Download No Slide Title

Refraction
Preview
Section 1 Refraction
Section 2 Thin Lenses
Section 3 Optical Phenomena
© Houghton Mifflin Harcourt Publishing Company
Section 1
Refraction
TEKS
Section 1
The student is expected to:
7D investigate behaviors of waves, including
reflection, refraction, diffraction, interference,
resonance, and the Doppler effect
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 1
What do you think?
• Suppose you are reaching for swim goggles
floating below the surface of a pool or trying to
net a fish while out in a lake. Would you reach at
the point where you see the object, or above it,
or below it?
– Describe personal experiences that helped you
answer this question.
– Make a sketch showing how you think light behaves
when leaving the goggles, passing into the air, and
then entering your eyes.
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 1
Refraction
• Why does the lawnmower turn when it strikes the grass?
– The right wheel slows down before the left one.
– Light waves behave in the same way.
• Refraction is the bending (change in direction) of light
when it travels from one medium into another.
– Caused by a change in speed
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 1
How does it bend?
Upper
Lower
edge
edge
• Wave fronts (dashed lines) slow down when entering
glass.
– The lower edge slows before the upper edge, so the wave turns
to the right.
– Also, the wavelength is shortened.
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 1
Wave Model of Refraction
Click below to watch the Visual Concept.
Visual Concept
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 1
Ray Diagrams
• Light rays reflect and refract.
• If the light slows down, it bends toward the normal line (glass < air).
– Angles are measured with the normal line.
• Light rays are reversible.
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 1
Law of Refraction
• c = 3  108 m/s
• v is always less than c, so n >1 for all media.
– nair = 1.000293
• n is dimensionless.
• n is a measure of the optical density of a material.
© Houghton Mifflin Harcourt Publishing Company
Refraction
Indices of Refraction
© Houghton Mifflin Harcourt Publishing Company
Section 1
Refraction
Section 1
Snell’s Law
• Angles must be
measured with the
normal.
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 1
Classroom Practice Problems
• Find the angle of refraction of a light ray (589 nm)
entering diamond from water at an angle of 30.00° with
the normal.
– Answer: 15.99°
• A light ray (589 nm) traveling through air strikes an
unknown substance at 60.00° and forms an angle of
41.42° with the normal inside. What material is it?
– Answer: n = 1.309, so the material is ice
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 1
Refraction
• Where does the cat see the fish?
• Where does the fish see the cat?
• Objects appear to be in line with the observed rays.
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 1
Now what do you think?
• Suppose you are reaching for swim goggles
floating below the surface of a pool. Would you
reach at the point where you see the object, or
above it, or below it?
– Make a sketch showing how light behaves.
• If you are under water looking at a person
standing on the side of the pool, where is the
image?
– Make a sketch showing how light behaves.
© Houghton Mifflin Harcourt Publishing Company
Refraction
TEKS
Section 2
The student is expected to:
7D investigate behaviors of waves, including
reflection, refraction, diffraction, interference,
resonance, and the Doppler effect
7E describe and predict image formation
as a consequence of reflection from a
plane mirror and refraction through a thin
convex lens
7F describe the role of wave characteristics
and behaviors in medical and industrial
applications
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 2
What do you think?
• How will the light bend as
it enters and leaves the
three glass blocks?
• Draw the rays as they
change direction. Make sure
your drawing includes
normal lines at each
interface.
• Would you describe the
combination of blocks as
converging or diverging with
respect to the incoming
light?
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 2
Lenses
• A lens is a transparent object that
converges or diverges light by
refraction.
– A converging lens is thicker at the middle.
– A diverging lens is thinner at the middle.
• Light actually bends at each surface.
However, for thin lenses, we can show
light bending only once at the center of
the lens.
• Focal length (f) is the distance from the
focal point (F) to the center of the lens.
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 2
Converging and Diverging Lenses
Click below to watch the Visual Concept.
Visual Concept
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 2
Ray Diagrams for Lenses
• Complete the
ray drawing
to locate the
image using
the rules
above.
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 2
Ray Tracing for a Converging Lens
Click below to watch the Visual Concept.
Visual Concept
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 2
Images Created by Converging Lenses
• Configurations 1 and 2:
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 2
Images Created by Converging Lenses
• Configurations 3 and 4:
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 2
Images Created by Converging Lenses
• Configurations 5 and 6:
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 2
Diverging Lens Diagram
• Complete the
ray diagram
for the lens
shown to the
left using the
three rules
from Table 2.
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 2
Ray Tracing for a Diverging Lens
Click below to watch the Visual Concept.
Visual Concept
© Houghton Mifflin Harcourt Publishing Company
Refraction
Thin-Lens Equations
© Houghton Mifflin Harcourt Publishing Company
Section 2
Refraction
Section 2
Sign Conventions
• p is positive if the object is in front of the lens.
• q is positive if the image is behind the lens (real
and inverted).
• q is negative if the image is in front of the lens
(virtual and upright).
• f is positive for converging lenses and negative
for diverging lenses.
• h and h’ are positive if upright and negative if
inverted.
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 2
Classroom Practice Problems
• When an object is placed 3.00 cm in front of a
converging lens, a real image is formed 6.00 cm
in back of the lens. Find the focal distance of the
lens.
– Answer: 2.00 cm
• Where would you place an object in order to
produce a virtual image 15.0 cm in front of a
converging lens with a focal length of 10.0 cm?
How about a diverging lens with the same focal
length?
– Answers: 6.00 cm, -30.0 cm
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 2
The Eye and Corrective Lenses
• Light is refracted at both the cornea (outer
surface) and the lens.
– When functioning properly, the converging lens can
adjust so that the image is focused on the retina.
• Muscles adjust the thickness of the lens.
• Many people are nearsighted (myopia) and can’t
see distant objects clearly.
• Older people are often farsighted (hyperopia)
and can’t see nearby objects.
– The lens becomes inflexible with age and can’t be
made thicker to focus on nearby objects.
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 2
Nearsightedness
• The image forms in front
of the retina, possibly
because the retina is too
long.
• What type of lens is
needed in front of the eye
to correct the problem,
converging or diverging?
Explain your reasoning.
– Answer: a diverging lens
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 2
Farsightedness
• The image forms behind
the retina, possibly
because the lens is
inflexible.
• What type of lens is
needed in front of the eye
to correct the problem,
converging or diverging?
Explain your reasoning.
– Answer: a converging lens
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 2
Combinations of Lenses
• Microscopes and refracting telescopes use two lenses.
– The objective lens forms a real image that is located inside the
focal point of the eyepiece.
– The eyepiece magnifies the first image, creating a large virtual
image.
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 2
Compound Light Microscope
Click below to watch the Visual Concept.
Visual Concept
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 2
Refracting Telescope
Click below to watch the Visual Concept.
Visual Concept
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 2
Now what do you think?
• How will the light bend as it enters and
leaves the three glass blocks?
• Draw the rays.
• How is this similar to a lens?
• Which type of lens?
• How would the rays exit the
three blocks if there were six equally
spaced rays instead of three?
• How would those same six rays exit a
converging lens?
© Houghton Mifflin Harcourt Publishing Company
Refraction
TEKS
Section 3
The student is expected to:
7D investigate behaviors of waves, including
reflection, refraction, diffraction, interference,
resonance, and the Doppler effect
7E describe and predict image formation
as a consequence of reflection from a
plane mirror and refraction through a thin
convex lens
7F describe the role of wave characteristics
and behaviors in medical and industrial
applications
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 3
What do you think?
• Suppose a beam of light entering a tank of water
strikes at a 60.00° angle with the normal. What
angle does it make with the normal after
entering the water? Sketch it.
• Suppose a beam of light emerging from beneath
the water surface strikes at a 60.00° angle with
the normal. What angle does it make with the
normal after entering the air? Sketch it.
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 3
Total Internal Reflection
• Total internal reflection occurs
if the angle in the denser
medium is too great.
– Light can’t emerge so it is
reflected back internally.
– Occurs if the angle is greater than
the critical angle (c).
• Used in fiber optics, right angle
prisms, and diamond cutting.
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 3
Critical Angle
• c occurs when the angle in the less
dense medium is 90°.
– At the critical angle, the emerging ray
travels along the surface.
– At greater angles, the rays are totally
internally reflected.
© Houghton Mifflin Harcourt Publishing Company
Refraction
Total Internal Reflection
Click below to watch the Visual Concept.
© Houghton Mifflin Harcourt Publishing Company
Section 3
Refraction
Section 3
Classroom Practice Problems
• Find the critical angle for light emerging from a
diamond into air. The index of refraction for
diamond is 2.419. Repeat for cubic zirconium
with n = 2.200.
– Answers: 24.42° for diamond and 27.04° for cubic
zirconium
• Which material is more likely to trap light
entering the top surface in such a way that it
reflects many times internally before emerging?
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 3
Atmospheric Refraction
• Make a sketch like that above. On your drawing, show
how light will bend when it strikes the atmosphere.
– Remember that this is a very slight change in the index of
refraction, and it occurs gradually as the atmosphere becomes
denser.
– This bending allows us to see the sun before it rises and after it
sets.
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 3
Mirages
• Mirages are caused by the refraction of light as it strikes
the hot air near the earth’s surface.
– This phenomena can be observed when driving on blacktop
roads on hot summer days.
• Inverted cars can be seen approaching, with the actual cars up
above them.
© Houghton Mifflin Harcourt Publishing Company
Refraction
Dispersion
• Refraction or n depends on the
wavelength.
– Longer wavelengths refract less.
• Prisms disperse the light into a
spectrum.
• Chromatic aberration is a lens
problem where different colors
focus at different points.
– Can lead to imperfect images for
cameras with less expensive
lenses.
© Houghton Mifflin Harcourt Publishing Company
Section 3
Refraction
Rainbows
© Houghton Mifflin Harcourt Publishing Company
Section 3
Refraction
Section 3
Dispersion of Light
Click below to watch the Visual Concept.
Visual Concept
© Houghton Mifflin Harcourt Publishing Company
Refraction
Section 3
Now what do you think?
• How do fiber optic cables keep the light trapped
inside the cable as it travels great distances and
bends around corners?
• What phenomena is responsible for trapping the
light?
• Why do different people see different colors for a
water drop when observing a rainbow?
• What phenomena is responsible for the rainbow?
© Houghton Mifflin Harcourt Publishing Company