Light WavesUnlike sound, light does NOT need a

medium to propagate or move through.

Light travels the fastest through vacuum space – whereas sound does not exist in a vacuum!

The speed of light in vacuum space is 300,000,000 m/s (3.0 x 108 m/s).

Unlike sound, light is a transverse propagation of energy.

Light is an electromagnetic wave. It has an electric part and a magnetic part.

contains all of the frequencies or wavelengths of visible and invisible light .

The Electromagnetic Spectrum or EM spectrum

Waves with the longest wavelengths (λ) have the lowest frequencies and the lowest energy. Ex: Radio, Microwave, Infrared

Waves with the shortest wavelengths (λ) have the highest frequencies and the highest energy.Ex: Ultraviolet, X-Ray, Gamma

Frequency vs. Wavelength

Selective TransmissionSome substances allow only some

frequencies of light to pass through; also called partial transparency.

Ex: Blue glass allows blue light frequencies to pass through but absorbs all other frequencies and turns them into heat. The glass warms up a little.

Ex: The earth’s atmosphere allows us to send radio waves to operate spacecraft.

Reflection of LightLight waves bouncing off a surface

or at the boundary of a medium.

Law of reflection:

Angle of incidence = angle of reflection

Selective Reflection

Things reflect the colors they do NOT absorb.

The reflected color (frequency) is visible to the human eye.Ex: The leaves of plants have a

chemical called chlorophyll that absorbs all colors (light energy), except the green frequencies they reflect.

Visible Light = R O Y G B V e

drange

e l l ow

reen

l ue

i o l et

Only reflected colors are visible to the human eye.

V B G Y O R

Chlorophyll b

Chlorophyll a

Chlorophyll Light Absorption

Refraction of Light Light refracts (bends) when it goes

from one medium into another, like from air to water.

This pencil appears broken in water because of the refraction of light.

When white light passes through a narrow slit and then through a prism, each wavelength (λ) refracts (or bends) at a different angle. Violet (the shortest wave) bends the most and red (the longest wave) bends the least, hence the rainbow.

Illusions caused by refraction

During sunsets, the sun appears higher in the sky than it actually is because of refraction.

The amount of bending depends on the material’s index of refraction.

Diamonds have a high index of refraction (lots of bending), while glass has a lower index of refraction (less bending).

Wavelengths (λ) of visible light

Human eyes send a different signal to the brain for

each λ. The brain interprets each as a different color.

Polarization of LightLight is polarized when all its

planes of travel are reduced to one single plane.

We can use this property of light to make sunglasses or other polarized materials.

Doppler Effect:The shift in frequency when the light

source and the observer are moving toward or away from each other.

Ex:Galaxies that are moving away from

ours appear red (redshift) , while those moving toward ours appear blue (blueshift).

Doppler Effect:

Light Diffraction

Light diffraction is a problem to scientists who study things with a light microscope.

A light beam going through a very narrow opening spreads out as the opening gets smaller. The light blurs and loses its brightness.

Lenses

A convex lens focuses parallel light rays onto a focal point.

A concave lens spreads parallel light rays away from each other.

Compare:

Light and sound can be refracted.

Light and sound can be reflected.

Light and sound can be diffracted.

Light and sound have a Doppler effect.

Light and sound are both waves of

energy.

Contrast:

Light can be polarized; sound cannot.

Light speed is almost a million times faster

than sound.

Light is a transverse wave; sound is

longitudinal.

Light can go through a vacuum; sound

cannot.

Light is electromagnetic; sound is

compressional.