Chapter 6 ± Telescopes
The Eye Forming an Image
x The eyes basic components are a lens, a pupil, and a retina.
x The retina contains light-sensitive cells (cones and rods) that send signals to the brain
via the optic nerve when triggered by light.
x The lens creates an image on your retina because it bends light.
o ie, a light wave is coming to you from a large distance ± a ray of light in a
straight line. When the wave hits your eye, it slows down due to denser matter.
This change in speed is called refraction. If the light wave hits at an angle, the
side of the wave nearest your eye slows down first, allowing the far side to
catch up, changing the direction the light is traveling.
DIAGRAM 6.2, pg 176.
x The bending of light that happens in your eye is precisely tuned to make vision
x 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. This allows parallel rays of light to
converge to a point called the focus.
x Light rays that are not parallel, like those from a nearby object, enter a lens from
different directions. The rays end up bending to form an image of the original object.
x The place where the image appears in focus is called the focal plane of the lens.
x In an eye with perfect vision, the focal plane is on your retina.
x The image formed by a lens is upside down but is flipped right side up by your brain.
x The pupil of the eye controls the amount of light that enters the eye by adjusting the
size of its opening.
x The pupil dilates/opens wider in low light levels, allowing your eye to gather as many
photons of light as possible; it constrict/narrows in bright light so that your eye is not
overloaded with light.
x &DPHUD¶VRSHUDWLRQLVVLPLODUWRWKDt of an eye, but it makes a permanent record of the
image on film or by using some type of electronic detector.
x The camera lens plays the role of the lens of the eye and the detector, the role of the
x The shutter is like an eyelid: light can reach the detector only when the shutter is open.
x Advantage of using a camera:
1. Much more reliable and detailed than a drawing.
2. Have control over exposure time ± the amount of time during which light
collects on the detector. A long exposure time means more photons reach the
detector, which means better recording of the details.
x CCDs (charge-coupled devices) are mostly used today to record images, because it is a
chip of silicon made to be extraordinarily sensitive to photons.
x The chip is divided into a grid of squares called picture elements or pixels.
x When a photon strikes a pixel, it causes a bit of electric charge to accumulate until an
exposure is complete, where a computer measures the total electric charge in each
pixel and determines the amount of photons that have struck each pixel.