Refraction & Refraction & LensesLenses

Chapter 18

Refraction of LightRefraction of LightLook at the surface of a

swimming poolObjects look distortedLight bends as it goes from

one medium into another\Why?

When a medium causes a wave to slow down it is more optically dense

Entering more Entering more optically denseoptically dense

Waves moving into a more optically dense medium will cause the wave speed to slow down and then bend toward the normal

Therefore, the angle of refraction is smaller than the angle of incidence

Entering less Entering less optically denseoptically dense

Waves moving into a less optically dense medium will cause the wave speed to increase and then bend away from the normal

Therefore, the angle of refraction is larger than the angle of incidence

Snell’s LawSnell’s LawDutch Scientist Willebrord Snell

discovered that a ray of light bends in such a way that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, index of refraction (n)

ni sin i nr sinr

ni index of refraction for the incident medium

i angle of incidence

nr index of refraction for the refracting medium

r angle of refraction

Index of refractionIndex of refraction ratio of speed of light in a

vacuum, c, to another medium, vn=c/v index of refraction in a vacuum is

1.00 table on page 486

If ray travels from a more optically dense into a less optically dense medium the angle of refraction is larger than the angle of incidence

Eventually the incidence angle is so great that all the light reflects back into the medium

No refraction takes place

Total Internal Total Internal ReflectionReflection

The incident angle that causes the refracted ray to lie right along the boundary of the substance

Unique to each substance

Critical AngleCritical Angle

Critical AngleCritical Angle

water

air critical ray

Calculate the critical angle

c

When light enters a thin glass rod light is internally reflected (fiber optics)

Telephone, computer and video signals

Explore the human body Plants use internal reflection

Total Internal Total Internal ReflectionReflection

Effects of Effects of RefractionRefractionMirages

Summertime - the sun hits to road and causes the air above the road to heat up

The index of refraction for warm air is 1.00026

The index of refraction for cool air is 1.00028

This small change in index of refraction causes the rays to bend

This bending makes the road look like there is a puddle on it

Because light travels slightly slower in Earth’s atmosphere than in outer space the sun rays bend causing the sun to reach us before the sun is actually above the horizon

Same in the evening, the rays bend and reach us after the sun has actually set

Dispersion of LightDispersion of LightLight of all wavelengths travels at

the same speed in a vacuumOther media causes the speed to

slow downThe wavelength also determines

the speed & index of refraction

In most materials red light travels fastest

It also has the smallest index of refraction

Violet is the slowest and has the largest index of refraction

Red is bent the least, violet the most

DispersionDispersionThe separation of light into a

spectrum by refractionDiamondRainbow (water)

18.2 Convex & Concave 18.2 Convex & Concave LensesLenses

1303 French physician wrote of using lenses to correct eyesight

1610 Galileo used two lenses to make a telescope

He then discovered the moons of Jupiter

Now; microscopes, cameras, limitless uses

Type of LensesType of Lenses

Lens-piece of transparent material used to focus light and form an image

Refractive index needs to be larger than air

Convex LensConvex Lens

Thicker at the center than at the edges Sometimes called a converging lens Why? What type of image will convex lenses

produce? Similar to concave mirrors

Concave LensConcave Lens

Thinner in the middle than at the edges Rays pass through the lens and spread

out Sometimes called a diverging lens What type of images are formed? Why? Similar to convex mirrors

Lens EquationsLens Equations

Same as the mirror equations Real images on the opposite side of the

lens and the image Real images di is positive Virtual images are always on the same

side of the lens and di is negative Negative magnification means image is

inverted and real

Real Images Formed by Real Images Formed by Convex LensConvex Lens

No center of curvature, just F and 2F F is the focal point and f is the focal length Object needs to be placed beyond the focal

point

Convex Lenses and Virtual Convex Lenses and Virtual ImagesImages

Object needs to be between the lens and the focal point

All rays diverge Virtual image on the same side of the

lens and larger in size

Concave LensConcave Lens

Rays will always diverge Always produce a virtual image Same side of lens Image will always be smaller

Ray DiagramsRay Diagrams

Ray 1: Passes parallel to the principal axis to center of lens then bends to pass through the focal point

Ray 2: Passes through the focal point to the center of the lens and bends parallel to principal axis

Ray 3: Straight through the center of the lens at the principal axis

18.3 Applications of Lenses18.3 Applications of LensesLens in the EyeLens in the Eye

Light travels through the cornea then the lens and then the image focuses on the retina

Retina works like the screen The cells in the retina absorb this light

and sends information about the image along the optic nerve to the brain

Focusing ImagesFocusing Images

Light is focused mostly by the air cornea boundary because this is the greatest difference in index of refractions

Lens does the fine focusing by accommodation (muscles around the lens contract, close up, or relax, far away)

changing the shape changes the focal length of the eye

NearsightednessNearsightedness

Myopia, can’t focus on far away objects Focal length too short so image forms in

front of the retina Use a diverging lens, concave lens, to

spread the rays out so that the image distance increases

FarsightednessFarsightedness

Hyperopia, can’t focus on close up objects The focal length of the eye is too long and

the image forms behind the retina Use a converging lens, convex lens, to

produce a virtual images farther from the eye the image then becomes the object for the eye

Also happens as a person gets older and the lens becomes more rigid, the muscles can’t change the shape of the lens