Chapter 25 - Light

Section 25.1 – Properties of Light

pp. 604-611

TERMS TO LEARN

• Electromagnetic (EM) wave– a wave that can travel through space and matter. It consists of changing electric and magnetic fields.

• Radiation – the emission of energy in the form of EM waves

LIGHT

• Light is an EM wave.• Light can travel through space and

matter.• A field is a region around an object that

can exert a force on another object without actually touching it (ex.: a magnet is surrounded by a magnetic field that can pull a paper clip toward it).

AN ELECTROMAGNETIC WAVE

25.1 Properties of light

• Light travels fast over long distances and carries energy and information.

• Light travels in straight lines, but can be bent by lenses or reflected by mirrors heat and warmth.

• Light has color and can be bright or dim.

25.1 The electromagnetic spectrum

• Light, like sound and heat, is a form of energy.

• The visible light we see is part of the electromagnetic spectrum.

25.1 Properties of light• You see book pages because light in the

room reflects from the page to your eyes.

• Your eyes and brain use the information carried by the light to make a mental picture.

25.1 Light is produced by atoms

• Most light is produced by atoms.• When you put some energy into the

atom, it excites the atom’s electrons.• Light is produced when the electron

releases this energy.

25.1 Incandescent light• Making light with heat

is called incandescence.

• Atoms in the filament convert electrical energy to heat and then to light.

• Incandescent bulbs are inefficient, but their waste heat can be useful.

25.1 Fluorescent light• To make light,

fluorescent bulbs use high-voltage electricity to energize atoms of gas in the bulb.

• These atoms release the electrical energy directly as light (not heat), in a process called fluorescence.

25.1 Color and energy

• When all the colors of the rainbow are combined, we see light without any color.

• We call the combination of all colors white light.

25.1 Color and energy

• Compare the hot, blue flame from a gas stove to the orange flame of a match.

• The light from a gas flame is blue (high energy) and the light from a match is red-orange (low energy).

25.1 The speed of light• The speed at which light travels through

air is about 300 million meters per second. • The speed of light is so important in

physics that it is given its own symbol, a lower case “c”.

25.1 Speed of light

• The speed at which electromagnetic waves travel through air is about 300 million meters per second.

• The speed of light is so fast that when lightning strikes a few miles away, we hear the thunder after we see the lightning.

25.1 Wavelength and Frequency of Light• Because the wavelength

of light is so small, scientists measure it in nanometers.

• One nanometer (nm) is one billionth of a meter (0.000000001 m).

25.1 Wavelength and Frequency of Light

25.1 What kind of wave is light?

• A sound wave is a oscillation of air. • A water wave is an oscillation of the

surface of water. • An oscillation of electricity or

magnetism creates electromagnetic waves.

25.1 Electromagnetic waves

• If you could shake the magnet up and down 450 trillion times per second, you would make waves of red light with a frequency of about 450 THz.

25.1 Electromagnetic spectrum

• The entire range of electromagnetic waves, including all possible frequencies, is called the electromagnetic spectrum.

• This spectrum includes visible light and invisible waves:• radio wave• microwaves• infrared light• ultraviolet light• X-rays• gamma rays

RADIO WAVES

• Have some of the longest wavelengths and lowest frequencies of all the EM waves

• Are low energy waves

AM VS. FM WAVES

Amplitude Modulation (A.M.)

Frequency modulation (F.M.)

Can travel farther than FM

More information can be encoded than AM

Can reflect off the ionosphere

Passes through the ionosphere

Have longer wavelengths than FM

Have shorter wavelengths than AM

TV AND RADIO WAVES

• TV uses radio waves that have shorter wavelength and higher frequency than those used by radio stations.

• Most TV stations are broadcast using A.M. and F.M.

• F.M. waves carry sound; A.M. waves carry pictures

MICROWAVES• Have shorter

wavelength and higher frequency than radio waves.

• Carry more energy than radio waves.

• Microwaves are also used in radar (radio detection and ranging).

INFRARED WAVES

• Have shorter wavelengths and higher frequency than microwaves.

• Can carry more energy than microwaves and radio waves

INFRARED WAVES• The Sun is a

source of IR waves.

• Stars, planets, buildings, humans and trees also emit IR waves.

IR WAVES• The amount of IR

radiation emitted by an object depends on the temperature of the object. Warmer objects give off more IR radiation than cooler ones.

• Your eyes cannot see IR radiation but there are devices that can detect it (i.e. night vision goggles)

VISIBLE LIGHT

UV LIGHT

• Also produced by the Sun.

• Has shorter wavelength and higher frequency than visible light.

• Carries more energy than visible light.

EFFECTS OF UV WAVESPositive Effects Negative Effects

Used to kill bacteria in food

Overexposure can cause sunburn, skin cancer, eye damage

Causes skin cells to produce Vitamin D

Overexposure can cause wrinkles and premature aging of the skin

X-RAYS

• Have the shortest wavelengths and highest frequency of all EM waves.

• X-rays carry a lot of energy and can penetrate many materials.

• Too much exposure to x—rays can damage or kill living cells

GAMMA RAYS

• Carry large amounts of energy and can penetrate materials very easily.

• Every day you are exposed to small amounts of gamma rays that do not harm you.

• Radiologists focus these on tumors to kill cancer cells.

• A negative effect of gamma radiation is that they kill healthy cells.