Light & Waves

Mrs. Wolfe

Recall that Light is Defined by Wavelength

• The Electromagnetic Spectrum

• Where does light come from?

• Electrons jumping• Space• Gamma Radiation

Electromagnetic Spectrum

• The light we can see is called Visible light. But the spectrum includes mostly light that we can’t see…

• For example, the sun gives us other “rays.” Like what?

Gamma Rays

• Emitted by radioactive substances

• Most dangerous types of EM waves

• Why??

• Highest Energy/Frequency

X-rays

• Also emitted by unstable, radioactive atoms

• Penetrate through flesh but not bone, so they’re useful in medicine.

• Prolonged exposure can cause damage to body tissues. Why??

Ultra-violet rays

• Primarily from the sun• Produces Vitamin D in

human skin..so it’s GOOD!

• Causes darkening of the skin

• Overexposure causes sunburn and can burn the retina of the eye

• Used to sterilize things

Visible Light• White light• Only light the human eye

can see• Wavelengths range from:

4 x 10-7 to 7 x 10-7 metersEach wavelength is a certain

colorR.O.Y. G. B.I.V.Red – longest wavelength Violet – shortest wavelength

The Visible Light Spectrum

Infra-red rays• Emitted by all objects• “Heat waves”• Hotter objects emit more

waves than cooler objects• Infra-red waves can

penetrate darkness, clouds, mist, etc.

• Used in military• Used in your remote

controls

Microwaves

• Most easily absorbed by water in food. When that happens, heat is produced.

• That’s how we cook our food…by energizing the food with “invisible” light!

Radio Waves

• Used in TVs and radios

• Police radio communication

• Navigation systems

How can I prove that these invisible light rays exist?

• Time for a Demo!

• Michael Faraday– Great electricity scientist

– 1791-1867

• The Faraday Cage!

Energy and Light

• Some types of light have more energy than others.

• • Which has more energy – Gamma rays or

Microwaves?

• Which has less energy – orange light or green light?

Practice Problem

• Plants use energy from sunlight to make their food during photosynthesis.

• A packet of light energy, a photon, is changed into food.

• Which would make a better grow-light: red or violet light?

Wavelength (nm)

% ABS

Why can’t we see all the light?

• We have rods and cones in our eyes that can only detect certain wavelengths of light.

• Other animals see different wavelengths. Snakes, for example, see infrared light.

Go to http://science.howstuffworks.com/eye2.htm

Scroll down and watch the video!

A few last words on light

• Light can travel through a vacuum.

• Example?• Space!

• All light travels at the same speed…3 x 108

m/s in a vacuum!

• That’s REALLY fast!

Practice Problem

• What is the wavelength of light has a frequency of 0.75 x 1015?

• S = f x w• S = 3 x 108 m/s, F=0.75 x 1015 Hz• 3 x 108 m/s = 0.75 x 1015 Hz x W• W= 4 x 107 m

• What color is this? • Violet!!

Take Home Messages

• Each type of light has its own wavelength, frequency, and energy.

• Visible light is the only light we can see.

• ROY G BIV (longest to shortest wavelength)

• Light can travel through a vacuum!

• The speed of light is CONSTANT (3 x 108 m).

• Can you use the calculator to solve math problems for frequency, wavelength, and speed?