Post on 03-Jan-2016
transcript
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?