Reflection and Reflection and RefractionRefraction

ReflectionReflection

The Law of The Law of ReflectionReflection

€

θi =θ rLaw of Reflection –Law of Reflection – the angle of the angle of

incidence and the angle of incidence and the angle of reflection are equalreflection are equal

The Law of The Law of ReflectionReflection

Reflection - Reflection - a wave bounces back a wave bounces back into the first medium when hitting a into the first medium when hitting a boundary of a second mediumboundary of a second medium

Our brain thinks light travels in Our brain thinks light travels in straight-lines.straight-lines.

Angle of Incidence –Angle of Incidence – angle made by the angle made by the incident ray and the normalincident ray and the normal

Angle of Reflection –Angle of Reflection – angle made by the angle made by the reflected ray and the normalreflected ray and the normal

Reflection of Sound Reflection of Sound

An echo is reflected soundAn echo is reflected sound

Sound reflects from all surfaces Sound reflects from all surfaces of a roomof a room

Acoustics is the study of the Acoustics is the study of the way sound reflects off of way sound reflects off of objects in a roomobjects in a room

Reverberations – Reverberations – Multiple Multiple reflections of sound within a reflections of sound within a roomroom

The walls of concert halls are The walls of concert halls are designed to make the reflection designed to make the reflection of sound diffuseof sound diffuse

Ex:Ex: Sitting in her parlor one night, Gerty sees Sitting in her parlor one night, Gerty sees the reflection of her cat, Whiskers, in the the reflection of her cat, Whiskers, in the living room window. If the image of Whiskers living room window. If the image of Whiskers makes an angle of 40° with the normal, at what makes an angle of 40° with the normal, at what angle does Gerty see him reflected?angle does Gerty see him reflected?

Reminder: Speed of Reminder: Speed of LightLight

What speed does light travel at in a What speed does light travel at in a vacuum? In air?vacuum? In air?

3 x 103 x 1088 m/sm/s

Does light speed up, slow down, or Does light speed up, slow down, or stay the same speed when it travels stay the same speed when it travels through different media?through different media?

Light travels Light travels fastestfastest in a in a vacuum and vacuum and slows downslows down in in water, glass, and other mediawater, glass, and other media

RefractionRefraction

Refraction:Refraction: Bending of Bending of waves as they change waves as they change speeds.speeds.

This change in direction This change in direction is caused by the fact that is caused by the fact that light travels at different light travels at different speeds in different media.speeds in different media.

The imaginary line The imaginary line (perpendicular to surface) (perpendicular to surface) to which we measure the to which we measure the angles is called the angles is called the normal line.normal line.

RefractionRefraction

Snell’s LawSnell’s Law

Relationship between the angle of Relationship between the angle of incidence (θincidence (θ11) and angle of refraction ) and angle of refraction (θ(θ22) can be explained by Snell’s Law.) can be explained by Snell’s Law.

Speed of light is greatest in a vacuum Speed of light is greatest in a vacuum (c = 3 x 10(c = 3 x 1088 m/s), so it is convenient m/s), so it is convenient to compare to this value. This ratio to compare to this value. This ratio is called the is called the index of refraction (n).index of refraction (n).€

sinθ1sinθ2

=v1v2

€

n =c

v=speed _of _ light _ in _vacuum

speed _of _ light _ in _medium

Snell’s Law and Snell’s Law and RefractionRefraction

Another way to write Snell’s Law:Another way to write Snell’s Law:

Crossing a boundary, the Crossing a boundary, the frequencyfrequency of a of a wave does not change (to conserve wave does not change (to conserve energy). θ, v, and λ do change.energy). θ, v, and λ do change.

Our brain thinks light travels in Our brain thinks light travels in straight lines. Ex: We see the fish straight lines. Ex: We see the fish shallow and far from the boat.shallow and far from the boat.

€

n1 sinθ1 = n2 sinθ2

RefractionRefraction

Ex: Ex: Hickory, a watchmaker, is interested in an Hickory, a watchmaker, is interested in an old timepiece that’s been brought in for a old timepiece that’s been brought in for a cleaning. If light travels at 1.90 x 10cleaning. If light travels at 1.90 x 1088 m/s in m/s in the crystal, what is the crystal’s index of the crystal, what is the crystal’s index of refraction? refraction?

Ex: Ex: While fishing out on the lake one summer While fishing out on the lake one summer afternoon, Amy spots a large trout just below afternoon, Amy spots a large trout just below the surface of the water at an angle of 60.0° to the surface of the water at an angle of 60.0° to the vertical, and she tries to scoop it out of the vertical, and she tries to scoop it out of the water with her net. a) Draw the fish where the water with her net. a) Draw the fish where Amy sees it. b) At what angle should Amy aim for Amy sees it. b) At what angle should Amy aim for the fish? (the fish? (nnwaterwater = 1.33). = 1.33).

Ex:Ex: Water has an index of refraction (n) of 1.33. Water has an index of refraction (n) of 1.33.

a)a) Find the speed of light in water.Find the speed of light in water.

b)b) Light from the tuna’s snout heads to the surface at Light from the tuna’s snout heads to the surface at 383800 and refracts to Carl’s eye. Find θ. and refracts to Carl’s eye. Find θ.

c)c) Extend Carl’s line of sight and sketch an imaginary Extend Carl’s line of sight and sketch an imaginary tuna at the location that he envisions it.tuna at the location that he envisions it.

Mirrors and LensesMirrors and Lenses

LensesLenses

Lens –Lens – a piece of glass which bends a piece of glass which bends parallel rays so that they cross and parallel rays so that they cross and form an imageform an image

Lenses work because light bends as it Lenses work because light bends as it goes from the air into the glass. goes from the air into the glass. (THINK: Refraction)(THINK: Refraction)

Eye glasses and telescopes use lenses Eye glasses and telescopes use lenses to help us see better.to help us see better.

EyesEyes

Your lenses in your eyes change size all the Your lenses in your eyes change size all the time.time.

When you look at objects real close up, the When you look at objects real close up, the lens gets thicker. If you look at objects far lens gets thicker. If you look at objects far away, it gets thinner. It does this to help away, it gets thinner. It does this to help you focus the correct image on the retina. you focus the correct image on the retina.

After light passes through the lens it shines After light passes through the lens it shines through the vitreous humor to the back of the through the vitreous humor to the back of the eye where it hits the retina. The retina eye where it hits the retina. The retina takes the light and changes it into nerve takes the light and changes it into nerve impules so the brain can understand what the impules so the brain can understand what the eye sees.eye sees.

In both the camera and the eye, the image is In both the camera and the eye, the image is upside down, our brain flips the image right-upside down, our brain flips the image right-side up for us.side up for us.

MirrorsMirrors

Your eye cannot tell the difference Your eye cannot tell the difference between an object and its virtual between an object and its virtual imageimage

Virtual Image –Virtual Image – the point located the point located behind a mirror where an object behind a mirror where an object appears to originateappears to originate

The image is as far behind a mirror as The image is as far behind a mirror as the object is in front of the mirrorthe object is in front of the mirror

Mirror ImageMirror Image