Characteristics of Light

Characteristics of Light

Electromagnetic Waves

An electromagnetic wave is a wave that consists of oscillating electric and magnetic fields, which radiate outward from the source at the speed of light.

The spectrum includes more than visible light.

• Not all light is visible to the human eye.

• A variety of forms of radiation -- including X rays, microwaves, and radio waves -- have many of the same properties as visible light.

Electromagnetic waves vary depending on frequency and wavelength.

• In classical electromagnetic wave theory, light is considered to be a wave composed of oscillating electric and magnetic fields.

Electromagnetic waves cont.

• These fields are perpendicular to the direction in which the wave moves.

• Therefore, electromagnetic waves are transverse waves.

• The electric and magnetic fields are at right angles to each other.

Multiple Choice

Electric and magnetic fields are ____________________ to the direction of the electromagnetic wave.

• parallel

• perpendicular

• 45 degrees

• 15 degrees

Multiple Choice

Electromagnetic waves are

• longitudinal waves

• transverse waves

• circular waves

Multiple Choice

Electromagnetic waves

• require a medium to travel

• cannot travel through empty space

• can travel trough a vacuum and a medium

• can only travel through a vacuum

Electromagnetic waves cont.

• Electromagnetic waves are distinguished by their different frequencies and wavelengths.

• Types of electromagnetic waves are listed on the right.

• Note the different frequencies and wavelengths.

All electromagnetic waves move at the speed of light.

• The current accepted value for light traveling in a vacuum is 2.99792458 x 10⁸ m/s

• Light travels slightly slower in air with a speed of 2.99709 x 10⁸ m/s.

Multiple Choice

How long does it take light from the sun to reach the Earth?

• about 1 second

• about 8 minutes

• about 1 hour

• about 24 hours

Wave Speed Equation

• For calculations, we use the value for the speed of light in a vacuum and air as 3.00 x 10⁸ m/s.

Multiple Choice

The speed of light is

• 3.00 x 10⁸ m/s

• 3.00 x 10⁸ km/h

• 3.00 x 10⁶ m/s

Sample Problem A

The AM radio band extends from 5.4 x 105 Hz to 1.7 x 106 Hz. What are the longest and shortest wavelengths in this frequency?

  $c\ =f\lambda\ \ \ \ \ \ \lambda=\frac{c}{f}$     $\lambda=\frac{3.0\times10^8m\div s}{5.410^5m\div s}\ =5.6\times10^2m$     $\lambda=\frac{3.0\times10^8m\div s}{1.7\times10^6m\div s}\ =\ 1.8\times10^2m\div s$  

 

What is the frequency of an electromagnetic wave if it has a wavelength of 1.0 x 10³ m?

 $c\ =\ f\lambda\ \ \ \ \ \ \ \ f\ =\ \frac{c}{\lambda}\ \ \ \ \ \ \ f\ =\ \frac{3.0\times10^8m\div s}{1.0\ \times\ 10^3m}\ =\ 3.0\times10^{5^{ }}Hz$  

Multiple Choice

What is the frequency of highly energetic ultraviolet radiation that has a wavelength of 1.25 x 10^-7 m?

• 2.40 x 10^{15 Hz}

• 5.4 x 10¹⁴ Hz

• 3.0 x 10⁵ Hz

Multiple Choice

What is the wavelength for a FM radio band of 88 MHz (8.8 x 10⁷ Hz)?

• 2.4 m

• 2.8 m

• 3.4 m

Waves can be approximated as rays.

• Waves can be treated as a straight line perpendicular to the wave front.

• This line is called a ray, and this simplification is called the ray approximation.

Illuminance decreases as the square of the distance from the source.

• The rate at which light is emitted from a source is called the luminous flux and is measured in lumens (lm)

Illuminance cont.

• The luminous flux divided by the area of the surface, which is called illuminance (measured in lm/m² or lux), decreases as the radius squared when you move away from a light source.

Sample Problem

How bright would the sun appear to an observer on Earth if the sun were four times farther from Earth than it actually is?

 $I\ =\ \frac{1}{x^2}\ \ \ \ \ \ \ I\ =\ \frac{1}{4^2}\ \ \ \ \ \ \ \ \ \ \ I\ =\ \frac{1}{16}$  

Multiple Choice

A light bulb has a illuminance of 440 lumens at a distance of 1 meter. What would be its illuminance at distance of 3.0 meters?

• 220 lumens

• 49 lumens

• 94 lumens

Multiple Choice

A light bulb has a illuminance of 680 lumens at a distance of 1 meter. What would be its illuminance at distance of 4.0 meters?

• 76 lumens

• 34 lumens

• 43 lumens

End