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Home , Fluorescence, Incandescence, Incandescent light bulb

Physics 106: Physics of

Some Choice Slides (#2)

• Blackbody Radiation • Sources of light • Dispersion • Convex

Blackbody radiation

• Everything (including you!) emits radiation • Everything (including you!) absorbs radiation. • The of a perfect blackbody only depends on the . • It doesn’t really matter what the source is made of.

A black cavity • What does it mean to be totally absorbing? – All light that hits is absorbed • What does it look like? – Its comes only from its radiation – no reflection

Blackbody Radiation

• Something “glowing” from internal heat is a visible example of the blackbody effect…

Blackbody spectrum

• http://webphysics.davidson.edu/Applets/BlackBody/Bl ackBody.html

Color Temperature

A single number to characterize the spectrum of a light source EVEN WHEN IT’S NOT A BLACKBODY… i.e., this is the temperature a blackbody would have to be to glow in that color you see

Color Temperature 1200 K: a 2800 K: (100W household bulb), sunrise and sunset 3000 K: studio , photofloods, 5000 K: electronic , average daylight. A designation of D50 stands for "Daylight 5000K" and is the most common standard for professional light booths for photography, and graphic arts. 6000 K: bright midday 7000 K: lightly overcast sky 8000 K: hazy sky 10,000 K: heavily overcast sky Sources of Light

• • • • •

Incandescent Light bulb

●7% efficiency (other 93% goes to heat). ●Higher temp, tungsten filament will melt. ●Partial , filled with and , but eventually the filament evaporates.

Incandescent Light bulb

●7% efficiency (other 93% goes to heat). ●Higher temp, tungsten filament will melt. ●Partial vacuum, filled with argon and nitrogen, but eventually the filament evaporates.

Halogen bulb

●Operates at a higher temp, tungsten does melt off but is redeposited by gas prolonging life (iodine, bromine, mixed with inert gas argon or nitrogen) ●Still not that efficient; tends to operate at higher wattage for more lumens (“brighter”); ●Higher color temperature (light is bluer)

Halogen bulb

●Operates at a higher temp, tungsten does melt off but is redeposited by halogen gas prolonging life (iodine, bromine, mixed with inert gas argon or nitrogen) ●Still not that efficient; tends to operate at higher wattage for more lumens (“brighter”); ●Higher color temperature (light is bluer)

Incandescent bulb ~ “blackbody”

Fluorescent Lamps

• Using discharges to produce specific band of radiation (black light) in gas ( vapor) • Not a blackbody! Much less wasted heat! • Using (fluoresces) to absorb the UV radiation and re­emit it as visible light. ● A 40 Watt bulb fluorescent is ~4 times brighter than a 40 Watt incandescent bulb.

Fluorescent Lamps

• Using discharges to produce specific band of ultraviolet radiation (black light) in gas (mercury vapor) • Not a blackbody! Much less wasted heat! • Using phosphors (fluoresces) to absorb the UV radiation and re­emit it as visible light. ● A 40 Watt bulb fluorescent is ~4 times brighter than a 40 Watt incandescent bulb.

Fluorescent bulb NOT a “blackbody”

Fluorescent bulb NOT a “blackbody” (compare this spectrum to the simplified incandescent one)

Rainbows

• Because of the dispersion of water, droplets of water can break up the sun light into a spectrum ⇒ rainbow

Dispersion in a droplet

refraction reflection

refraction

42°

Confusion in a droplet – T, P, S • Wait! Why is on top and refraction red on bottom? reflection

refraction

42°

All rays from a rainbow come from different raindrops.

The Arc of a rainbow

Double Rainbows

refraction reflection

reflection refraction

Dispersing Prism • Careful! The blue light bends MORE TOWARDS THE NORMAL than the red as it goes into the (so θ gets smaller) and MORE AWAY FROM THE NORMAL as it leaves the glass back into the air (so θ gets bigger). • The geometry of the prism does the rest • A picture is worth all 49 of these words…

Since nblue > nred, blue is bent to “greater angle” Halos and Sun Dogs (Parhelia)

Note 22o halo

Sun dogs

Refraction in ice Again, picture (and some math) is worth a load of words.

False Sunrise

Rare Rainbows

Refraction Review rays bend toward the normal going from low index to high index (air to glass) rays bend away from normal going from high index to low index (glass to air) the angle of incidence where the angle of refraction = 90 deg. is the critical angle ­ above this everything is totally internally reflected

Refraction Review •The index of refraction depends on

• Different refract by different angles (usually similar but not the same) = dispersion

• Dispersion is responsible for the rainbow (not so much Noah?)

Parallel Light From “Faraway” (read: Infinity)

• All rays are nearly parallel. • Rays parallel to each other are imaged on the focal plane.

What kind of image?

Real image

object

More on (thin) lenses later…

Images • Real Images – Image is formed by intersecting rays that came from source; specifically, if your eyes trace rays straight back to the image, it’s real • Virtual Images – Image is formed by rays that appear to intersect at the image; if your eyes trace back rays which don’t go to the image, it’s virtual • The question is settled by the rays between the image and YOU.

(virtual) image

object

mirror

Plane Mirror Image

image

object

mirror • virtual image • upright (non­inverted) • same size as object (magnification =1) • same distance from mirror as object

Convex mirror

1 C ­ center F ­ focal point

1 Focal length f O F f = OF = 1/2 OC C 1

1 Ray Rule I ­ all rays parallel to the axis are reflected so that they appear to be coming from the focal point F.

Convex mirror

1 C ­ center 2,2 F ­ focal point

1 Focal length f O F f = OF = 1/2 OC C 1

2,2 1 Ray Rule II ­ all rays (when extended) aimed at C are reflected back on themselves

Convex mirror

3 C ­ center F ­ focal point

3 Focal length f O F f = OF = 1/2 OC C 3

3 Ray Rule III ­ all rays (when extended) aimed at F are reflected back parallel to the axis

This is just Rule I reversed

Finding the image

Q’ Q

P O

F C

­ draw ray from object point Q towards C ­ draw ray from Q parallel to axis; extend from mirror surface to F ­ where the extended rays intersect is Q’ ­Where does a line to F go (T,P,S)? Image from a convex mirror

­Magnification? M < 1 (demagnify!)

http://www.microscopy.fsu.edu/primer/java/mirrors/convex.html

Image from a convex mirror

­Where does an object have to be to have its image at “F”? ­How big is the image? (What is the magnification?http://www.microscopy.fsu.edu/primer/java/mirrors/convex.html

Properties of the image • It is a virtual image • It is an upright (or erect) image. • It is closer to the mirror than the real object • The image is smaller than the real object (M<1) • Applications: store and car mirrors – (allows to see wider angles)