Light Unit

David B. Brown6C

Wave Definitions

There is the crest, the point at the top The trough, the point at the bottom, (opposite of crest) Medium the material of which a wave moves, (for an

ocean wave it is the ocean) Wavelength, the distance between two crests

measured in meters. The Greek letter lambda (λ) is used to abbreviate wavelength.

Frequency: the number of crests that pass a given point in one second. Unit is hertz (waves/second)

Scientists use frequency and wavelength interchangeably when describing EM waves.

Parts of a Wave

Frequency/Wavelength The lower the

frequency, the longer the wavelength.

The higher the frequency, the shorter the wavelength.

Electromagnetic Waves Electromagnetic waves: transverse

waves made of vibrating magnetic and electric fields. These two fields are at right angles to each other.

Travel at the speed of light.

Electromagnetic Waves Electromagnetic waves do not

require a medium. They will travel through the vacuum of space.

We know this as we see light from the sun and other stars.

Electromagnetic Spectrum The electromagnetic spectrum is the full

range of all the EM-waves arranged from the lowest to the highest frequency.

Radio Waves Radio waves: long wavelength, low

frequency, low energy waves. Used to carry information and entertainment through radio and television.

Microwaves Microwaves have

shorter wavelengths and higher frequencies than radio waves. So microwaves carry more energy than radio waves.

They are used in microwave ovens, cell phones, and radars.

Infrared Waves Infrared means

“below red.” Infrared waves are known as heat waves.

When you sit outside on a sunny day, you feel warm because of the infrared waves from the sun.

Campfires give off lots of IR light.

Infrared Light Many animals use

infrared light to find their prey. They see the heat waves given off by the body heat of their victim.

The Predator used infrared to find his victims in the movie.

Thermal imaging devices use this principle.

Visible LightNarrow range of EM waves that humans can

see. Red is the lowest frequency (longest λ). Violet is the highest frequency (shortest λ).

ROYGBIV

Rainbows The colors of light

are easy to remember by the acronym ROYGBIV.

Red, orange, yellow, green, blue, indigo, and violet.

Ultraviolet Light Means “beyond violet.” Shorter wavelengths,

higher frequency, and higher energy than visible light.

Our sun produces a lot of this light.

Causes suntans/sunburns. Can be used to disinfect

medical instruments. The ozone layer protects

us from much of the ultraviolet radiation.

X-Rays High energy, will

penetrate soft parts of your body.

Used to make images of the inside of your body.

Too many can be harmful. That is why you wear a lead apron to shield the parts of your body they are not interested in.

Gamma Rays Highest frequency,

highest energy waves. This makes them the most penetrating.

Radioactive materials emit gamma rays, so they can be very dangerous as they can kill cells as they pass through.

In controlled conditions they can be used to treat cancer.

Gamma Ray Emitters Many deep space objects

such as supernova’s, black holes, and neutron stars emit gamma rays. Fortunately our atmosphere shields us from them.

Closer to Earth radioactive materials and nuclear weapons emit gamma rays.

What causes light? All light is

produced by “excited” atoms.

When an atom absorbs energy, an electron will be bumped to a higher level. This produces an unstable or “excited” atom.

Emitting Light Atoms do not want

to be unstable. The electron will lose

the extra energy by giving off a photon, allowing it to fall back to its original level.

Light Emission