Reflection Reflection and and

RefractionRefraction

Light interacts with matter

• Interaction begins at surface and depends on– Smoothness of surface– Nature of the material– Angle of incidence

• Possible interactions– Reflection– Refraction – Absorption– Transmission

Transparent materials transmit light

Opaque materials do not allow transmission of light

Reflect, absorb or combination

ReflectionReflection• Most objects we see reflect light rather than

emit their own light.

Principle of Least TimePrinciple of Least Time

• Fermat's principle - light travels in straight lines and will take the path of least time

MIRROR

A B

Wrong PathTrue Path

Law of ReflectionLaw of Reflection

• “The angle of incidence equals the angle of reflection.”

• This is true for both flat mirrors and curved mirrors.

Reflection

• Angles measured with respect to the “surface normal”– Line perpendicular

to the surface

i = r

Law of ReflectionAngle of incidence = Angle of reflection

Image formation

• Real image– Can be viewed or

displayed at its location– Example - movie image

on a screen• Virtual image

– Appears to come from a location where it is not directly visible

– Examples: plane mirror, convex mirror, concave mirror

Types of ReflectionTypes of Reflection

• Specular Reflection - images seen on smooth surfaces (e.g. plane mirrors)

• Diffuse Reflection - diffuse light coming from a rough surface (cannot see a reflection of yourself)

RefractionRefraction• Refraction is the bending of light when it

passes from one transparent medium to another.

• This bending is caused by differences in the speed of light in the media.

RefractionRefraction

WATER

AIR

Normal LineFast

Slow

Light Beam

Refraction • Light crossing a boundary

surface and changing direction • Reason: change in light

propagation speed– Moving to a medium with a

slower propagation speed

• Light bends toward surface normal

– Moving to a medium with a faster propagation speed

• Light bends away from the normal

Magnitude of refraction depends on: •Angle that light strikes surface•Ratio of speed of light in the two transparent materials

Incident ray perpendicular to surface not refracted

Refraction ExamplesRefraction Examples

• Light slows down when it goes from air into water and bends toward the normal.

• An Analogy: A car slows down when it goes from pavement onto gravel and turns toward the normal.

• An Illusion : Fish in the water appear closer and nearer the surface. (See Figure 28.27)

RefractionRefraction

WATER

AIR

Observer

True Fish

False Fish

Atmospheric RefractionAtmospheric Refraction

• Our atmosphere can bend light and create distorted images called mirages.

Refraction• Mirages • Critical angle

– Light refracted parallel to surface

– No light passes through surface - “total internal reflection”

– Applications - fiber optics, gemstone brilliance

• Index of refraction– A measure of light

speed

Substance Index of refraction

Light speed

Air Approx. 1 ~c

Water 1.333 0.75c

Glass 1.5 0.67c

Diamond 2.4 0.42c

BE condensate

18,000,000 38 mph!

Total Internal Reflection...Total Internal Reflection...

• …is the total reflection of light traveling in a medium when it strikes a surface of a less dense medium

WATER

AIR

Light Source

Critical Angle

Total Internal Reflection

LensesLenses• Converging Lens

• A lens that is thicker in the middle and refracts parallel light rays passing through to a focus.

• Diverging Lens

• A lens that is thinner in the middle than at the edges, causing parallel light rays to diverge.

Optics

• The use of lenses to form images

• Concave lenses

– Diverging lenses

– Vision correction/in association with other lenses

• Convex lenses

– Converging lenses

– Most commonly used lens

– Magnifiers, cameras, eyeglasses, telescopes, …

Myopia (Near-Sightedness)

People with near-sightedness cannot see clearly at distance.

Hyperopia (Farsightedness)

People with far-sightedness cannot see clearly up close.

The human eye

• Uses convex lens with muscularly controlled curvature to change focal distance

• Nearsightedness (myopia) - images form in front of retina• Farsightedness (hyperopia) - images form behind retina• Correction - lenses (glasses, contacts) used to move images onto retina

• Total Internal Reflection occurs when the angle of incidence is greater than the critical angle.

• The critical angle for an air-water interface is 48°.

• Fish can see everything above the water’s surface in a 96° cone of vision.

Dispersion...Dispersion...• …is the separation of white light into pure colors (ROY G. BIV).

• Dispersion Examples: • Prisms• Diffraction Gratings• CD’s• Raindrops

Dispersion and colors

• White light

– Mixture of colors in sunlight– Separated with a prism

• Dispersion

– Index of refraction varies with wavelength

– Different wavelengths refract at different angles

– Violet refracted most (blue sky)

– Red refracted least (red sunsets)

– Example: rainbows• Wavelength/frequency related

RainbowsRainbows• Raindrops refract, reflect and disperse sunlight.

• Rainbows will always appear opposite of the Sun in the sky.

• You cannot run from or run to a rainbow!

Rainbows

The nature of lightWave-like behavior

Interference• Young’s two slit experiment

• Interference pattern - series of bright and dark zones

• Explanation - constructive and destructive interference

Wave-like behavior - polarization

• Alignment of electromagnetic fields

• Unpolarized light - mixture of randomly oriented fields

• Polarized light - electric fields oscillating on one direction

• Two filters - passage depends on alignment

• Reflection polarization

PolarizationPolarization

• The alignment of the electric vectors that make up electromagnetic radiation.

• See Figure 29.31.

PolarizationPolarization

• Light can be polarized by:

– (1) passing light though a polarizer material or

– (2) reflecting light off of a solid or liquid surface.

• For example, reflected light from a lake is mostly horizontally polarized.