Section 1 Refraction Section 2 Thin Lenses Section …...converging lens, a real image is formed...

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Refraction

Section 1

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Section 1 Refraction Section 2 Thin Lenses Section 3 Optical Phenomena

Refraction

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The student is expected to: TEKS

7D investigate behaviors of waves, including reflection, refraction, diffraction, interference, resonance, and the Doppler effect

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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.

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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

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How does it bend?

•   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.

Lower

edge

Upper

edge

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Click below to watch the Visual Concept.

Visual Concept

Wave Model of Refraction

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Ray Diagrams

•   Light rays reflect and refract. •   If the light slows down, it bends toward the normal line (θglass < θair).

–   Angles are measured with the normal line. •   Light rays are reversible.

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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.

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Indices of Refraction

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Snell’s Law

•   Angles must be measured with the normal.

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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

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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.

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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.

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Section 2



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

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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?

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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.

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Visual Concept

Converging and Diverging Lenses

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Ray Diagrams for Lenses

•   Complete the ray drawing to locate the image using the rules above.

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Visual Concept

Ray Tracing for a Converging Lens

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Images Created by Converging Lenses

•   Configurations 1 and 2:

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Diverging Lens Diagram •   Complete the

ray diagram for the lens shown to the left using the three rules from Table 2.

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Visual Concept

Ray Tracing for a Diverging Lens

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Thin-Lens Equations

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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.

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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

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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.

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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

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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

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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.

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Visual Concept

Compound Light Microscope

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Visual Concept

Refracting Telescope

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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?

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Section 3



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

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.

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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 (θc).

•   Used in fiber optics, right angle prisms, and diamond cutting.

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Critical Angle

•   θc 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.

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Total Internal Reflection

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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?

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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.

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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.

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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.

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Rainbows

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Visual Concept

Dispersion of Light

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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?

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Section 1 Refraction Section 2 Thin Lenses Section …...converging lens, a real image is formed...

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