11.3Optical Phenomena in

Nature

• rainbows and how they are formed• the phenomenon known as apparent depth• shimmering and what causes it• mirages and what causes them

Today’s learning goals:

Rainbows

Rainbows§ rainbow – an arc of colours of the visible spectrum appearing

opposite the Sun, caused by reflection, refraction, and dispersion of the Sun’s rays as they pass through raindrops

Formation of a Rainbow§ a rainbow forms when sunlight enters a water droplet

and refracts off the inner surface of the droplet, and then refracts again when leaving the droplet

§ the two refractions result in dispersion of the light

Formation of a Rainbow§ when you see a rainbow, the colours that you see come

from different droplets at different heights in the sky

Formation of a Primary Rainbow§ because the violet light is “bent” the most, it appears to

always be on the bottom of the rainbow, while the red light, “bent” the least, always appears on top

Formation of a Secondary Rainbow

§ a secondary rainbow always appears:

§ higher in the sky

§ much fainter

§ reversed in colour

Formation of a Secondary Rainbow§ a secondary rainbow is caused when sunlight reflects

twice inside rain droplets

Why does a rainbow appear as an arch?

Apparent Depth§ an optical effect in which the image of an object appears

closer than the object

§ the chest at the bottom of the sea looks like it is higher and closer than it actually is

§ the bottom of the sea is deeper than it appears to be

Apparent Depth

Shimmering§ the apparent movement of objects seen through air that

is unevenly heated and moving

§ you can see shimmering in air above very hot surface

§ objects look wavy

§ caused by refraction of light in unevenly heated air (hot air is less dense than cooler air)

Mirages§ an optical effect caused by the bending of light rays

passing through layers of air that have extremely different temperatures

Mirages§ when the hot ground heats the air just above it, it makes

the lower layer of air much hotter than the higher air§ when sunlight reaches the hot air near the ground, the

sunlight is refracted upward§ your brain assumes that light travels in a straight line, so

you interpret the origin of the light as being on the ground

Homework§ Read p. 468-474 in the textbook.§ Complete Q. 2-5, 7 p. 475.§ Review Chapter 10 Quiz