Lenses and the Formation of Images. Basic Lens Shapes Converging Lens -thickest in the middle...
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Transcript of Lenses and the Formation of Images. Basic Lens Shapes Converging Lens -thickest in the middle...
Lenses and the Formation of Images
Basic Lens ShapesConverging Lens-thickest in the middle (thinnest at edge)-causes parallel light rays to converge (come
together) through a single point after refraction
Diverging Lens-thinnest in the middle (thickest at edge)-causes incident parallel rays to spread apart
after refraction
• We will only study the incident ray coming into the lens and the ray leaving the lens
(in actuality there are two refractions in a lens; air to glass, glass to air)
Converging Lenses Words
2F’ 2F
Converging Lenses WordsOptical Centre (O)- Centre of the LensPrincipal Axis (PA)- The line through the optical centre that
is perpendicular to the central dashed line of the lensPrincipal Focus (F)-the point on the PA of a lens where light
rays parallel to the PA converge after refractionSecondary Principal Focus (F’)- the focus that is on the same
side of the lens relative to the incident rays2F- twice the distance from the principal focus2F’-twice the distance from the secondary principal focus
Diverging Lens Words
2F2F’
-if you project the diverging rays backward off of a diverging lens, it looks as if they come from a virtual focus, this point now becomes the principal focus
Secondary Principal Focus (F’)-now on the other side of the lens where the rays actually diverge
Converging vs. Diverging Lenses
Converging Lens Diverging Lens-brings parallel light rays together through a focus after refraction
-spreads parallel light rays apart after refraction so that it looks as though they have come from a virtual focus
-the principal focus is on the OPPOSITE side of the lens as the incident rays
-the principal focus is on the same side of the lens as the incident rays
What affects what an image will look like?
• The type of lens (converging or diverging)• The location of the object
With lenses, we will use ray diagrams to draw incident rays and refracted rays.
Emergent rays-the ray that leaves the lens, refracted as it goes from the lens back to the air
• In a thin prism, the emergent ray is almost unaffected by the presence of the prism
• We will work with thin prisms. Thick Prism Thin Prism
How to locate the image in a converging Lens
A ray parallel to the principal axis is refracted through the principal focus.
• A ray through the F’ is refracted parallel to the principal axis.
• A ray through O continues straight through without being refracted
• Where the two refracted lines cross, the image is produced.
Object beyond 2F’
• Smaller• Inverted• Between F and 2F• Real
Object at 2F’
• Same Size• Inverted• At 2F• Real
Object between F’ and 2F’
• Larger• Inverted• Beyond 2F• Real
Objects beyond 2F’
-the image is smaller than the object and between 2F and F
-as you move slowly toward the lens, the image gets larger and larger
-eventually the image and the object are the same size when the object is located at 2F’ the image would be at 2F
Object at F
• No clear image formed
Object between F’ and the Lens
• Larger• Upright• Behind the Lens• Virtual