Light and the

Electromagnetic Spectrum

The Electromagnetic Spectrum

The electromagnetic spectrum represents the range of energy from low energy, low frequency radio waves with long wavelengths up to high energy, high frequency gamma waves with small wavelengths.

Visible light is a small portion of this spectrum. This is the only part of this energy range that our eyes can detect. What we see is a rainbow of colors.

RedOrangeYellowGreenBlueIndigoViolet

ROY G BIV

Frequency Ranges• Wavelengths• 104 101 1 10-2 10-5 10-6 10-8 10-10 10-12

• Frequencies (cycles per sec) 3 x 106 3 x 1010 3 x 1014 3 x 1016 3 x1018 3 x10 22

Frequency Ranges of Visible LightRed light has a frequency of roughly 4.3 × 1014 Hz, and a wavelength of about

7.0 × 10 7 m (700nm).

Violet light, at the other end of the visible range, has nearly double the frequency—7.5 × 1014 Hz—and (since the speed of light is the same in either case) just over half the wavelength—

4.0 × 10 7 m (400nm).

The radiation to which our eyes are most sensitive has a wavelength near the middle of this range, at about

5.5 x 10-7m (550 nm), in the yellow-green region of the spectrum.

It is no coincidence that this wavelength falls within the range of wavelengths at which the Sun emits most of its electromagnetic energy—our eyes have evolved to take greatest advantage of the available light.

c = fλ

• The frequency (f) of a wave is the number of waves to cross a point in 1 second (units are Hertz – cycles/sec or sec-1)

• λ is the wavelength- the distance from crest to crest on a wave

• The product of wavelength and frequency always equals the speed of light.

c = v = λf• Why does this make sense?

• NOTE:

c is a constant value= 3.00 x 108 m/s

• Calculate the wavelength of yellow light emitted from a sodium lamp if the frequency is

5.10 x 1014 Hz (5.10 x 1014 s-1)

List the known info List the unknownc = 3.00 x 1010 cm/s wavelength (λ) = ?

cmFrequency (v) = 5.10 x 1014 s-1

C = λv λ = c v

λ = 3.00 x 1010 cm/s = 5.88 x 10-5 cm

5.10 x 1014 s-1

PROBLEMS

YOUR TURN1- What is the wavelength of radiation

with a frequency of 1.50 x 1013 s-1?

2- What frequency is radiation with a wavelength of 5.00 x 10-6 cm? In what region of the electromagnetic spectrum is this radiation?

RADIO WAVES• A. Have the longest wavelengths and lowest

frequencies of all the electromagnetic waves.• B. A radio picks up radio waves through an

antenna and converts it to sound waves.• C. Each radio station in an area broadcasts at a

different frequency. # on radio dial tells frequency.• D. MRI (MAGNETIC RESONACE IMAGING)

– Uses Short wave radio waves with a magnet to create an image

MRI of the Brain

AM=Amplitude modulation—waves bounce off ionosphere can pick up stations from different cities.

(535kHz-1605kHz= vibrate at 535 to 1605 thousand times/second)

+

FM=Frequency modulation—waves travel in a straight line & through the ionosphere--lose reception when you travel out of range.

(88MHz-108MHz = vibrate at 88million to 108million times/second)

+

Bands of Radio/TV/Microwaves

MICROWAVES• Microwaves—have the shortest wavelengths

and the highest frequency of the radio waves.– Used in microwave ovens.

• Waves transfer energy to the water in the food causing them to vibrate which in turn transfers energy in the form of heat to the food.

– Used by cell phones and pagers.– RADAR (Radio Detection and Ranging)

• Used to find the speed of an object by sending out radio waves and measuring the time it takes them to return.

INFRARED RAYS• Infrared= below red• Shorter wavelength and higher frequency than

microwaves.• You can feel the longest ones as warmth on your

skin• Heat lamps give off infrared waves.• Warm objects give off more heat energy than cool

objects.• Thermogram—a picture that shows regions of

different temperatures in the body. Temperatures are calculated by the amount of infrared radiation given off. Therefore people give off infrared rays.

http://www.snellinfrared.com/library/_view_msg.asp?refid=19

VISIBLE LIGHT• Shorter wavelength and higher frequency than

infrared rays.• Electromagnetic waves we can see.• Longest wavelength= red light• Shortest wavelength= violet (purple) light• When light enters a new medium it bends

(refracts). Each wavelength bends a different amount allowing white light to separate into it’s various colors ROYGBIV.

ULTRAVIOLET RAYS• Shorter wavelength and higher frequency than

visible light• Carry more energy than visible light• Used to kill bacteria. (Sterilization of

equipment)• Causes your skin to produce vitamin D (good

for teeth and bones)• Used to treat jaundice ( in some new born

babies.• Too much can cause skin cancer. • Use sun block to protect against (UV rays)

X- RAYS• Shorter wavelength and higher frequency than UV-

rays• Carry a great amount of energy• Can penetrate most matter.• Bones and teeth absorb x-rays. (The light part of

an x-ray image indicates a place where the x-ray was absorbed)

• Too much exposure can cause cancer – (lead vest at dentist protects organs from unnecessary

exposure)• Used by engineers to check for tiny cracks in

structures.– The rays pass through the cracks and the cracks appear

dark on film.

GAMMA RAYS

• Shorter wavelength and higher frequency than X-rays

• Carry the greatest amount of energy and penetrate the most.

• Used in radiation treatment to kill cancer cells.

• Can be very harmful if not used correctly.

Using the EM waves to view the Sun

Animation—View a Galaxy at different wavelengths

• Brief SUMMARY• A. All electromagnetic waves travel at the

same speed. (300,000,000 meters/second in a vacuum.

• B. They all have different wavelength and different frequencies.– Long wavelength-lowest frequency– Short wavelength highest frequency– The higher the frequency the higher the

energy.

• The colors we see in objects are the colors that are reflected, all other colors are absorbed. A red t-shirt appears red because red is reflected to our eyes and the other colors are absorbed.

• When all colors are being reflected we see white light (white isn’t really a color)

• When all wavelengths of light are being absorbed we see black (black also, isn’t really a color)

• A false-color image is made when the satellite records data about brightness of the light waves reflecting off the Earth's surface.