Chapter 14: Light and Reflection

14.1 Objectives

• Be able to discuss the historical developments and understanding of light.

• Know the speed of light.• Be able to explain how light waves are

produced and how they transfer energy.• Know the different forms of electromagnetic

radiation.

What is Light?

Greek philosophers discussed thestructure of light.

Pythagoras: particles

Empedocles: waves

Euclid: beams

Particle or Wave?Isaac Newton—particles

Christian Huygens—waves

Thomas Young(1801): “proved” waves.

Albert Einstein (1905):“proved” particles!

Light has wave-like and particle-like characteristics.

The Speed of LightGalileo: light is too fast to measure!

Olaus Roemer (1676): light from Jupiter takes 16 minutes to cross earth’s orbit.

Albert Michelson (1880)• c = 3.00 x 108 m/s • c: celeritas (Latin for swiftness)• exactly 299,792,458 m/s

Electromagnetic Waves

• James Maxwell: how fast do EM waves travel?(see it)

• James Maxwell: light is an EM wave!• made by vibrating electric charges EM cause

electric charges to vibrate, transferring energy• require no medium, travel at c • c = f

Electromagnetic Spectrum

14.2 Objectives (continued)

• Be able to make calculations related to the intensity of light.

• Understand the difference between wattage and brightness.

Light Intensity

24 r

PI

light power in lumens (lm)

intensity in lux (lx)

but same lumens.

More watts…

Fewerwatts…

• Understand the concept of reflection• Understand how curved mirrors form images.• Draw ray diagrams.

14.2/14.3 Objectives

Reflection

• reflection: a wave bouncing off a surface• law of reflection: i = r

• virtual image: reflect rays diverge; extend to a point behind the mirror

• real image: rays meet in space; can project on a screen• concave mirrors focus or converge light so they can

form real images.• convex mirrors diverge light rays so they only form

virtual images.

Curved Mirrors

CONCAVE CONVEX

Ray diagrams can be used to determine…•type of image•location of image•orientation of image•size of imageDraw ray diagrams for…•Object beyond C, concave lens•Object inside F, concave lens•Object in front of convex lens

Ray Diagrams

14.2/14.3 Objectives (continued)

• Be able to use the Gauss equation as it applies to mirrors.

• Be able to use the magnification equation as it applies to mirrors.

1 1 1

f d d

mh

h

d

d

o i

i

o

i

o

A 7.5 cm tall candle is placed 22.4 cm from a convex mirror with a 15.0 cm focal length. Calculate the image distance, the magnification, and the image height. Describe the image.

(+) di = real (-) di = virtual(+) f = converge (-) f = diverge(+) hi = upright (-) hi = inverted

Mirror Math

14.4 Objectives

• Understand how our eyes see color through the process of color addition.

• Understand how pigments absorb colors through the process of color subtraction.

Colorwhite light = all colors (Newton)400 nm — 700nmblack = no color

Color addition occurs in the eyes/brain.Color is a human perception.Cone cells respond to blue, green, and red light (RGB).

Color Addition

B + G = C (cyan)G + R = Y (yellow)R + B = M (magenta) B + G + R = W (white)

Complementary Colorscomplementary colors: any two colors that add to make white.red + cyan = white R + (G + B) = white

Retinal fatigue flag

Color Subtraction

pigments absorb certain colors (convert to heat) and reflect othersThe subtractive primary colors are CYM.

C + Y = (W – R – B) = G

C = B + G = (W − R)Y = G + R = (W − B)M = R + B = (W − G)

CMYK (K is for blacK)

Printers also use a black pigment.

Objectives

• Be able to explain why materials are transparent or opaque to various frequencies of EM.

• Understand why the sky is blue, why the sun is yellow, why sunsets are red, and why the ocean is blue-green.

• Understand how polarizers work.• Understand the concept of a “red shift” or

“blue shift.”

• opaque: dissipative absorption; solid or liquid particles resonate with EM, energy is converted into KE (heat) • scattering: gas particles resonate with EM, but EM is emitted in random directions (less converted to KE)• transmission (transparent) or reflection: particles don’t (or barely) resonate, EM quickly re-emitted• net speed of light in materials < c

Light and Resonance

Blue Skies and Red SunsetsRayleigh scattering: violet and blue scattered by N2 and O2

The sun appears Y because R and G scatter less.

white light

At sunset, sunlight travels through more atmosphere; more green scatters, leaving a red sunset.

Blue-Green Oceans

Water absorbs IR and some red.Water reflects B and G, appearing greenish-blue.

Polarization

unpolarized light: oriented randomly some surfaces / materials polarize (align) the waves

Red Shift (Doppler Effect for Light)

Edwin Hubble:the red shift of galaxies increasewith distance(expanding universe)