Light Touch

Sugar WaterMirage

The thin blanket of air that envelopes the Earth is rarefied at high altitudes blue and densifies with proximity to the surface. Thus the refractive index of the atmosphere increases in a continuous manner from unity in the vacuum of space to approximately n=1.000293 at sea level (h=0.5893 prn). Everywhere the gradient of the refractive index points toward the Earth's center, and so light passing from the sun to an observer on Earth will take straight or curved paths F~QURE1. (a) AN OBSERVER CAN SEE depending on whether the light DIRECT SUNLIGHT (SOLIDLINE) EVEN travels parallel or perpendicular to WHEN A STRAIGHT RAY PATH (DASHED the index gradient. At high noon, the LINE) IS BLOCKED; (b) DURING A LUNAR direct sunlight observed from the ECLIPSE, SOME LIGHT IS LENSED ground travels through the atmo- AROUND THE EARTHAND DIMLY . sphere along a straight trajectory. ILLUMINATES THE MOON; sunset has taken a curved path, WHEN THE SUN IS LOW IN THE SKY. bending toward the hgh refractive blGHT FROM POINT A IS BENT SO AS TO index layers near the ground. APPEAR TO BE COMING FROM POINT B. The curved ray trajectory at sunrise or sunset leads to a variety of sight is the flattening of the sun's interesting effects. For example, we disk at sunrise or sunset. Again, the can see direct sunlight when the sun appearance is the result of curved ray is behind the Earth in the moments paths along the Earth's atmosphere before sunrise and after sunset (see (see Fig. lc). Fig. la),During a lunar eclipse, the All of these effects are mirages moon is in the Earth's shadow, but because they arise from inhomogene- some direct sunlight reaches the ities in the atmosphere's refractive FIGURE2. (a) rMFERlOR MIRAGE: moon anyway. The reason is that index. Mirages can also arise from LIGHT BENDS AWAY FROM THE some sunlight is bent around the local index inhomogeneities near the LOW REFRACTIVE LAYER NEAR Earth; in other words, the gradient- Earth's surface. Hot asphalt warms THE WARM ROAD SURFACE AND index atmosphere acts as a weak lens. the air near a road surface and the ENTERS THE EYE AT AN ANGLE Most blue light is scattered in the refractive index decreases locally as SO AS TO APPEAR TO HAVE atmosphere, but some red light the air becomes rarefied. Light ORIGINATED FROM BELOW THE makes it through to pass through to passing through this region will bend ROAD SURFACE; (b) SUPERIOR MIRAGE:LIGHT BENDS TOWARD the moon, and from there is scattered away from the road to the cooler air +WE COOL AIR OVER THE SEA SO back to our eyes on Earth. Knowing above (see Fig. 2a). This is commonly THE EYE SEES THE IMAGE the moon's approximate position in seen on a still day after the sun has LOOMING IN THE: SKY. the sky, the dark adapted eye can warmed a long stretch of road. There find a very dim, dull red moon after seem to be wet patches on the the moon becomes fully eclipsed (see pavement; these arise from deflected Fig. Ib). Perhaps a more familiar skylight and are called "Inferior Mirage" image$ "Superior Mirage" images are usually seen over a cool days. Fill it half-way with tap water. ocean on a warm, still day. The result Carefully pour one or two 16 oz. is inverted images that loom over the bottles of Karo syrup or 1-2 lbs. of SUSAN HOUDE-WALTER isAssistant horizon (See Fig. 2b). granulated sugar into the tank, taking Professor of Optics a f the University of Mirage effects can be created care so as to disturb the water as little Rochester, GREG PIERCEis Senior with household materials. Place a as possible. Being heavier than water, Technical Associafefor the Institute of clean fish tank in a quiet spot where the sweetener will settle at the Optics, University of Rochester. it won't get jostled for a couple of bottom of the tank. However, the

OPTICS & PHOTONICS NEWS/JULY 19 TOPICAL MEETING

JANUARY 18-21, 1993 SALT LAKE CITY, UTAH

Sponsored by Optical Society of America

-- ABSTRAC11" DEADLINE: October 2, 1992

I SCOPE I A topical meeting on high resolution spectroscopy will provide a forum to bring together scientists from a wide range of disciplines and research en- vironments. Talks and discussions will range from technical details of ex- perimental approaches to funda- mental scientific problems, including: experiments techniques, approaches to data analysis, applications of high resolution spectroscopy, and current topics of research.

The topics to be considered at the conference will include (but will not be limited to) those listed below: Electronic Spectroscopy m Surface Science Atomic Spectroscopy la Infrared and Rarnan FlGURE 3. (a) COLLIMATED BEAM FROM A DIODE LASER BENDS INTO HIGH REPRAG- Spectroscopy TlWE lNDEX REGION OF SUGAR WATER IN FISH TANK. A SMALL FRACTION OF THE BEAM 1s SENT THROUGH THE HOMOGENEOUS TANK WALL AS A REFERENCE FOR THE rr Ti me-Resolved Spectroscopy EYE; (b) SEVEN '*DDTS'"AS W~EWEDTHROUGH A FISHTANK FILLED WITH KARD SYRUP I I m Lineshape Analysis AND WATER. THE CIRCULAR DOTS APPEAR TO FLATTEN WHEN VIEWED THRQU6H THE rr Fourier Transform Techniques GRADLENT-INDEX REGION, MUCH LIKE THE SUN'S D1SC FLATTENING NEAR THE I i i i HORIZON. r Far Infrared Spectroscopy m Energy Transfer Processes sweeteners are water-soluble and the using a laser pointer or some other dissolved sugar water will slowly laser beam (see Fig. 3a). Viewing m Intramolecular Vibrational Energy diffuse upward. A gradient-index objects through the tank gives rise to Redistribution region near the bottom of the tank distortions similar to those seen at r Remctte Sensing will be apparent after 8-24 hours. sunrise and sunset (see Fig. 3b). A pencil beam pointed parallel to the index gradient travels in a REFERENCES -- . For further information contact: straight line from the top to the 1. M. Minneart, The Nature of Light and Color in Opical 90eiey of America 2010 Massachusetts Ave., NW bottom of the tank. A pencil beam fhe Open Air, Dover, 1954. Washington, DC 20036-1023 pointed perpendicular to the index (202) 223-0920 gradient will bend toward the high Fax: (202) 416-6100 index region, in this case to the bottom of the tank. This is easily seen

OPTICS & PHOTONICS NEWS/JULY 1992