Download 4.6 Lenses

p. 344 – 351
Try the Starting Point Activity on p. 345
What does a lens have to do with optics?
• similar to mirrors, but surface is not shiny (reflective)
• light passes through a lens, unlike a mirror
• light refracts instead of reflecting
• used in cameras, telescopes, microscopes, and
projectors
• also used to help people see clearly
• eyeglasses, contact lenses
• reading stones are early examples of lenses (p. 350)
What is a lens?
• A transparent object with at least one curved side that
causes light to refract
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this is why a window is not a lens
have plane (flat), concave and/or convex surfaces
have two sides
either side can be plane, concave, or convex (i.e. can have two
different types of sides)
• Many types, but we will focus on converging
and diverging lenses
Converging & Diverging lenses
• Converging lenses cause light rays to come together at
a common point
• have a convex shape
• Diverging lens = cause light rays to spread away from a
common point
• have a concave shape
Converging Lenses
• B ring parallel light rays toward a common point
• Have a convex shape
• Are thicker in the centre than on the edges
• When rays move from air to the glass lens, they move
from a fast medium to a slow medium
• rays refract toward the normal
• When the rays leave the lens on the other side, they
refract away from the normal, but this time the normal
is positioned so that this actually causes rays to
continue converging
Diverging Lenses
• Cause parellel light rays to spread apart from a
common point
• Have a concave shape
• Are thinner in the centre than on the edges
• We will focus on converging lenses in this class
Applications of Lenses
• Cameras are essentially a bunch of
different lenses stacked together
• Microscopes
• Projectors
• Telescopes
• Corrective eye glasses,
contacts, etc.
Optics terms review
• Principal axis, PA
• is a straight line that passes through the
centre of the lens, that intersects it at 90 (called a normal)
• when rays that are parallel to the PA pass through a converging lens, the
rays intersect the focal point (F)
• Vertex
• called the optical centre (O) in a lens
• Focal point, F
• where light rays come together when they pass through a lens
• because light can pass through both sides of a lens, lenses have two focal
points (principal focus = F & secondary focus = F’)
• Focal length, f
• distance from optical centre to the focal point
• the thicker the lens, the shorter its focal length
Drawing ray diagrams of lenses
• Light rays actually refract twice in a lens, but we
approximate by drawing a single refraction
• draw a vertical line perpendicular to the PA to approximate this on your
ray diagrams
• Images form in lenses in much the same way as in
curved mirrors
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L  closer to or farther from lens (compared to object)
O  upright or inverted
S  larger, smaller, or the same size as the object
T  virtual or real
• Images are considered virtual when the image forms on
the object side of the lens, and it cannot be projected onto
a screen
Setting up ray diagrams
• draw the lens, vertical line thru the lens, PA, F, F’, 2F, O
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principal focus (F) is on the object side of the lens
secondary focus (F’) is on the far side of the lens from the object
2F is sort of like the centre of curvature (C) in mirrors
O is the optical centre of the mirror, where the vertical line through
the lens & the PA intersect at 90 (like the vertex, V, in mirrors)
Refraction rules
1. Rays parallel to the PA refract through F’
2. Rays that pass through O travel straight through the
lens, without bending
3. Rays passing through F refract parallel to the PA
• the top of the image is where refracted rays intersect
• if refracted rays need to be extended on object side so they meet,
the image that forms is virtual
Setting up a ray diagram
Draw:
• vertical line through the centre of the lens (will act as
the refraction boundary)
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PA = principal axis
F = principal focus (on object side of lens)
F’ = secondary focus (on far side of lens from object)
2F and 2F’ = twice the focal length of the lens on both sides
O = optical centre
Steps to drawing ray diagrams
Draw a ray from the top of the object:
1. parallel to the PA, which refracts thru F’ (blue ray in textbook)
2. straight thru O, which doesn’t refract but continues straight (red
ray in textbook)
3. to the lens as though the ray passed thru F (the focal point on the
object side of the lens), which refracts parallel to PA (green ray in
textbook)
LOST every image that forms
Object beyond 2F
Object between 2F and F
Object on F
Object between F and the lens