Nature of Light

WARMUP

What do you think light is?

Is light made of matter? Can light travel through space?

THE NATURE OF LIGHTAPPLIED PHYSICS: BOOK O, CHAPTER 3

SECTION 1: WHAT IS LIGHT?

By the end of this section, you should be able to:

Describe light as an electromagnetic wave.

Calculate distances traveled by light by using the speed of light.

Explain why light from the sun is important.

LIGHT: AN ELECTROMAGNETIC WAVE

Light is a type of energy that travels as a wave. But unlike most other types of waves, light does not require matter through which to travel.

Light is an electromagnetic wave (EM wave).

LIGHT: AN ELECTROMAGNETIC WAVE

An electromagnetic wave is a wave that consists of electric and magnetic fields that vibrate at right angles to each other.

LIGHT: AN ELECTROMAGNETIC WAVE

An electric field surrounds every charged object. You see the effect of electric fields whenever you see objects stuck together by static electricity.

A magnetic field surrounds every magnet. Because of magnetic fields, paper clips and iron filings are pulled toward magnets.

LIGHT: AN ELECTROMAGNETIC WAVE

An EM wave can be produced by the vibration of an electrically charged particle.

This vibration makes electric and magnetic fields vibrate also. Together, the vibrating fields are an EM wave that carries energy.

The transfer of energy as electromagnetic waves is called radiation.

LIGHT FROM THE SUN

Scientists have yet to discover anything that travels faster than light.

In the near vacuum of space, the speed of light is about 300,000 km/s. Light travels slightly slower in air, glass, and other types of matter.

HOW FAST IS LIGHT?

LIGHT FROM THE SUN

EM waves from the sun are the major source of energy on Earth. For example, plants use photosynthesis to store energy from the sun.

Animals use and store energy by eating plants or by eating other animals that eat plants.

Even fossil fuels store energy from the sun. Fossil fuels are formed from the remains of plants and animals that lived millions of years ago.

LIGHT FROM THE SUN

Only a very small part of the total energy given off by the sun reaches Earth. The sun gives off energy as EM waves in all directions. Most of this energy travels away in space.

SECTION 1 REVIEW

p.65 #2-9

WARMUP

Describe the weather conditions necessary to see a rainbow.

Why do rainbows form?

SECTION 2: THE ELECTROMAGNETIC SPECTRUM

By the end of this section, you should be able to:

Identify how electromagnetic waves differ from each other.

Describe some uses for radio waves and microwaves.

List examples of how infrared waves and visible light are important in your life.

Explain how ultraviolet light, x-rays, and gamma rays can be both helpful and harmful.

CHARACTERISTICS OF EM WAVES

The light that you can see is called visible light. However, there is light that you can’t see.

The light that you can see and light that you cannot are both kinds of electromagnetic (EM) waves.

Other kinds of EM waves include x-rays, radio waves, and microwaves.

CHARACTERISTICS OF EM WAVES

All EM waves travel at 300,000 km/s in a vacuum.

The entire range of EM waves is called the electromagnetic spectrum. The electromagnetic spectrum is divided into regions according to the length of the waves.

RADIO WAVES

Radio waves cover a wide range of waves in the EM spectrum. Radio waves have some of the longest wavelengths and the lowest frequencies of all EM waves.

Radio waves are any EM waves that have wavelengths longer than 30 cm. Radio waves are used for broadcasting radio signals.

RADIO WAVES

Radio stations can encode sound information into radio waves by varying either the waves’ amplitude or frequency.

Changing amplitude or frequency of a wave is called modulation. AM stands for amplitude modulation, and FM stands for frequency modulation.

RADIO WAVES

RADIO WAVES

AM radio waves have longer wavelengths than FM radio waves. AM radio waves can bounce off the atmosphere and thus can travel farther than FM radio waves.

But FM radio waves are less affected by electrical noise than AM radio waves, so music broadcast from FM sounds better than music from AM stations.

RADIO WAVES

TV signals are also carried by radio waves. Most TV stations broadcast radio waves that have shorter wavelengths and higher frequencies than those from radio stations.

Some waves carrying TV signals are transmitted to artificial satellites orbiting Earth. The waves are amplified and sent to ground antennas. They the signals travel through cables to TVs in homes.

MICROWAVES

Microwaves have shorter wavelengths and higher frequencies than radio waves. Microwaves have wavelengths between 1 mm and 30 cm.

Microwaves are used to send information over long distances.

MICROWAVES

MICROWAVES

Cellular phones send and receive signals using microwaves. Signals sent between Earth and artificial satellites in space are also carried by microwaves.

Microwaves are used in radar. Radar (radio detection and ranging) is used to detect the speed and location of objects.

MICROWAVES

Radar sends out microwaves that reflect off an object and return to the transmitter. The reflected waves are used to calculate speed.

INFRARED WAVES

Infrared waves have shorter wavelengths and higher frequencies than microwaves. The wavelengths of infrared waves vary between 700 nanometers (nm) and 1 mm.

Almost everything give off infrared waves, including the sun, buildings, trees, and your body. The amount of infrared waves an object emits depends on the object’s temperature. Warmer objects give off more infrared waves than cooler objects.

INFRARED WAVES

VISIBLE LIGHT

Visible light is the very narrow range of wavelengths and frequencies in the EM spectrum that humans’ eyes respond to. Visible light waves have wavelengths between 400 nm and 700 nm.

The visible light from the sun is white light. White light is visible light of all wavelengths combined.

VISIBLE LIGHT

Humans see different wavelengths of visible light as different colors. The longest wave-lengths are seen as red light. The shortest wave-lengths are seen as violet light.

The range of colors is called the visible spectrum.

ULTRAVIOLET LIGHT

Ultraviolet light (UV light) is another type of EM wave produced by the sun. Ultraviolet waves have shorter wavelengths and higher frequencies than visible light.

The wavelengths of UV light wave vary between 60 nm and 400 nm.

ULTRAVIOLET LIGHT

Too much UV light can cause sunburn. UV light can also cause skin cancer and wrinkles, and damage the eyes.

Ultraviolet waves produced by UV lamps are used to kill bacteria on food and surgical tools. Small amounts of UV light are beneficial to your body, causing skin cells to produce vitamin D.

X-RAYS & GAMMA RAYS

X-rays have wavelengths between 0.001 nm and 60 nm. X-rays can pass through many materials, making them useful in the medical field.

However, too much exposure to x-rays can damage or kill living cells.

X-RAYS & GAMMA RAYS

X-RAYS & GAMMA RAYS

Gamma rays have wavelengths shorter than 0.1 nm. They can penetrate most materials easily.

Gamma rays are used to treat some forms of cancer. Doctors focus the rays on tumors inside the body to kill the cancer cells.

Gamma rays are also used to kill harmful bacteria in foods, such as meat and fresh fruits.

SECTION 2 REVIEW

p.73 #2-12