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Chapter 28Refraction

Lecture 32

Midterm 2 – Monday Nov. 15Chapters 15-28 (Lectures 17-32)

Review Lecture on Friday

RefractionWhen light bends in going obliquely from one medium to another, we call this process refraction.

Refraction• Refraction occurs to

minimize the time taken by light to travel from A to B.

• Just as if you wanted to save someone from drowning, the quickest path would not be a straight line – it would be the dashed path shown.

RefractionLight follows a less inclined path in the glass.• Light travels slower in glass than in air, so it

minimizes the time it spends in the glass.

RefractionLight rays pass from air into water and water into air.• Pathways are reversible for both reflection and

refraction.

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Demo: Refraction thru BlockLight is refracted entering the block and

refracted back on leaving the block.

Refraction

Refractive index:Index of refraction, n, of a material• indicates how much the speed of light differs

from its speed in a vacuum.• indicates the extent of bending of rays.• ratio of speed of light in a vacuum to the speed

in a material.

RefractionRefractive index (continued):

• In equation form:

• Medium with a high index means high bending effect and greatest slowing of light.

• Some values of index of refraction n:– Air 1.0– Water 1.33– Glass 1.5 (depends on type of glass)– Diamond 2.4

n = speed of light in vacuum

speed of light in material

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Law of RefractionLight passing from one

material to another is refracted to an angle of refraction that depends on the refractive index of each material.

Angle is smaller in the material with higher refractive index (lower speed of light).

Refracted light that bends toward the normal is light that has

A. slowed down.B. sped up.C. nearly been absorbed.D. diffracted.

RefractionCHECK YOUR NEIGHBOR

Refracted light that bends toward the normal is light that has

A. slowed down.B. sped up.C. nearly been absorbed.D. diffracted.

RefractionCHECK YOUR ANSWER

Refracted light that bends away from the normal is light that has

A. slowed down.B. sped up.C. nearly been absorbed.D. diffracted.

RefractionCHECK YOUR NEIGHBOR

Refracted light that bends away from the normal is light that has

A. slowed down.B. sped up.C. nearly been absorbed.D. diffracted.

Explanation:This question is a consistency check with the question that asks about light bending toward the normal when slowing.

RefractionCHECK YOUR ANSWER

RefractionIllusions caused by refraction

• Objects submerged in water appear closer to the surface.

RefractionIllusions caused by refraction (continued)

• Objects such as the Sun seen through air are displaced because of atmospheric refraction.

Refraction

Illusions caused by refraction (continued)

• Atmospheric refraction is the cause of mirages.

Dispersion

Dispersion• Process of separation of light into colors arranged by

frequency

• Components of white light are dispersed in a prism (and in a diffraction grating).

Dispersion Example

When white light passes through a prism, green light is bent more than

A. blue light.B. violet light.C. red light.D. None of the above.

DispersionCHECK YOUR NEIGHBOR

When white light passes through a prism, green light is bent more than

A. blue light.B. violet light.C. red light.D. None of the above.

DispersionCHECK YOUR ANSWER

RainbowsRainbows are a result of dispersion by many drops.• Dispersion of light by a single drop

Rainbows• Sunlight incident on two sample raindrops, as shown,

emerges from them as dispersed light. • The observer sees the red light from the upper drop and

the violet light from the lower drop. • Millions of drops produce the whole spectrum of visible

light.

Rainbow from Water Drop Dispersion

LensesLenses• Two common types:

– Converging (convex) lens• thicker at the center than edges• converges light (brings light rays together)

– Diverging (concave) lens• thinner at the center than edges• diverges light

Lenses

• Image formation is a consequence of light traveling in straight lines.

• The first camera—the pinhole camera—illustrates this fact.

Lenses

A lens nicely bends the straight-line paths of light.

9-Nov-10 Physics 1 (Garcia) SJSU

Real Image of Convex Lens

Image formed by convex lens can be observed on a screen.

The action of lenses depends mainly on

A. reflection.B. refraction.C. Both A and B.D. Neither A nor B.

LensesCHECK YOUR NEIGHBOR

The action of lenses depends mainly on

A. reflection.B. refraction.C. Both A and B.D. Neither A nor B.

LensesCHECK YOUR ANSWER

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LensesCurvature of a lens surface produces a

continuous, variable angular refraction.

Concave lens shrinks its image

Convex lens magnifies its image

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Concave LensesCurved surface of a concave lens causes

light rays to diverge, shrinking images.

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Convex LensesCurved surface of a convex lens causes

light rays to converge, magnifying images

Eyeglasses (& Contact Lenses)• Farsightedness (poor near vision) corrected with

converging lens.– The higher the converging power of a lens, the

greater its ability to bring rays to a focus. – Optometrist prescribes the lens power needed to

correct vision to normal; e.g., Left, +2.0; Right +2.5• Nearsightedness (poor distant vision) corrected

with diverging lens.– Diverging lens has a negative power.– The greater the negative lens power, the more

divergence it adds to rays.

Key Points of Lecture 32Key Points of Lecture 32

Before Friday, read Hewitt Chap. 28 (second half).

Homework #23 due by 11:00 PM Friday Nov. 12

• Refraction• Index of Refraction• Law of Refraction• Optical Illusions due to Refraction• Dispersion & Rainbows• Lenses: Converging & Diverging