colour white light is not a single colour; it is made up of a mixture of the seven colours of the...

ColourColour

White light is not a single colour; it is made up of a mixture of the seven colours of the rainbow.

We can demonstrate this by splitting white light with a prism:

This is how rainbows are formed: sunlight is “split up” by raindrops.

Adding coloursAdding coloursWhite light can be split up to make separate

colours. These colours can be added together again.

The primary colours of light are red, blue and green:Adding blue and

red makes magenta (purple)

Adding blue and green makes cyan

(light blue)

Adding all three makes white again

Adding red and green makes yellow

Seeing colourSeeing colourThe colour an object appears depends on the

colours of light it reflects.

For example, a red book only reflects red light:

White

light

Only red light is

reflected

A white hat would reflect all seven colours:

A pair of purple trousers would reflect purple light (and red and blue, as purple is made up of red and

blue):

Purple light

White

light

Using coloured lightUsing coloured light

If we look at a coloured object in coloured light we see something different. For example, consider a football kit:

White

light

Shorts look blue

Shirt looks red

In different colours of light this kit would look different:

Red

lightShirt looks red

Shorts look black

Blue

light

Shirt looks black

Shorts look blue

Some further examples:

Object Colour of lightColour object seems to be

Red socks

Red Red

Blue Black

Green Black

Blue teddy

Red Black

Blue

Green

Green camel

Red

Blue

Green

Magenta book

Red

Blue

Green

Using filtersUsing filtersFilters can be used to “block” out different colours of

light:

Red Filte

r

Magenta

Filter

Investigating filtersInvestigating filters

Colour of filter Colours that could be “seen”

Red

Green

Blue

Cyan

Magenta

Yellow

Red

Magenta

White

Yellow

Blue Green

Cyan

RefractionRefraction

Refraction is when waves ____ __ or slow down due to travelling in a different _________. A medium is something that waves will travel through. When a pen is placed in water it looks like this:

In this case the light rays are slowed down by the water and are _____, causing the pen to look odd. The two mediums in this example are ______ and _______.

Words – speed up, water, air, bent

Dispersion – different colors refract by different amounts

Ex.: glass prism bends blue blue travels the slowest

Diagram of the eye

• Cornea- the protective cover over the the eye

• The pupil is the hole that lets the light in

• Iris- this is the color of your eye and expands or contracts to let more or less light in

• Retina- this is where the image is projected on to the eye.

• Fovea- is make up of rods and cones to send the image to your brain

Optical eye illusion

• Concentrate on the four dots in the middle of the picture for about 30 seconds.

• Close your eyes, and tilt your head back.

• Keep them closed... you will see a circle of light.

• Continue looking at the circle... what do you see?

Explanation…

• Because this grid is covered with black with white dots in it-your brain is confused and it sees the grid as essentially all black

• This is why it tries to add black color to the white spots-and this is why the dots dance when your not looking at them

Why is the sky blue?

Notes: March 8th 2001

Something to ponder…?

• When you look at the sky at night it is black, with the stars and the moon forming points of light on that black background. So why is it that, during the day, the sky does not remain black with the sun acting as another point of light? Why does the daytime sky turn a bright blue and the stars disappear?

Light scattering• 1. Sun is an extremely

bright source of light.

• 2. We have an atmosphere. Small atoms of oxygen and nitrogen in the atmosphere effect sunlight.

• Think of ringing bells!

Rayleigh scattering

• Physical phenomenon that causes light to scatter when it passes through particles.

• Tiny particles bend high frequency light. (oxygen, nitrogen)

• Large particles bend low frequency light. (methane, sulfur)

Blue sky… why not violet?• This scattering of the higher frequencies of light

illuminates the skies with light on the BIV end of the visible spectrum.

• Violet is scattered most easily, so why isn’t the sky violet?

Sensitivity • Our eyes are more sensitive to light

with blue frequencies.

• Three primary color cones in our eye: Blue, green, and red.

Why is the sun yellow?

• The lower frequencies of sunlight (ROY)

tend to reach our eyes unscattered as we look directly at the

midday sun. Although sunlight is a combo of

all colors, yellow is most intense.

Appearance of sun• Appearance of the sun changes with the time of

day. • As the path which the light takes through the

atmosphere increases ROYGBIV encounters more atmospheric particles, and scatters more light.

Sunset• During sunset hours, the light passing through

our atmosphere to our eyes tends to be concentrated with red and orange frequencies. This is why sunsets have a reddish-orange hue.

What effects a sunset?• The effect of a red sunset becomes more

pronounced if the atmosphere contains more and more sulfur aerosols (industrial pollutant)

Blue haze• Some mountain ranges are famous for their blue

hazes which result from aerosols from the vegetation reacting with ozone to form small particles (200 nm) which scatter blue light.

Why are clouds white?• Clouds appear white

because they consist of water droplets that are many sizes and scatter all wavelengths equally.

• (Mie scattering)

Why is sea water greenish blue?• Water absorbs

infrared waves (heats water)

• Absorption of red light by water.

• What is left is the complement of red, which is cyan. (Greenish blue)

Why are sea creatures red?• In deep sea water, since

there is no red light coming in to be reflected, the lobsters and other sea creatures look black, and through evolution have avoided being eaten by their predators.

In Summary• The sky is blue because blue from sunlight is

scattered in all directions by molecules in the atmosphere.

• Sunsets are red because all the other color frequencies are filtered out.

• Water looks greenish blue because water absorbs red light.

• The color of things depend on what colors are reflected or absorbed by molecules.

The Electromagnetic

Spectrumc = f

c = speed of light

f = frequency

= wavelength

Luminescence = "light emission not due to heating something”

1. Fluorescence

2. Phosphorescence

“delayed fluorescence”

Fluorescence

• Object only glows if shine some light on it

• The light shining on it may be “invisible”!

• The light glowing may also be “invisible”

• But the light glowing will always have a lower energy (i.e. lower frequency, longer wave)

• Ex: Shine sunlight (contains uv rays) on your clothes• fluorescent detergent emits white light

Fluorescence doesn’t last!

• Incoming light excites electrons to a higher energy level, but then they quickly fall back down and release their energy as light

Fluorescence used in forensics, drug research, basic chemistry

Fly parts

Semen on arm minerals

Blacklights : electron energy in, out is uv light

Phosphorescence keeps glowing!

• Incoming light excites electrons to a higher energy level, but then electron falls only partly to a middle metastable level and more slowly release extra energy as light

Phosphorescence lasts in the dark!