Electromagnetic Spectrum

The Electromagnetic Spectrum is all of the frequencies or wavelengths of

electromagnetic radiation

Electromagnetic WavesThese are waves of electrical energy and unlike water and sound waves, they can travel through space; they do not need a medium to travel through.

The wavelength is the distance between the crest of one wave and the crest of another wave.

As you move to the right of the spectrum, what happens to the wavelength?

The shorter the wavelength the higher the energy

Gamma Rays

• Very high energy and high frequency rays• Used in radiotherapy to treat cancer, by

destroying cancer cells• Sterilization of medical instruments (like

before surgery)

X Rays

• High frequency, short wavelength rays• Your doctor uses them to look at your bones• Your dentist uses them to look at your teeth

to detect cavities

Ultraviolet Rays

• Medium frequency and medium wavelength rays

• The sun emits these rays (UV rays)- this is what causes our skin to burn, and long term exposure can lead to premature aging of the skin and even skin cancer

• Hot gases in the universe also emit UV rays

Infrared Rays

• Used by doctors to treat skin diseases and relieve the pain of sore muscles; in these treatments, the infrared rays pass through the patient’s skin and produce heat

• Photographers use it to take pictures in places where there is no visible light

• Burglar alarms also use infrared

Radio Waves• These rays have the longest wavelengths in

the EM spectrum• Used to carry signals for your television and

cellular phones• Also used, obviously as their name implies, to

bring music to your radio

Visible Light- Makes up only a very small

portion of the electromagnetic spectrum

- It is the only portion of the electromagnetic spectrum that we can see.

- If you were to shine white light through a prism, it would produce rainbow colors because the prism separates the light into different wavelengths of colors

RainbowsThe spectrum of colors is also seen in a rainbow. Water droplets in the atmoshpere can separate white light into visible light of different wavelengths. As a result, you will see the colors of visible light in a rainbow!

White light entering a water molecule in the atmosphere can get separated into different colors of visible light

The colors we see are just different wavelengths of visible light. Cells in the human eye react differently to different wavelengths of light.

Because of this, humans see different wavelengths as colors.

Which color has the shortest wavelength?

• Violet

• Which has the longest wavelength?• Red

Behavior of Light

Light can be:

• Transmitted• Reflected• Refracted• AbsorbedLight travels through space in a straight line as

long as nothing disturbs it.

Absorption

• The transfer of energy carried by light waves to particles of matter is called absorption.

Transmission

• Transmission is the passing of light through matter. In fact, without the transmission of light you could not see. All the light that reaches your eye is transmitted through air and several parts of your eye

Describe three ways light interacts with a person looking out a windowYou can see objects because outside because light is transmitted through.

You can see the glass and your reflection in it because light is reflected off the glass.

The glass feels warm on a sunny day because some of the light is absorbed by the glass.

Reflection –when light rays bounce off an object; this is how we see objects

The angle of reflection for a light beam is equal to the angle of incidence

Types of Reflection

Regular Reflection• Occurs when light beams

are reflected at the same angle. When your eye detects the reflected beams, 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 the reflected light is directed towards your eyes.

Specular (regular) reflection is why you can see yourself in a mirror, but because of diffuse

reflection, you cannot see your reflection on walls

Remember the sun is a natural source of visible light waves

Our eyes see the reflection of sunlight off objects around us. The color of this apple appears red because red is reflected while

all other colors of the spectrum are absorbed

ScatteringThis is an interaction of light with matter that causes light to change directions.

Light scatters in all directions after colliding with particles of matter or undergoing diffuse reflection

As light moves through the atmosphere it continues to go in a straight line until it bumps into a molecule or dust particle. Then what happens to light depends on its wavelength and the size of

the particle that it bumps into• Larger particles like dust and water droplets

will reflect, or bounce light• Smaller particles like gas molecules absorb

some of the light. After a while it radiates, or releases, some of the color that was absorbed

Why Does the Sky Appear Blue?

All colors can be absorbed by the atmospheric gas molecules, but the higher frequencies, such as, blues are absorbed more often than the reds. This is called scattering.The absorbed blue light then gets scattered in different directions. Whichever direction you look some of the scattered blue light reaches you.

Types of Matter

• Transparent

• Translucent

• Opaque

Transparent means matter through which visible light can easily be transmitted through. The

wings of this butterfly are transparent.

Translucent objects allow some light to transmit, but also scatters some light as it passes through,

so the object seen is blurry

Opaque matter does not transmit any light. Wood and metal are examples of opaque material. You cannot see

through opaque objects

Colors of ObjectsWhen white light shines on a strawberry, only red light is reflected. Other colors of light are absorbed. Therefore, the strawberry appears red to you.

RefractionSometimes light rays bend, or change directions, like when they enter a new medium at an angle

For example, light rays change directions when they move from air to water

RefractionThis is why a pencil positioned at an angle in a glass of water looks bent.

Light slows down when it enters a new medium, in this case water, and so it bends slightly making the pencil appear as if it is bent.

Note Check

1. List three electromagnetic waves we discussed today.

2. Name an opaque object in this room.3. Name a transparent object you have at

home.4. How is diffuse reflection different from

regular reflection?5. How much of the electromagnetic spectrum

is made up of visible light? (a small portion or large)