Introduction to Electromagnetic waves, Light, and color

Objectives Describe a light wave. Understand qualitatively the relationship between color, frequency, and wavelength for light. Use the equation for the speed of light to calculate frequency and wavelength.

Physics terms Light – form of radiant energy consisting of electromagnetic waves traveling at the speed of light Medium – a method of transportation of specific wave Vacuum – an area such as space in which there is no air or air has been removed.

Physics terms Reflection – process by which wave bounce off or return by striking a surface or the boundary between two different materials. Refraction – process by which waves change direction by traveling into or thought a surface or boundary between two materials.

Electro- magnetic Waves

Equations: Speed of light The speed of light is a constant, and equals the product of its frequency and wavelength: c = speed of light = 3.0 × 108 m/s in a vacuum f = frequency (Hz) λ = wavelength (m)

Properties of light Light has seven observable properties.

Light waves Light waves can travel through many mediums: gases liquids solids Light is shown here traveling through air, glass and water. Students may mention plastic, oil, or even gems such as diamond and ruby.

Light waves vs. sound waves Sound waves need a medium, such as air, to propagate. Light waves don’t require a medium. Light waves can travel through the vacuum of empty space. Point out that light from the Sun reaches us through the vacuum of space.

Speed of light Light travels very, very fast! Nothing travels faster than light. The speed of light is the speed limit of the universe. The speed of light is so important that it gets its own symbol, c.

Engaging with the concepts What is the frequency of light that has a wavelength of 2.0 x 10-10 m? Frequency 2.00e-10

Engaging with the concepts What is the frequency of light that has a wavelength of 2.0 x 10-10 m? (This is two angstroms.) 5.0 x 1018 Hz These are x-rays. Frequency 5.00e+18 2.00e-10

Frequency and color The frequency of an electromagnetic wave equals the frequency of the oscillating electric charge that creates it. If the charge moves up and down 15 trillion times per second, the resulting wave is red light. If the charge moves up and down 25 trillion times per second, the resulting wave is blue light. The frequency of light determines its color.

Frequency and color Frequency Redder light has lower frequency and longer wavelengths. Bluer light has higher frequency and shorter wavelengths.

Particles scatter light When light strikes small particles in the atmosphere, it may be absorbed. If this happens, the particles subsequently re-emit the light—but usually in a different direction! Scattering is the process of small particles absorbing and re-emitting light.

Why is the daytime sky blue? Small particles in the atmosphere scatter blue light, because blue light has short wavelengths similar in size to the particles. The scattered blue light comes to us from every direction, all over the sky.

Why are sunsets red? At sunset, the Sun's light must pass through much more air to reach us. All that extra air results in more and more scattering of the blue wavelengths—until there is little or no blue light left.

Why is the night sky black? When all three additive primary colors (red, green, blue) appear in equal amounts, the result is white light. The absence of all three colors is black. At night there is no sunlight, so the night sky is black.

Digital images A digital image tricks the brain into seeing a full range of colors by adding together varying amounts of red, green, and blue.

Additive colors RGB = “Red, Green, Blue” When combinations of RGB are added together, you can create many different colors.

Additive colors and technology RGB additive colors are used in many technologies to create digital images. television computer screens digital cameras computer projectors

Additive colors and technology A digital image is made up of tiny dots called pixels. Each pixel is created by combining the three RGB colors. A one megapixel image has one million dots.

Additive colors in digital cameras In a digital camera, each pixel is captured by tiny red, green and blue sensors. Each sensor records a number from 0 to 255 corresponding to the light intensity for that color. A one megapixel digital image is a string of three million numbers which prescribe the intensity of red, green and blue for each pixel.

RGB color wheel representations Each of these color wheels are explanations of how colors of light combine to form new colors. Analyze each visual representation and describe in words how each explains how colors of light combine to create new colors. Critique how well each color wheel explains how colors combine, based on your experiments and experience with the interactive element. Evaluate these two representations. Which is more useful in helping you to determine the red, green, and blue values to match various colors during the investigation.

The technology of color We use two different methods for creating different colors. Digital devices create colors using emitted light in an additive process. Print images in books and newspapers use reflected light in a subtractive process.

Printed images Screens display color by emitting light. The pages of a book reflect light instead.

Printed images Screens display color by emitting light. The pages of a book reflect light instead. When you view a color page, you see the colors it reflects. Other colors have been absorbed.

Printed images Screens display color by emitting light. The pages of a book reflect light instead. When you view a color page, you see the colors it reflects. Other colors have been absorbed. Cyan, magenta and yellow pigments are used in this subtractive process to remove light upon reflection.

Compare color wheels RGB additive primary colors CMYK subtractive primary colors Each set of colors can be created from the other!

Light and color Where do the colors of a rainbow come from?

Light and color All the colors of the rainbow are contained in white light from the Sun. Raindrops disperse the white light into its colors. A glass prism can also disperse white light into its colors.

How do we sense color? The sensation of color is a perception of the energy in light.

How do we sense color? The sensation of color is a perception of the energy in light. Within the visible spectrum, lower energy light appears red to us, and higher energy appears blue.

How the eye sees color The eye’s retina has two kinds of light sensors. Rods detect the intensity of light. Cones detect the color of light.

How the eye sees color The eye’s retina has two kinds of light sensors. Rods detect the intensity of light. Cones detect the color of light. Point out that our eyes work like a digital camera with three sensors corresponding to the three primary colors. There are 3 types of cones: cones that detect red light, green light, and blue light. The brain combines these intensities and colors into an image.

How the eye sees color Our eyes work like a digital camera. In a digital camera, each pixel is captured by tiny red, green and blue sensors. In our eyes, the three types of cones are sensors corresponding to the three primary colors. Point out that our eyes work like a digital camera with three sensors corresponding to the three primary colors.