Post on 21-Mar-2016
description
Thin LensesThin LensesChapter 15Chapter 15
What is a lens?What is a lens? A transparent object that refracts A transparent object that refracts
light rays, causing them to light rays, causing them to converge or diverge to create an converge or diverge to create an imageimage
Converging LensesConverging Lenses A converging lens is thicker at the A converging lens is thicker at the
middle than it is at the rimmiddle than it is at the rim– The front of the lens is the side of The front of the lens is the side of
the lens where the object isthe lens where the object is
Converging lenses can produce Converging lenses can produce real or virtual imagesreal or virtual images
Converging LensesConverging Lenses
The focal points are equidistant from the center of the lens. That distanceis the focal length of the lens.
F F 2 F2 F
Converging Lens Converging Lens Reference Rays Reference Rays (p.569)(p.569)
RayRay From object From object to lensto lens
From lens to From lens to imageimage
Parallel RayParallel Ray Parallel to Parallel to principal axisprincipal axis
Passes Passes through focal through focal point Fpoint F
Central RayCentral Ray To the center To the center of the lensof the lens
From the From the center of the center of the lenslens
Focal rayFocal ray Passes Passes through focal through focal point Fpoint F
Parallel to Parallel to principal axisprincipal axis
Converging LensesConverging Lenses
The focal points are equidistant from the center of the lens. That distanceis the focal length of the lens.
F F 2 F2 F
Ray 1
Ray 2
Ray 3
Images Created by Converging Lenses Images Created by Converging Lenses (p.571)(p.571)
Images produced by lenses Images produced by lenses (p. 570-571)(p. 570-571) An object An object
infinitely far infinitely far away from a away from a converging lens converging lens will create a will create a point image at point image at the focal pointthe focal point– i.e. light from the i.e. light from the
SunSun
Images produced by lenses Images produced by lenses (p. 570-571)(p. 570-571)
As a distant object As a distant object approaches the focal approaches the focal point, the image point, the image becomes larger and becomes larger and farther awayfarther away
Images produced by lenses Images produced by lenses (p. 570-571)(p. 570-571)
When the object When the object is at the focal is at the focal point, the light point, the light rays exit parallel rays exit parallel to each other to each other and the image is and the image is “at infinity”“at infinity”– i.e. a lighthouse i.e. a lighthouse
or a searchlightor a searchlight
Images produced by lenses Images produced by lenses (p. 570-571)(p. 570-571) When the object is inside the When the object is inside the
focal point, the rays are drawn focal point, the rays are drawn the same way but you must the same way but you must extend them backwards in order extend them backwards in order to find the imageto find the image
Converging Lens: Objects inside the Converging Lens: Objects inside the focal pointfocal point
F F 2 F2 FThese rays are diverging and won’t cross
Draw lines extending backwards to form the image
Thin Lens and Thin Lens and Magnification Equations Magnification Equations (p.574)(p.574)
fqp111
pqM
Sign conventions for Sign conventions for lenses p.574lenses p.574
++pp Object distance for Object distance for
lenseslensesqq Image in back of Image in back of
the lensthe lensImage in front Image in front of the lensof the lens
ff Converging LensConverging Lens Diverging LensDiverging Lens
Sample Problem p.576 Sample Problem p.576 #2#2 Sherlock Holmes examines a clue Sherlock Holmes examines a clue
by holding his magnifying class by holding his magnifying class (with a focal length of 15.0 cm) (with a focal length of 15.0 cm) 10.cm away from an object. Find 10.cm away from an object. Find the image distance and the the image distance and the magnification. Describe the magnification. Describe the image he observes. Draw a ray image he observes. Draw a ray diagram to confirm your answers.diagram to confirm your answers.
What kind of lens is a What kind of lens is a magnifying glass?magnifying glass? Magnifying glasses produce Magnifying glasses produce
enlarged images. Therefore, a enlarged images. Therefore, a magnifying glass is a converging magnifying glass is a converging lens. lens.
f = + 15 cm, p= +10 cmf = + 15 cm, p= +10 cm
Solve the problemSolve the problem
301
101
151111
pfq cm 30q
31030
cmcm
pqM
The image is virtual and upright
Magnifying Lens Ray DiagramMagnifying Lens Ray Diagram
F F 2 F2 FObject
Notice that the image is located at 2F (30 cm) and it is also in front of the lens which is why q is negative.
Diverging LensesDiverging Lenses A diverging lens is thinner at the A diverging lens is thinner at the
middle than it is at the rimmiddle than it is at the rim
A diverging lens has two focal A diverging lens has two focal points but only one focal lengthpoints but only one focal length
Diverging lenses only produce Diverging lenses only produce virtual imagesvirtual images
Diverging LensesDiverging Lenses
The focal points are equidistant from the center of the lens. That distanceis the focal length of the lens.
F F 2 F2 F
Diverging Lens Reference Rays (p.569)Diverging Lens Reference Rays (p.569)
RayRay From object From object to lensto lens
From lens to From lens to imageimage
Parallel RayParallel Ray Parallel to Parallel to principal axisprincipal axis
Directed away Directed away from focal from focal point Fpoint F
Central RayCentral Ray To the center To the center of the lensof the lens
From the From the center of the center of the lenslens
Focal rayFocal ray Proceeding Proceeding toward back toward back focal point, Ffocal point, F
Parallel to Parallel to principal axisprincipal axis
Drawing the RaysDrawing the Rays
F F 2 F2 F
Ray 1
Ray 2
Ray 3
Sample Problem (p.579 Sample Problem (p.579 #1)#1) Using a ray diagram, find the Using a ray diagram, find the
position and height of an image position and height of an image produced by a viewfinder in a produced by a viewfinder in a camera with a focal length of 5.0 camera with a focal length of 5.0 cm if the object is 1.0 cm tall and cm if the object is 1.0 cm tall and 10.0 cm in front of the lens. A 10.0 cm in front of the lens. A camera viewfinder is a diverging camera viewfinder is a diverging lens.lens.
Ray DiagramRay Diagram
F F 2 F2 F
Object
The image is about 1/3 of the object height (.33 cm). The image location is about 3 or 4 cm from the lens. So, q is approximately -3.5 cm.
Smart people verify!Smart people verify!
103
101
51111
pfq
cmq 3.3
pq
hh
Mo
i
cmcmcmcm
pqh
h oi 33.
10)33.3(1)(