Optical Phenomena in Nature
How do Rainbows form?
Rainbows explained
• Refraction of sun light through water dropplets.
• The centre of the rainbow is directly opposite the sun
• The order of colours is always the same due to the different
angles of refraction. (red refracts the least and violet refracts
the most.
Notice anything special about double
rainbows?
A Secondary rainbow has the colours
reversed!!!
• Because the angle of sun light is “shallower” it
refracts differently in the water droplets
Why is the sky blue and clouds white?
• This is actually a hard one…
• Very small particles disperse light well (smaller than the
wavelengths of light (390-750 nm).
• These are gas particles of oxygen and nitrogen in the
atmosphere. Blue to violet are the dominant colours
which are dispersed but human eyes are more sensitive
to the blue wavelengths.
• Larger particles do not disperse light well (the larger
cloud water droplets are in this range) and therefore
reflect all wavelengths in the spectrum and light retains
its white appearance.
Apparent depth, an illusion…
• The refraction of light from
water to air makes an object
appear shallower than it
really is.
Shimmering…
Shimmering…
• Hot air and cold air have different optical
densities (index of refraction).
• When air is very hot (hot pavement, campfire,
above a bbq or desert) it moves around and
mixes with colder air constantly changing the
direction of light giving the ``wavy``
appearance of objects.
Why do mirrages happen?
A Mirrage
• Works on the same principle as shimmering.
• hot air is less dense than cold air and has a lower
index of refraction (light travels faster).
• When the air directly above the ground is much
hotter than the air above. Light is bent upwards
(away from the normal).
• This causes you to interpret light coming from the
sky or a distant object as something on the ground.
Sun/moon rise and set
• When the sun or moon is on the horizon the light must
pass through much more atmosphere before it reaches
you. Remember the blues and violets are being
diffracted by atmospheric gases and buy the time the
light reaches you all that is left are the red, yellow, and
orange wave lengths.
Sun/moon rise and set