LIGHT & COLOR

Very Important Concepts When light reaches the boundary between two media, some of its energy is reflected, some is transmitted, and some is absorbed. The relative amounts of each depends on… the frequencies of light, the angle the light reaches the boundary, and the nature of the two media. Examples: Light traveling through air reaches colorless glass Light traveling through air reaches colored glass Light traveling through air reaches a red sweater We only “see” what reaches our eyes!

What is your mental model of light… reflecting from a surface? transmitting through a material? being absorbed by a material? link and link

can clearly see objects through them Based on these interactions of light with matter, we classify materials as: transparent - readily transmits light; can clearly see objects through them translucent - transmits, but diffuses, light; cannot see objects clearly through them opaque - transmits no light; cannot see through them

Visible light is that portion of the electromagnetic spectrum which stimulates the retina of the human eye. Visible spectrum wavelengths range from about 400 nm (violet) to 760 nm (red). Light travels at about 3 x 108 m/s through empty space and slightly slower through air. Remember that for all waves, v = f.

WHITE light is composed of all colors. Red, orange, yellow, green, blue, violet is the order of increasing frequency or decreasing wavelength. Frequencies directly above this spectrum are ultraviolet. Frequencies directly below this spectrum are infrared.

colors that combine to form white light. Complimentary colors are two colors that combine to form white light. Red and cyan, blue and yellow, green and magenta are pairs of complimentary colors. Red, blue, and green are called primary colors or secondary pigments. Cyan, yellow, and magenta are called primary pigments or secondary colors.

Activity: Color Mixing These sites let you simulate mixing colors and pigments of light: link1, link2, link3 “But I learned that the primary colors are red, blue, and yellow – not red, blue, and green.” Read about that and more here. Activity: Color Mixing

The color of an opaque object depends on the colors (frequencies) of light incident upon it and on the colors (frequencies) of light reflected. The color of a transparent object depends on the colors (frequencies) of light incident upon it and on the colors (frequencies) of light transmitted.

Now that we know more about color and what colors are seen when light passes through or reflects from various materials, let’s look at how colors are revealed by various natural phenomena.

The property of light responsible for the separation of light into colors by a prism is known as refraction. Refraction is defined as the change of direction of a ray of light as it passes obliquely from one medium into another of different transmission speed.

The properties of light responsible for the separation of light into colors by a rainbow are reflection and refraction. Reflection is defined as the turning back of a wave when it reaches the boundary of the medium through which it is traveling. Learn more about rainbow formation here and here.

Soap Bubbles (and other thin films) The properties of light responsible for the separation of light into colors by a soap bubble (or other thin films) are reflection, refraction, and interference. Interference is defined as the result of the superposition of two or more waves. Explore thin film interference here and here.

Thin Film Interference The color spectrum seen in a soap bubble, or any thin film, results from the interference of the reflections of light from the front and back surfaces of the film. The colors seen depend on the thickness of the film. The light most strongly seen has a wavelength such that the film thickness is an odd multiple of l/4. Other wavelengths will suffer partial or total destructive interference.

Spectroscopes and “Rainbow Glasses” The properties of light responsible for the separation of light into colors by a spectroscope (or diffraction grating) are diffraction and interference. Diffraction is defined as the spreading of a wave around the edge of a barrier or through an opening. View simulations of diffraction here and here. Activity: Spectroscopes