Chapter 14: Light and Reflection

The Sombrero Galaxy

(www.nasaimages.org)

Electromagnetic Waves

• What’s comes to mind when you think of the word light?

• Generally people only think about sources that produce light that is visible to humans

What is light?

• Light waves are electromagnetic waves– Remember that electromagnetic waves do not

need a medium to propagate

• Electromagnetic Wave: A transverse wave consisting of oscillating electric and magnetic fields at right angles to each other.

EM Waves

There’s more than what the eye can see

• Not all light is visible to the human eye

• The complete electromagnetic spectrum contains many different types of radiation

The electromagnetic spectrum

Remembering the Spectrum

• Rabbits Mate In Very Unusual Expensive Gardens• Red Martians Invaded Venus Using X-Ray Guns• From Longest Wavelength to Shortest Wavelength:

Radio, Microwave, Infrared, Visible, Ultraviolet, X-Ray, Gamma Ray

Why can’t we see the other parts of the EM spectrum?

• The Sun’s surface is brightest in the portion of the spectrum that is visible to humans

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter2/plank_e_sun.html

Does the Sun emit more than visible light radiation?

X Rays

Infrared

White Light

Microwave/Radio

UltravioletUltraviolet

http://sohowww.nascom.nasa.gov/

The Milky Way

Galaxy M101 Across the Spectrum

All EM waves travel at the speed of light

• Speed of light in a vacuum: c= 3.00 x 108 m/s

• Speed of light = frequency x wavelength

fc

Brightness

• Brightness at any point is the power per unit area at that point (like sound intensity)

• Brightness decreases by the square of the distance from the source– i.e. if you move an object twice as far away

from a light source, one-fourth as much light falls on it

How bright?

• Let’s pretend that a person is looking at a candle that is 1 m away from her. If the candle were move to a distance of 3 m away, how would the brightness change?– The candle would be 1/9 as bright

Section 14.2: Flat Mirrors

• Reflection: The turning back of an electromagnetic wave at the surface of a substance

Enceladus(Moon of Saturn)

• Reflects ~ 99% of EM waves

Iapetus (Moon of Saturn)

• Light regions: Reflect 50-60% of EM Waves

• Dark Regions: Reflect 2-5% of EM Waves

Reflection

• The texture of a surface affects how it reflects light

• Diffuse Reflection: Rough, textured surfaces reflect light in many different directions

• Specular Reflection: Smooth, shiny surfaces (i.e. water) reflect light in one direction

Specular vs Diffuse Reflection

Specular Reflection Diffuse Reflection

•When the book says reflection, it means specular reflection

Basic Reflection

• Angle of Incidence (θ) = Angle of Reflection (θ’)

Angles

Angle of Incidence: (θ) The angle between the incoming light ray and the line perpendicular to the surface

Angle of Reflection: (θ’) The angle between the reflected light ray and the line perpendicular to the surface

Flat Mirrors

• Flat Mirrors are the simplest type of mirror

• When an object is placed in front of a flat mirror, light rays will spread out from the object and reflect from the mirror’s surface

• Virtual Image: An image formed by light rays that only appear to intersect

Flat (Plane) Mirror Images

• The image produced is upright

• The image is the same size as the object (i.e., the magnification is m = 1)

• The image is the same distance from the mirror as the object appears to be (i.e., the image distance = the object distance)

• The image is a virtual image