Wave Nature of Light

15.2 (L) The Wave Nature of Light

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Main Idea

Like all waves, light diffracts around objects, has a wavelength and frequency, and can be Doppler shifted.

Essential Questions

• How does diffraction demonstrate that light has wave properties?

• What are the effects of combining colors of light and mixing pigments?

• How do phenomena such as polarization and the Doppler effect occur?

Diffraction and the Wave Model

When you are around the corner from a band room, you can hear the music because the sound bends. You can't see the musicians because the light reflecting from them doesn't reach your eyes. Does the light not bend at all, or does it not bend as much as the sound?

In 1685 Italian scientist Francesco Maria Grimaldi observed that the edges of shadows are not perfectly sharp. He introduced a narrow beam of light into a dark room and held a rod in front of the light such that it cast a shadow on a white surface.

The shadow cast by the rod was wider than the shadow should have been if the light traveled in a straight line past the edges of the rod. Grimaldi also noted that the shadow was bordered by colored bands. He determined that both of these observations could be explained if light bent slightly. He called the bending of light as it passes the edge of a barrier diffraction.

Huygen's Principle

In 1678 Dutch scientist Christiaan Huygens used a wave model to explain diffraction. His wave model explained the diffraction Grimaldi saw.

Color

• In 1666 Newton performed experiments on the colors produced when a narrow beam of sunlight passed through a glass prism.

• Newton called the ordered arrangement of colors a spectrum.

Color

• Newton allowed the spectrum from one prism to fall on a second prism.

• If the spectrum was caused by irregularities in the glass, he reasoned that the second prism would increase the spread in colors.

• Instead, the second prism reversed the spreading of colors and recombined them to form white light.

Color

• Newton concluded that white light is composed of colors and that a property of the glass other than unevenness caused the light to separate into colors.

• The work of Grimaldi, Huygens, Newton and others suggested that the color of light is related to wavelength.

Color

Color

As white light crosses the boundary from air into glass and back into air, its wave nature causes each different color of light to be bent at a different angle. The shorter the wavelength, the more the light is bent. This unequal bending of the different colors causes the white light to be spread into a spectrum.

​(Ever heard of Roy G. Biv?)

Polarization of Light

• Polarization is the production of light with a specific pattern of oscillation.

• Nonpolarized light rays vibrate randomly in every direction perpendicular to the direction they travel.

• Polarization can occur by filtering and by reflecting.

Malus's Law

Speed, Wavelength, and Frequency of Light

Speed, Wavelength, and Frequency of Light

• The Doppler effect describes how light frequency changes if an observer and a light source are moving toward or away from each other.

• If observed wavelengths are longer than the original wavelength, light appears closer to the red end of the spectrum.

• This means the object is moving away from the observer, and the light is redshifted.

Speed, Wavelength, and Frequency of Light

• If observed wavelengths are shorter than the original wavelength, light appears closer to the violet end of the spectrum.

• This means the object is moving towards the observer, and the light is blueshifted.

• Astronomers can determine how objects, such as galaxies, are moving relative to Earth by observing the Doppler shift of their light.