Oct 11 - Intro to Light and Intro to Telescopes Flashcards
How do we experience light in a form of energy
Radiative Energy: energy that light carries (one of three categories of energy, along with kinetic and potential energy)
Power:
rate of energy flow
Watts:
units of the rate of energy flow
Spectrum:
rainbow of light; red, orange, yellow, green, blue, violet
We see white when these are mixed in equal proportions
Light from the Sun or a light bulb is often called white light
Black - perceived with no light and no color
Primary colors of vision:
red, green, blue - colors directly detected by cells in your eyes
Diffraction grating:
piece of plastic or glass is etched with many closely spaced lines
How do light and matter interact?
4 basic ways:
- Emission
- Absorption
- Transmission
- Reflection
Emission:
A light bulb emits visible light; the energy of the light comes from electrical potential energy supplied to the light bulb.
Absorption:
When you place your hand near an incandescent light bulb, your hand absorbs some of the light, and this absorbed energy warms your hand
Materials that absorb light are called opaque
Transmission
Some forms of matter (such as glass or air) transmit light, allowing it to pass through.
Materials that transmit light are said to be transparent
Reflection
scattering: Light can bounce off matter, leading to what we call reflection when the bouncing is all in the same general direction or scattering when the bouncing is more random
Light as a Wave
Waves - throwing pebble into pond
Waves consist of peaks, where the water is higher than average, and troughs, where the water is lower than average
As the waves pass by a floating leaf, you’ll see the leaf rise up with each peak and drop down with each trough (through frequency), but the leaf itself will not travel across the pond’s surface with the wave.
We conclude that even though the waves are moving outward, the particles (molecules) that make up the water are moving primarily up and down (along with a bit of sloshing back and forth).
That is, the waves carry energy outward from the place where the pebble landed but do not carry matter along with them.
In essence, a particle is a thing, while a wave is a pattern revealed by its interaction with particles.
3 properties of waves:
- Wavelength
- Frequency
- Speed
Wavelength - 3 properties of waves:
Wavelength is the distance from one peak to the next
Frequency
The number of peaks passing by any point each second
Cycles per second: are often called hertz (Hz)
Speed
Speed: (of the waves) tells us how fast their peaks travel across the pond
Because the waves carry energy, the speed essentially tells us how fast the energy travels from one place to another.
A simple formula relates the wavelength, frequency, and speed of any wave
wavelength x frequency = sound
What’s a field?
The concept of a field is a bit abstract, but it is used to describe the strength of force that a particle would experience at any point in space.
Electricity and magnetism also create forces, so their strength in different places can be described in terms of electric fields and magnetic fields
What type of waves are light waves?
Light waves are traveling vibrations of both electric and magnetic fields, so we say that light is an electromagnetic wave
Just as the ripples on a pond will cause a leaf to bob up and down, the vibrations of the electric field in an electromagnetic wave will cause any charged particle, such as an electron, to bob up and down
If you could set up electrons in a row, they would wriggle like a snake as light passed by
The distance between peaks in this row of electrons would tell us the wavelength of the light wave, while the number of times each electron bobbed up and down would tell us the frequency
All light travels through empty space at the same speed…
the speed of light
Relationship between wavelength and frequency for light: The longer the wavelength, the lower the frequency, and the shorter the wavelength, the higher the frequency.
What are photons?
We say that light comes in individual “pieces,” called photons, that have properties of both particles and waves
Just as a moving baseball carries a specific amount of kinetic energy, each photon of light carries a specific amount of radiative energy
Like waves, each photon is characterized by a wavelength and a frequency
To sum up, our modern understanding maintains that:
(1) light is both a particle and a wave, an idea we describe by saying that light consists of individual photons characterized by wavelength, frequency, and energy
(2) the wavelength, frequency, and energy of light are simply related because all photons travel through space at the same speed—the speed of light.
What is the electromagnetic spectrum?
In fact, the light that we can see is only a tiny part of the complete spectrum of light, usually called the electromagnetic spectrum; light itself is often called electromagnetic radiation
What do we call the light we can see?
Visible light, is found near the middle of the spectrum, with wavelengths ranging from about 400 nanometers at the blue or violet end of the rainbow to about 700 nanometers at the red end
What’s infrared light?
Light with wavelengths somewhat longer than those of red light is called infrared, because it lies beyond the red end of the rainbow
Radio Waves
are the longest-wavelength light
Microwaves
Light in the region near the border between infrared and radio waves, where wavelengths range from micrometers to centimeters, is often called
Ultraviolet
On the other side of the spectrum, light with wavelengths somewhat shorter than those of blue light is called ultraviolet, because it lies beyond the blue (or violet) end of the rainbow
X-rays & Gamma Rays
Light with even shorter wavelengths is called x-rays, and the shortest-wavelength light is called gamma rays
Bending Light
The lens of the eye creates an image by bending light in much the same way as a simple glass lens.
The result is bending (more technically known as refraction)—a change in the direction in which the light is traveling
Image Formation
Light rays that enter the lens farther from the center are bent more, and rays that pass directly through the center are not bent at all.
In this way, parallel rays of light, such as those from a distant star, converge to a point called the focus (or focal point)
The fact that parallel rays of light converge to a sharp focus explains why distant stars appear as points of light to our eyes or on photographs
Image Formation - Unparalleled Lines (CONT)
Light rays that are not parallel, such as those from a nearby object, enter a lens from different directions. These rays do not all converge at the focus, but they still follow precise rules as they bend at the lens
The result is the bending of rays to form an image of the original object. The place where the image appears in focus is called the focal plane of the lens
Recording Images
The basic operation of a camera is quite similar to that of an eye. The camera has a small opening for light to enter, much like the pupil of the eye. The camera lens bends the light, bringing it to a focus on a detector (or image sensor)that makes a permanent record of the image.
We can use the shutter to control the exposure time of an image, the amount of time during which light collects on the detector
Light Collecting Area - Telecopes
A telescope’s light-collecting area tells us how much total light it can collect at one time.
Telescopes are generally round, so we usually characterize a telescope’s size by the diameter of its light-collecting area. For example, a “10-meter telescope” has a light-collecting area that is 10 meters in diameter.
Angular Resolution - Telecopes
Angular resolution is the smallest angle over which we can tell that two dots—or two stars—are distinct.
The human eye has an angular resolution of about 1 arcminute , meaning that two stars can appear distinct only if they have at least this much angular separation in the sky.
The ultimate limit to a telescope’s resolving power comes from…
The properties of light. Because light is an electromagnetic wave, beams of light can interfere with one another like overlapping sets of ripples on a pond
This interference limits a telescope’s angular resolution even when all other conditions are perfect. That is why even a high-quality telescope in space cannot have perfect angular resolution
The angular resolution that a telescope could achieve if it were limited only by the interference of light waves is called its…
Diffraction limit. (Diffraction is a technical term for the effects of interference that limit telescope resolution.)
The diffraction limit depends on both the diameter of the telescope’s primary mirror and the wavelength of the light being observed
2 basic designs of telescopes:
- Refracting
- Reflecting
Refracting Telescope
operates much like an eye, using transparent glass lenses to collect and focus light
uses a precisely curved primary mirror to gather light (FIGURE 6.11). This mirror reflects the gathered light to a secondary mirror that lies in front of it. The secondary mirror then reflects the light to a focus at a place where the eye or instruments can observe it—sometimes through a hole in the primary mirror and sometimes through the side of the telescope (often with the aid of additional small mirrors)
Nearly all telescopes used in current astronomical research are reflectors, mainly for two practical reasons
First, because light passes through the lens of a refracting telescope, lenses must be made from clear, high-quality glass with precisely shaped surfaces on both sides (In contrast, only the reflecting surface of a mirror must be precisely shaped; the quality of the underlying glass is not a factor)
Second, large glass lenses are extremely heavy and can be held in place only by their edges. Because the large lens is at the top of a refracting telescope, it is difficult to stabilize refracting telescopes and to prevent large lenses from deforming.
