Chapter 5

The Nature of Light

Light is a type of energy that travels by

electromagnetic waves.

An electromagnetic wave (EM) consists of

electric and magnetic fields that vibrate

at right angles to each other.

Remember, electromagnetic waves do not

need a medium. It can travel through

empty space or matter, and consists of

changing electric and magnetic fields.

A field exists around certain objects and can

exert a force on an object without touching it.

FIELDS ARE NOT MADE OF MATTER

Example- The earth’s gravitational force

� An electric field surrounds every charged

object. The electric field around an object pulls

oppositely charged objects toward it and repels like-charged objects.

Example – Static Electricity

� A magnetic field surrounds every magnet.

� Radiation is the transfer of energy as

electromagnetic waves.

Think about This:

The speed of light is about 300,000,000 m/s.

Light travels about 880,000 times faster than

sound.

If you could run at the speed of light, you could

travel around the earth 7.5 times in 1 second.

That’s Fast!!!!

In case you were wondering, it takes

about 8.3 minutes for light to travel from

the sun to Earth. The Earth is

approximately 93 million miles from the

sun.

Only a small amount of energy is given off

by the sun when it reaches Earth. Most of

the sun’s energy is lost in space.

5-2

Electromagnetic Spectrum

� The entire range of EM waves is called the

electromagnetic spectrum.

� The electromagnetic spectrum is divided

into regions according to the length of the

wave.

Long Wave Length –Short Wave Length or

Low Frequency- High Frequency

� Radio waves have some of the longest

wavelengths and lowest frequencies of all

EM waves. Radio waves are any EM wave

that has wavelengths longer than 30 cm.

Modulation is changing amplitude or frequency.

AM radio waves have longer wavelengths than FM radio waves. However, FM radio

waves are less affected by electrical noise.

� Microwaves have shorter wavelengths and

higher frequencies that radio waves.

Microwaves are used to send information

over long distances.

Example – Cellular Phones, Radar

Infrared waves have shorter wavelengths and

higher frequencies than microwaves do.

Visible light is the very narrow range of

wavelengths and frequencies in the

electromagnetic spectrum that humans can

see.

White light is visible light of all wavelengths.

Examples- lamps, fluorescent bulbs, visible

light from the sun.

Humans see the different wavelengths of visible

light as different colors. The longest wavelength

is red light and the shortest is violet light.

The range of colors is called the visible spectrum.

ROY G. BIV

Ultraviolet light is produced by the sun.

Ultraviolet light affects your body in both good

and bad ways.

Bad

� Sunburn

� Skin Cancer

� Wrinkles

� Eye Damage

Good

� Kills bacteria on food and surgical tools

� Small amounts cause skin cells to produce

Vitamin D-allows intestines to absorb Calcium

X-rays and gamma rays have some of the

shortest wavelengths and highest

frequencies of all EM waves.

X-rays can pass through many materials.

X-rays cannot pass through lead

Gamma rays can penetrate most materials

easily and are used to treat some forms of

cancer. Some healthy cells may be killed

during process.

5.3

� Reflection happens when light waves bounce off an object. Light reflects off surfaces the same way a ball bounces off the ground.

Law of Reflection – The angle of incidence is equal to the angle of reflection.

Incidence is the arrival of a beam of light at a surface. Pg 146

Beam of light

traveling

toward the mirror.

The line perpendicular

to the mirrors surface

is called the normal.

Regular reflection occurs when light beams are

reflected at the same angle. Your eyes detect the

reflected beams and you can see a reflection on

the surface.

Diffuse reflection occurs when light beams reflect

at many different angles. You can’t see a

reflection because not all of the reflected light is

directed towards your eye.

Regular Reflection

vs.

Diffuse Reflection

Objects that produce visible light are called

luminous.

A visible object that is not a light source but you

can still see because the reflection of light

travels to your eyes is illuminated.

Absorption is the transfer of energy carried by

light waves to particles of matter. The farther

the light travels the more of it is absorbed.

Scattering is an interaction of light with matter

that causes light to change direction after

colliding with particles of matter.

Why is the sky blue?

Light with shorter wavelengths is scattered more

than light with longer wavelengths. Sunlight is

made up of many different colors of light but

blue light has very short wavelengths and is

scattered more than any other color. When you

look at the sky, you see a background of blue

light.

Refraction is the bending of a wave as it passes

between two substances in which the speed of

the wave differs. Ex- Rainbows (colors disperse)

Refraction of light occurs because the speed of

light varies depending on the material that the

wave travels through. Pg 149

Diffraction is the bending of waves around barriers

or through openings.

Interference happens when two or more waves

overlap that results in a single wave.

Overlapping waves combine by constructive and

destructive interference.

5.4

� Transmission is the passing of light through

matter. All of the light that reaches your eyes is

transmitted through air.

Transmission-Reflection-Absorption

Light is transmitted , reflected, and absorbed

when it strikes the glass in a window.

� Matter through which visible light is easily

transmitted is called transparent.

Examples- air, glass, and water

Translucent matter transmits light but also

scatters the light as it passes through the matter.

Example - wax paper

Matter that does not transmit any light is called

opaque.

Examples – wood, metal, your book

How We See Color

Humans see different wavelengths of light as

different colors. Humans see long wavelengths

as red and short wavelengths as violet. Some

colors, like pink and brown are seen when

certain combinations of wavelengths are

present.

So, the color that an object appears to be is

determined by the wavelengths of light that

reach your eye. Your eyes send signals to your

brain and is interpreted into colors.

Primary colors - red, blue, and green.

When two primary colors of light are added

together, you see a secondary color of light.

Secondary colors:

Cyan (blue plus green)

Magenta (blue plus red)

Yellow ( red plus green)