Light and Color Chapters 16 & 19

Light and the Electromagnetic Spectrum

Visible Light is an electromagnetic wave that stimulates the retina of the eye. Its wavelengths are between 400 nm (violet) and 700 nm (red).

The Electromagnetic Spectrum is the full range of wavelengths and frequencies at which all electromagnetic radiation exists and the corresponding names that we give to certain “zones”.

How do scientists analyze light? A spectroscope

(containing a diffraction grating) is used to measure light from glowing elements and separate that light into its various frequencies. It is able to measure Doppler shift of the wavelengths of the light from elements.

What does a spectroscope do?

Helps chemists determine the elemental composition of heated gases or materials using the spectra they emit.

Astronomers can use spectra seen to determine what elements make up the stars and galaxies and to see if they are moving towards (blue- shifted) or away (red-shifted) from us.

Relative Speed of Light and the

Doppler Effect with Light

Doppler Effect with light determined based on relative speed

Relative speed – magnitude of the difference

between the velocities of the light source and the observer of the light

Blue-Shifted or Red-Shifted? Blue-shifted - When light

source approaches observer, there is an increase in measured frequency, so wavelength of light shortens. Called blue-shifted because the increase is towards the higher frequency (or blue) end of the color spectrum. (Ex: side of a star turning towards us as it spins)

Red, green and blue channels represent the red-shifted and blue-shifted motions of the ionised material in the halo. The positions of the two galaxies C11 and C15 are marked.

Blue-Shifted or Red-Shifted?

Red-shifted - When a light source recedes (goes away) from observer, there is a decrease in measured frequency, so wavelength lengthens. Called red-shifted because the decrease is towards the lower frequency (or red) end of the color spectrum (Ex: the side of a star spinning away from us; distant galaxies show a red shift as they move away)

Speed of Light

c = f l wavelength (m)

frequency (Hz)

speed of light

3.00 x 108 m/s

All EM radiation travels at c.

Light speed – History of Discovery

In a vacuum, light has a speed of 3.00 x 108m/s (symbol c), which is about 670 million mph!!!

Before 17th century, people believed light traveled instantaneously or that speed of light was too fast to be measured.

Roemer used the orbit of Jupiter’s moon, Io, and the orbit of Earth around the sun to first measure the speed of light in 1674.

Michelson, using a specially designed experiment sending light between two mountains, more closely estimated the speed of light in 1926. His measurement is almost the exact same that we use today!

Light…

has a dual personality - it can behave as

both a particle (advanced physics) and a wave.

can travel fastest in a vacuum! The speed of light through the other types of

mediums (solid, liquid, and gas) is dependent on the density of that medium. More dense substances slow down light more than less dense. (Opposite of sound!)

Speed of Sound vs. Speed of Light

Light travels almost a million times faster than sound.

Light travels around the earth 7.5 times per second.

The speed of light Speed of light in a vacuum is equal to a

constant c, which equals 3.00 x 108 m/s.

Rearrange the speed of light formula to find wavelength of a light wave by dividing c by frequency of that light.

We can also rearrange the formula to find frequency, if we know the wavelength and speed.

Remember a nanometer is 10-9 m.

/c fl

c fl

Absorbing and Emitting Light

White paper absorbs and then emits photons of all colors of light.

Black ink absorbs the photons of all colors and emits none.

Transparent - allow most light to pass through them

Translucent - allow some light to pass through, but some is absorbed and some is reflected

Opaque - do not allow any light to pass through, but instead absorb or reflect all light.

Absorbing vs. Reflecting Materials

Colors of Light

White light is a combination of the spectrum of colors, each having different wavelengths.

When combined, the three primary light colors of red, blue, and green will produce white light.

In other combinations, they will produce other colors.

Primary and Secondary Pigments Primary and Secondary Light

Why do we see colors?

Pigments reflect color of light that we see. So… a

shirt that has been dyed to appear red is only red because it reflects mostly red light and absorbs most of the other two primary light colors of blue and green. So that shirt must contain cyan pigment.

When mixed, the primary pigments create the secondary pigments (red, blue, and green).

Examples

White light is incident on the three shapes below. Explain why they appear to be the color they are by using reflected and absorbed colors as well as identifying which pigments must be present in the object.

Yellow color is seen. Red

and Green light are reflected,

Blue light is absorbed. Blue

pigment must be present.

Blue color is seen. Blue light

is reflected, Red and Green

light are absorbed. Yellow

pigment must be present.

Magenta color is seen. Red

and Blue light are reflected,

Green light is absorbed. Green

pigment must be present.

What’s the difference between light

and pigments?

Unlike light, pigments are referred to as subtractive colors. For instance, when you mix the primary pigments of yellow and cyan, the color of green is what you will see. That is because green pigment absorbs the light with colors of blue and red and reflects back only green.

Complimentary Colors of Light

Complimentary colors are two colors that add together to produce white light.

RED + CYAN = WHITE

GREEN + MAGENTA = WHITE

BLUE + YELLOW = WHITE

Polarized Light Normal, unpolarized light (just like all

electromagnetic radiation) vibrates in two dimensions as it travels.

Polarized light consists of waves vibrating in a one particular plane. Polarization is done by using VERY small filters (slits) that block vibrations from other planes.

Application of Polarization – sunglasses polarized

to reduce glare reflected off water or off the road

Polarized Light

Thin Films

Colors in soap and oil films are caused by the interference of specific wavelengths of light reflected from the front and back surfaces of the thin films.

The colors actually show up because an antinode (constructive interference) is formed.

Diffraction

Diffraction is the bending of light around a barrier.

Diffraction pattern – a pattern of bright and

dark bands produced by constructive and destructive interference; white light shows all the colors of spectrum.

Diffraction gratings (ex: goggles used in demos) are devices made up of many single slits that bend light and form diffraction patterns.

Thin Slits and Diffraction

Real-Life Interference