydrogen and oxygen are diffusing through air. A small amount of each is released simultaneously. How much time passes before the hydrogen is 1.00 s ahead of the oxygen? Such differences in arrival times are used as an analytical tool in gas chromatography.
Ezekiel Reply
please explain
Samuel
what's the definition of physics
Mobolaji Reply
what is physics
Nangun Reply
the science concerned with describing the interactions of energy, matter, space, and time; it is especially interested in what fundamental mechanisms underlie every phenomenon
AI-Robot
what is isotopes
Nangun Reply
nuclei having the same Z and different N s
AI-Robot
Practice Key Terms 1

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'College physics' conversation and receive update notifications?

Ask