Optics
Gabrielle DePetro
Amy Chang
Tiffany Chau
Introduction to Optics
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Optics- study of how light behaves
Speed of light- 3 x 10^8 m/s
Speed of sound- 340 m/s
Light is both a wave and a particle
– Wave: can undergo destructive interference,
refraction, diffraction
– Particle: can travel through a vacuum
Reflection
• Reflection- the return of a wave back to its original
medium
• Law of Reflection- states the angle of incidence and the
angle of reflection are equal to each other
• Angle of Incidence- between the incident ray and the
normal
• Angle of Reflection- between the reflected ray and the
normal
• Normal- imaginary line perpendicular to a surface
• Angle of Incidence = Angle of Reflection
Reflection and Mirrors
• Mirrors produce only virtual images
• Virtual image- appears to be in location where
light does not really reach
• Eye cannot tell difference between object and
virtual images
• Image distance is same behind mirror as
object distance is in front of it
• Image and Object are the same size
Refraction
• Refraction- bending of a wave as it crosses the
boundary between two media at an angle
• Direction of light rays change because of
refraction
• Causes of refraction:
– Changes in speed of light crossing from one
medium to another
– Variations in temperatures and densities of same
medium
Refraction and The Normal
• Depending on what medium they enter, light
rays bend towards or away from the normal
• If light enters a medium in which it travels
from:
– Increasing to Decreasing speed, then rays bend
TOWARD the normal
– Decreasing to Increasing speed, then rays bend
AWAY from the normal
Snell’s Law and Refraction
• Refraction- bending of light at the interface of
2 different materials
• Snell’s Law if the “equation” for refraction
• Snell’s Law: n(in)sinƟ in = n(out)sinƟ(out)
• n= index of refraction/property of the material
• n (air)= 1.0
• n (water)= 1.3
Critical Angle
• Critical angle- incident angle that causes the
refracted ray to skim right along the boundary of
a substance
• Produces an angle of reflection of 90°
• Only exists when light is attempting to penetrate
a medium of higher optical density than it is
currently traveling in
• If the angle of incidence exceeds the critical
angle, the ray is completely reflected and does
not enter the new medium
Total Internal Reflection
• Occurs when light attempts to pass from a
more opticslly dense medium to a less dense
medium at an angle greater than the critical
angle
• There is no refraction, only reflection
• Used for fiber optics
Lenses
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Lens- piece of glass or plastic that refracts light
Lenses change the path of light
A light ray bends as it enters and leaves glass
Lenses form an image by bending parallel rays of
light that pass through it
• Light passing through certain shapes increases or
decreases image size and distance compared to
the object being viewed
• Focal Length- distance between center of lens
and its focal point
Converging Lenses
• Converging = Convex
• Thicker in the middle
• Causes rays that are initially parallel to meet at a single
point
• Focal point- point at which beam of light parallel to the
principal axis converges
• Any incident parallel beam converges to a point on the
focal plane
• Lens has 2 focal points and 2 focal planes
• Key features- principal axis, focal point, focal plane
Diverging Lenses
• Diverging = Concave
• Thinner in the middle
• Rays of light appear to originate from a single
point
• Incident beam of light parallel to the principal
axis is diverged
• Focal point- point at which light rays appear to
originate from
Ray Diagram Rules
• Ray diagrams show the principal rays that can be used to
determine the size and location of an image
• An arrow represents the object and one end is on principal
axis
• To locate position of image, know the paths of 2 rays from a
point on the object
• 3 Rules:
– Ray parallel to principal axis passes through the focal point on
the opposite side
– Ray passing through the center of the lens that is undeflected
– Ray through the focal point in front of the lens, emerges parallel
to principal axis after refraction
Image Formation
• Type of image formed depends on the shape of
the lens and the position of the object
• Magnification- when the use of a lens allows an
image to be observed through a wider angle than
would be observed without the lens
• Magnifying Glass- converging lens that increases
the angle of view and allows more detail to be
seen
• Magnification Equation: -image size/object size
Images Formation
FOR CONVERGING
• Object is beyond focal point of converging lens:
– Real image, inverted
– Slightly beyond focal point image is far away
– Far from focal point  image is nearer
• Object is between focal point and lens:
– Virtual image, magnified, farther from the lens than the object, rightside up
FOR DIVERGING
• No matter how far or near the object is:
– Virtual, right-side up, smaller than the object
SUMMARY
• For all Real Images, image and object are on opposite sides of the lens
• For all Virtual Images, image and object are on same side of the lens
Multiple Choice Review
1. Convex lenses are…
A)Convergent
B)Divergent
C)Mirrors
D)Polarized
2. The image formed my concave lenses is always….
A)Real and Inverted
B)Real and Upright
C)Virtual and Inverted
D)Virtual and Upright
3. For an object with dₒ = 12m and focal length = -4m, what is the
distance of the image?
A) 0.1m
B) -3m
C) -30m
D) 10m
4. ________ is an optical device that works by reflection and _______
is an optical device that works by refraction.
A) Mirror, lens
B) Lens, mirror
C) Mirror, mirror
D) Lens, lens
5. A concave lens is _______ and _______.
A)Convergent, magnifies
B)Convergent, reduces
C)Divergent, magnifies
D)Divergent, reduces
6. Nearsightedness can be corrected by…
A)Convex lenses
B)Convex mirrors
C)Concave lenses
D)None of the above
7. Calculate the critical angle for light going from water into air
A) 4.87°
B) 48.75°
C) 487.53°
D) 90°
8. (In the image below) As the image is moved from the red arrow to
the green arrow it…
A) is closer and increases in size
B) is closer and decreases in size
C) is farther and increases in size
D) is farther and decreases in size
9. If an object is 8m from a lens and produces an image 4m from the
lens, what is the magnification factor, M?
A) -0.5
B) 0.5
C) -0.05
D) 0.05
10. The angle of reflection is 30°. What is the angle of incidence?
A) 30°
B) 60°
C) 90°
D) 150°
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Answers
• ANSWERS: 1) A 2) D 3) B 4) A 5) D 6) C 7) B 8)
C 9) A 10) A