ʟᴇɴsᴇs

                27 - 30

lens - carefully shaped piece of transparent material,
refracts light rays to produce an image

ex: eyeglasses, cameras

series of connected prisms, each with slightly different
surface angle, work together to form focuses image

have focal points where they concentrate light

focus through refraction, not reflection

2 focal points, 1 on each side

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       Converging Lenses

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also known as convex lens, refracts incoming parallel 

light rays so they converge at focal point

thicker towards center than edges

object infinitely far away create point image at focal point

image is real, can be projected on screen

ex: magnifying glass

use lens to create image of distant light source, measure

distance from lens to image to find focal length

principal rays

1. parallel to principal axis - focused at focal point

2. center of lens - continue in straight light with no net

refraction, both side of lens are parallel to one another

3. through focal point - leave lens parallel to principal 

axis on opposite sides

produce real or virtual image depending on location

approach focal point - image is larger, farther away

between converging lens and focal point - light rays

diverge when they pass through lens, create enlarged

virtual image on same side as object

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          Pinhole Camera

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early cameras used small apertures the size of pinholes

remove one side of cardboard box, replace with semi
transparent sheet

poke hole on opposite side

inverted image with appear on sheet of paper

smaller pinhole = sharper image

make actual camera by using unexposed film

excellent fro viewing solar eclipses

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          Diverging Lenses

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concave lens, cause incoming rays to spread out

only produce virtual images

always virtual, upright, reduced in size

thicker toward edges

rays may need to be extended backward in order to 
determine point from which they originated

light rays leave every point on object (emission or reflection)
and pass through every part of lens

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    Spherical & Chromatic
             Aberration

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aberration - distortion of image caused by unintended lens

or mirror behavior

spherical aberration - light passing through edge of

lens focused at slightly different point

result in blurred image

reduced by using adjustable aperture, only center of lens 

used to focus light

reduce amount of light used, longer exposure time

spherical aberration corrected by using parabolic mirror

chromatic aberration - light of different colours refracted

different amounts by lens, converge at different points

corrected by combining lenses of different types of glass

curved mirrors can cause spherical aberration, but not

chromatic because no refraction takes place

eyes - pupils at most narrow, sharper image