Light and Color
Light one candle and chase away the night
Electromagnetic Waves

Non Mechanical Waves – this means
they do not need a medium to carry
them.
 Transverse
 They
 They
Wave
travel through a vacuum
work through the electrical and
magnetic fields
Electromagnetic Spectrum
Classification of electromagnetic
waves according to frequency
 No sharp boundary between regions
 All travel at the same relative speed
called the speed of light - c

Transparent Materials

How light penetrates transparent material such as glass:
Transparent Materials
Average speed of light through different materials
•
vacuum—c (300,000,000 m/s)
•
atmosphere—slightly less than c (but rounded off to c)
•
water—3/4 c
•
glass— 2/3 c, depending on material
•
diamond—4/10 c

All EMW travel at the same speed.
Transparent
So we see that glass is transparent to visible light, but not to ultraviolet and
infrared light.
Opaque Materials

Most things around us are opaque—they absorb light without re-emitting
it.


Books, desks, chairs, and people are opaque.
Vibrations given by light to their atoms and molecules are turned into
random kinetic energy—into internal energy.

These materials become slightly warmer.
Opaque Materials
Shadows

A thin beam of light is often called a ray.

When we stand in the sunlight, some of
the light is stopped while other rays
continue in a straight-line path.

We cast a shadow—a region where light
rays do not reach.
Seeing Light – The Eye
The retina is composed of tiny antennae that resonate to
the incoming light.
• Rods handle vision in low light.
– They predominate toward the
periphery of the retina.
• Cones handle color vision
and detail.
– They are denser toward the
fovea.
– There are three types of cones,
stimulated by low, intermediate
and high frequencies of light.
Seeing Light – The Eye

Although our vision is poor from the corner of our eye,
we are sensitive to anything moving there.
Seeing Light – The Eye
The brightest light that the
human eye can perceive
without damage is some 500
million times brighter than
the dimmest light that can
be perceived.
Lateral inhibition: We don’t
perceive the actual
differences in brightness.
The brightest places in our
visual field are prevented
from outshining the rest.
Seeing Light – The Eye
The brightest light that the
human eye can perceive
without damage is some 500
million times brighter than
the dimmest light that can
be perceived.
Lateral inhibition: We don’t
perceive the actual
differences in brightness.
The brightest places in our
visual field are prevented
from outshining the rest.
Seeing Light – The Eye
Seeing Light – The Eye
Seeing Light – The Eye
Seeing Light – The Eye
Seeing Light – The Eye
Seeing Light – The Eye
Color in Our World
Color we see depends on frequency of light.

Lowest frequency—perceived as red

In between lowest and highest frequency—
perceived as colors of the rainbow (red,
orange, yellow, green, blue, indigo,
violet)

Highest frequency—perceived as violet

Beyond violet, invisible ultraviolet (UV)
Stroop Effect
Read the following using the color of the
word – not the word itself.
Color in Our World
Color

Physiological experience

In the eye of the beholder
Selective Reflection
Objects reflect light of some frequencies
and absorb the rest.
•
Rose petals absorb most of the light and
reflect red.
•
Objects that absorb light and reflect none
appear black.
•
Objects can reflect only those frequencies
present in the illuminating light.
Selective Transmission
Color of transparent object depends on color of light it transmits.
Colored glass is warmed due to the energy of
illuminating the glass.

absorbed light
Mixing Colored Light
Additive primary colors:

Red, green, and blue

Produce any color in the spectrum
Mixing Colored Light
Subtractive primary colors

Combination of two of the three additive primary colors:

red + blue = magenta (subtracting green)

red + green = yellow (subtracting blue)

blue + green = cyan (subtracting red)
Mixing Colored Light
Only three colors of ink (plus black) are used to print color
photographs—(a) magenta, (b) yellow, (c) cyan, which
when combined produce the colors shown in (d). The
addition of black (e) produces the finished result (f).
Why is the sky Blue???
Sun emits white light.
Light passes through the atmosphere.
Blue light is scattered. Smaller wavelengths scatter
the most.
No atmosphere means no blue light scattering
Sun appears yellow because blue is removed –
green/red
Why Sunsets Are Red
Light that is least scattered is light of low frequencies,
which best travel through air.
•
Red
•
Orange
•
Yellow
Colors
title
title
Colors
title