Why Objects Have Color

In the first picture the tomato absorbs blue and green wavelengths and In the first picture the tomato absorbs blue and green wavelengths and reflects the red wavelength. reflects the red wavelength.

In the second picture red light is shone upon the tomato. In the second picture red light is shone upon the tomato.

The tomato is still reflecting the red wavelength and thus still looks red. The tomato is still reflecting the red wavelength and thus still looks red.

33rdrd picture blue light is shone upon the tomato, and since the tomato picture blue light is shone upon the tomato, and since the tomato absorbs the blue wavelength the tomato appears to be black.absorbs the blue wavelength the tomato appears to be black.

Visible light is a combination of many wavelengths (colors), which give it a white appearance. When light hits an object certain wavelengths are reflected and others are absorbed.

Light Transmission

Transparent: Materials, such as window glass, through which light can travel easily and through which other objects can clearly be seen.

Translucent: Materials, such as glass blocks, through which light can pass through but no clear image can be seen. Opaque: Materials which absorb and reflect light. Objects cannot be seen through the material. Most objects are opaque.

Primary and Secondary Colors

The primary light colors are Red, Blue, and Green (RGB).The primary light colors are Red, Blue, and Green (RGB).

The secondary light colors are Yellow, Cyan, and Magenta.The secondary light colors are Yellow, Cyan, and Magenta.

Combining pigments in painting is exactly the opposite: Combining pigments in painting is exactly the opposite:

The primary pigments are Yellow, Cyan, Magenta.The primary pigments are Yellow, Cyan, Magenta.

The secondary pigments are Red, Blue and Green.The secondary pigments are Red, Blue and Green.

Light Colors Vs. Pigments-Primary colors in light are red, green, and blue because when put together in the right intensities they form white light. -When lights these colors are combined in pairs they form the secondary colors for light.

-Pigment colors are seen by reflected light. -A primary pigment color is one that absorbs only one primary light color and reflects the other two primary colors. Thus yellow, magenta, and cyan are the primary colors for pigments. Yellow reflects red & green, cyan reflects green & blue, and magenta reflects red & blue. -Secondary pigments colors then are blue, green, and red because they absorb two primary light colors and reflect their own light color back.

Polarized LightBeam o’ Light

Beam o’ Light

A polarizing filter is made of a material with long molecules that A polarizing filter is made of a material with long molecules that allow electromagnetic waves of one orientation through. allow electromagnetic waves of one orientation through. If a wave has an electric field with any other orientation, the filter will If a wave has an electric field with any other orientation, the filter will only allow a component to pass through, absorbing the rest. Note only allow a component to pass through, absorbing the rest. Note that only transverse waves such as light can be polarized. that only transverse waves such as light can be polarized. Much of the light we see is at least partially polarized. Some Much of the light we see is at least partially polarized. Some sunglasses contain polarizing filters which helps to block glare (such sunglasses contain polarizing filters which helps to block glare (such as the glare that is noticeable when looking out over a lake on a as the glare that is noticeable when looking out over a lake on a sunny day).sunny day).

Electric Field Electric Field OrientationsOrientations

Light coming directly from the sun or other sources is unpolarized, meaning the electric and magnetic fields oscillate

in many different planes. Polarized light refers to light in which all waves have electric fields Polarized light refers to light in which all waves have electric fields oscillating in the same plane. oscillating in the same plane.

Polarized Light (cont.)

Unpolarized light propagates in all orientations. No particular orientation is preferred.

When it passes through a filter that only allows vertical components of electric fields to pass, its intensity is cut in half.

on average, the light is “half horizontal and half vertical” in terms of electric field components. All horizontal components are blocked, making the resulting polarized light half as bright.

Consider if another filter is perpendicular to the direction of the first one, i.e., a filter that only allows the horizontal components of electric fields to pass through. This would completely block the remaining light. Thus, any two perpendicular filters will block all incoming light.

Suppose now that the two filters are offset by some angle . Regardless of the angle, the first filter blocks half the light.

If = 0, the second filter has no effect.

If = 90, the second filter blocks the other half of the light. In general, when polarized light with an electric field of amplitude E passes through the second filter, the amplitude will drop to E cos.