PHYSICS AND THE VISUAL ARTS
Notes on Lesson 2
Things to remember from Lesson 2 and its demonstrations.
What is light? We said that one model for light is a traveling electromagnetic wave.
What do we mean by that? You saw wave pulses on a rope. You saw wave pulses on a spring, both transverse and
longitudinal. What was the distinguishing characteristic of the transverse wave? Of the longitudinal
wave? (Remember the demo with the long slinky.)
You also saw wave pulses on the wave machine. First we used the wave demonstrator to show reflections
from a free end and from a fixed end. (What happened to the phase of the wave upon reflection?)
Reflection also occurs when a wave (including light) encounters a change in medium. (The different
mediums have different waves speeds.) In that case there is both reflection and transmission.
With the machine we could see the effect of "hard" and "soft" reflections. When the wave speed in the
incident medium is slower than in the transmitting
medium the reflection is "soft" and the reflected pulse comes back with the same phase, that is, with
the pulse amplitude in the same direction. In other words, an upward incident pulse is reflected as an
upward pulse. Conversely, when the reflection is from faster medium to
slower medium, the reflection is "hard" and the phase is reversed. In that case the direction of the
waves amplitude is inverted (upward incident pulse gives downward reflection).
Light is a transverse wave. There is a traveling, oscillating electric field
that is the light wave and this field is transverse to the direction
the wave is propagating. The speed of light is 3.0 x 108 m/s.
Can you show that the speed of light is
approximately one foot per nanosecond; that is, 1 ft/ns?
Could you find the speed of light in other units?
You should know the meaning and symbols for the following quantities: wave
speed, wavelength, period, and frequency.
The relationship between them is
wave speed = (frequency)(wavelength)
The equation
above is a key to all wave motion. As the light passes from one material to
another, the wave speed can change but the frequency always remains
constant. Thus, from the equation we see that the wavelength must change
when the speed changes. You saw that with the waves on the wave machine. The wavelength increased
dramatically as the waves went from the slower moving section of the machine to the faster moving section.
When light passes through transparent material it interacts with the atoms of the material in such a way
that the speed of the light is less than the speed of light in a vacuum (also known as free space). We
characterize the material according to the index of refraction. The index of refraction, n is
defined as the ratio of the speed of light in free space, c, to the speed of light in the material,
v. Expressed as an equation it is:
n = c/v . The index of refraction of air at 0 degrees Celsius and atmospheric
pressure is 1.000293. For most purposes it is so close to 1.00 that you may use 1.00 for its value. Many
liquids and solids have indices ranging from 1.3 to 3.5. Some examples are listed in the table below.
| Substance | Index of Refraction |
|---|
| Air | 1.00029 |
| Ice | 1.31 |
| Plexiglas | 1.51 |
| Crown glass | 1.52 |
| Flint glass | 1.66 |
| Zircon | 1.923 |
| Diamond | 2.417 |
You will learn more about the effects of the index of refraction in Lesson 4.
Also, light is only a small part of a much larger spectrum of similar
electromagnetic radiation. Light is that portion of the
electromagnetic spectrum that we can see with our eyes. Roughly, you can see
radiation from wavelengths beginning around 400 nm in the violet to around
700 nm in the far red. Radiation in the wavelength region just beyond 700 nm
is called infrared while that in the region just below 400 nm is called
ultraviolet. If you remember your Latin, this terminology may seem backward.
Can you make sense of it? (Hint: Think of frequency in the equation given above.)
Microwaves are similar to light waves, just with longer wavelengths and
thus we cannot see them. Microwaves are transmitted by paper
and wood and human tissue. They are reflected by mirrors and metal sheets
and even by screen wire if the screen mesh is small compared to the
wavelength of the microwaves. You may have noticed that the windows on microwave ovens have a wire screen
inside. Its purpose is to reflect the microwaves so that they don't escape into the room.
You also heard a discussion of the photo or particle model for light. What was discovered in 1900 was that
light carried energy in little bundles that was proportional to the frequency of the light. The consequence of
the discoveries in the early part of the 20th century is that light must be treating as both wave and particle
at the same time. These light particles, the photons, carry discrete amounts of momentum and energy but they
still have a wavelength and show wavelike properties. The energy in the photon is given by
E = hf,
where h is the Planck constant = 6.626 x 10-34 J-s.
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Last Modified: 01/27/05
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