Download 15.4 How Light Behaves

15.4 How Light Behaves
The study of light is called optics. By understanding
how light behaves, scientists have learned to design a
variety of optical instruments that aid in the study of
the universe. For example, microscopes enable us to
examine extremely small objects, such as single-celled
organisms. With telescopes, we can observe distant but
very large objects, such as galaxies and planets. Optics
also enables us to understand vision. The colors of the
sky, the sparkle of a diamond, and many other parts of
the everyday world.
15.4.1 Illumination
A body that gives off light is called luminous, like
all those glow-in-the-dark toys, and a body that gives
off light when it is heated is called incandescent, like
the filament in a light bulb. A body is illuminated
when you can see it because it reflects light towards
your eyes.
A star, the pixels of your computer screen, and the
gas in a flourescent light are luminous. Why? Because
they give off light but not due to heating. The filament
of a bulb and molten lava are incandescent because
they give off light due to heating. The moon and your
book do not give off any light, but they are illuminated
by reflected light from the sun or from your lamp.
Radiant Flux – the amount of energy radiated in a
unit of time by an electromagnetic wave source.
Luminous Flux (F) – the electromagnetic radiation in
only the visible range of wavelengths. It is measured in
lumen (lm)
Illuminance (E) – the luminous flux per square meter
and is measured in lm/m2 or lux.
Luminous Intensity (I) of a source in a particular
direction, using the unit candela (cd).
One cd is equal to 1 lumen/steradian. The steradian
is the unit of solid angle, given by A/R² where A is the
area of the sphere and R is the radius.
15.4.2 Reflection
Refract – change direction.
Transparent (materials) – let's light rays pass
through it without mixing them up.
Translucent (materials) – allows rays pass through it,
but it mixes them up so that you cannot see clearly
through the material.
Opaque (materials) – blocks all light.
Normal – a line perpendicular to the reflecting surface.
The angle between the path of an incoming ray and a normal is
called the angle of incidence.
Wherever a ray reflects from a surface, it has an equal angle to
the normal at that spot as it had before the reflection.
If you have a nice, clean, flat, shiny mirror, the light will still
be recognizable as the beam from the flashlight is called
specular reflection.
Diffuse Reflection – the reflection will no longer look like the
beam of light before it was reflected.
15.4.3 Refraction
Waves that do not reflect back from a boundary but
travel into the new medium instead are said to refract.
In one dimension, the wave just move just moves
straight into the new material, adjusting only its speed.
In two dimensions, the angle formed by the refracted
ray and the boundary will change from the incident
angle, because the wave moves at a different speed in
the new medium.
When light passes through a surface, its speed
changes. If the light enters at any angle except a right
angle, the change in the light's speed changes it's
direction of travel.
The angle i between an approaching ray and the
interfere between two media is called the angle of
incidence of the ray. The angle r between the receding
ray at the interface has passed though is called the
angle of refraction.
15.4.3.1 Snell's Law
Snell's Law is a relationship between indices of
refraction, the angles of incidence and refraction, and
speeds in this problem:
sin i / sin r = v₁ / v₂
This result is known as Snell's Law after its discoverer,
the seventeenth – century Dustch astronomer
Willebrord Snell.
15.4.3.2 Index of Refraction
The ratio between the speed of light c and its speed v in
a particular medium is called index of refraction of the
medium. The greater the index of refraction, the greater
the extent to which a light beam is deflected on
entering or leaving the medium. The symbol for the
index of refraction is n, so that n = c / v.
Substance
Air
Benzene
Carbon Disulfide
Diamond
Ethyl alcohol
Glass, crown
Glass, flint
Ice
Lucite and plexiglas
Quartz
Water
Zircon
n
1.0003
1.5
1.63
2.42
1.36
1.52
1.63
1.31
1.51
1.46
1.33
1.92
It is easy to write Snell's law in terms of the indexes of
refraction n₁ and n₂ of two successive media. This is usually
written in the form n₁sin i = n₂ sin r
15.4.4 Absorption
Opaque materials absorb certain colors of light.
Transparent materials also absorb certain colors if they
contain dyes or pigments.
Scattering describes what happens when light rays
strike atoms, molecules, or other individual tiny
particles. These particles send the rays of light off in a
new directions ----- that is, they cause the rays to
scatter.
On a clear day, the ocean appears blue because of two
processes:
1. The ocean's surface reflects some of the blue light
from the sy toward the observer.
2. Light coming directly from the sun enters the water.
The water molecules then scatter more blue rays
toward the observer than they do the other colors in
sunlight.
Example:
A beam of parallel light enters a block of ice at an
angle of incidence of 30°. What is the angle of
refraction in the ice?
Solution:
The index of refraction of air and ice are,
respectively, 1.00 and 1.31
sin r = (nair / nice) sin i = 1.00 / 1.31 sin 30° = 0.382