Light

What is light? Early scientists thought it was made up of particles (protons, neutrons and electrons) Travels in a straight line instead of spreading out (like a wave would) Later scientists discovered light does spread so they thought it was a wave Einstein published his photoelectric effect which says that light is made up of particles that have wavelike tendencies

Photons and light The particles that light is made up of are known as photons Massless bundles of energy The energy comes from accelerating electrons that are typically found in atoms The energy is partially electric and partially magnetic Hence the term: electromagnetic waves

The speed of light Have you ever been to a baseball game where you see the bat hit the ball, but hear it a second later? That is because light travels about 100,000 times faster than sound!!! Light travels at approximately 300,000,000 m/s (300,000 km/s) in a vacuum It only takes light from the sun 8 minutes to reach the Earth, which is 93 million miles away!!

The Speed of Light Discuss with your neighbor what you think happens to the speed of light as it travels through our atmosphere or through water Light travels slightly slower through the atmosphere and 25% slower when it travels through water

The Electromagnetic Spectrum

What is the electromagnetic spectrum? The range of all electromagnetic waves All EM waves travel at the same speed (300,000,000 m/s = speed of light), but have different wavelengths

What is the electromagnetic spectrum? How is that possible? Discuss with a neighbor (hint: think about the wave speed formula) They have different frequencies! As you move from left to right on the spectrum, frequency increases

Radio Waves Any EM wave with a wavelength that is longer than 30 cm The longest wavelength of all EM waves What does that tell you about the frequency? They have low frequencies

Radio broadcasting Different radio stations are created through modulation Changing either the amplitude or the frequency of the wave AM radio = amplitude modulation Travel further distances FM radio = frequency modulation Shorter distances, but clearer sound due to less interference

Microwaves – not just for cooking Shorter wavelengths and higher frequencies than radio waves Wavelength is between 1mm and 30 cm Cell phone signals are carried on microwaves

Microwaves – not just for cooking Any signal sent between the earth and satellites in space are microwaves Also used in radar guns Reflected microwaves are used to calculate the speed of passing cars

Infrared Waves Shorter wavelengths and higher frequencies than microwaves Between 700 nm and 1 mm Most things give off infrared waves The amount given off depends on temperature of the object Warmer object give off more than cooler objects You can’t see IR waves with the naked eye Night vision glasses let you see IR waves

Visible Light A very small part of the EM spectrum that can be seen by humans Shorter wavelengths and higher frequencies than IR waves Wavelengths between 400 and 700 nm Only some of the light from the sun is visible light In other words, we can not see all of the light emitted from the sun We see white light from the sun A combination of all of the colors of light

Colors of light Different wavelengths of visible light are seen as different colors Longest wavelength = red Shortest wavelength = violet Roy G Biv – visible spectrum Red Orange Yellow Green Blue Indigo violet

Ultraviolet Light Also produced by the sun Shorter wavelengths and higher frequencies than visible light Wavelengths between 60 and 400 nm

Ultraviolet Light The good side: The bad side: Used to kill bacteria on food and tools Small amounts help your skin to produce vitamin D The bad side: Causes sunburn, skin cancer, wrinkles and eye damage Most UV rays from the sun do not reach the Earth

X-rays Some of the shortest wavelengths Wavelengths between .1 and 60 nm Can pass through materials because they are so short Too much exposure to them can be harmful to human cells

Gamma rays Wavelengths shorter than 0.1 nm Used to treat some forms of cancer May kill some healthy cells along with the cancer cells Used to kill bacteria in food