Chapter 27 Color. 04/14/04 Dr. Jie Zou PHY 3050G Department of Physics

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1 Chapter 27 Color Selective reflection Selective transmission Mixing colored light Why the sky is blue Why sunsets are red Why clouds are white Why water is greenish blue 1

2 Color The colors we see depend on the frequency or wavelength of the light we see. Lights of different frequencies or wavelengths are perceived as different colors. The lowest-frequency light we detect appears to most people as the color red, and the highest frequency as violet. Between them range the infinite number of hues that make up the color spectrum of the rainbow. These hues are grouped into the seven colors of red, orange, yellow, green, blue, indigo, and violet. These colors together appear white. The white light from the sun is a composite of all visible frequencies. 2

3 Selective reflection Selective reflection: Objects reflect specific colors of light. They reflect only part of the light that is incident upon them, the part that gives them their color. For example, the atoms of the petals of a red rose absorb all visible light except red, which they reflect, and that is why the petals appear red. An object that reflects light of all the visible frequencies, such as a white page, is the same color as the light that shines on it. If a material absorbs all the light that shines on it, it reflects none and hence it appears black. Color depends on the light source since an object can reflect only those frequencies present in the illuminating light. 3

4 Selective transmission The color of a transparent object depends on the color of the light it transmits. Selective transmission: Objects selectively absorb light of certain frequencies and selectively transmit others. Ordinary window glass is colorless because it transmits light of all visible frequencies equally well. 4

5 Mixing colored light Radiation curve of sunlight: the graphical distribution of brightness versus frequency. The intensity of light from the Sun varies with frequency, being most intense in the yellow-green part of the spectrum. All the colors combine make white. The perception of white by our eyes also can result from the combination of only red, green, and blue light. Red, green, and blue are also called the additive primary colors. Various amounts of red, green, and blue add to produce any color in the visible spectrum. 5

6 When the air is full of particles larger than molecules, lower-frequency light is also scattered, which add to give a whitish sky. Why does the sky appear deeper blue after a heavy rainstorm? Why the sky is blue Scattering: when light is re-emitted in all direction, we say the light is scattered. Why the sky is blue: Nitrogen and oxygen molecules in the atmosphere scatter high-frequency light more strongly than low-frequency ones. Since our eyes are not very sensitive to violet light, the blue scattered light is what predominates in our vision, and we see a blue sky. The blue of the sky varies in different places under different conditions. Dust and other particles larger than molecules also scatter light of lower frequencies strongly, which makes the sky appear whitish. 6

7 Why sunsets are red Light of lower frequencies, such as red, orange, and yellow is the least scattered by the atmosphere, and thus is better transmitted through the air. Red, which is scattered the least and thus transmitted the most, passes through more atmosphere than any other color. Sunlight travels through more atmosphere at sunset, and thus more time to scatter all the higherfrequency components of the light, leaving the Sun look redder, going from yellow to orange and finally to a red-orange at sunset. 7

8 Why clouds are white Water droplets in a variety of sizes make up clouds. The different-size droplets produce a variety of scattered frequencies. The tiniest scatter more blue than other colors. Slightly larger droplets scatter slightly more higher frequencies, say green. Still larger droplets scatter more red. The overall result is a white cloud- All the colors combined make white. 8

9 Why water is greenish blue Water is cyan because it absorbs red light. The froth in the waves is white because, like clouds, it is composed of a variety of tiny water droplets that scatter all the visible frequencies. While the sky is blue because blue is strongly scattered by molecules in the atmosphere, water is bluish green because red is absorbed by molecules in the water, hence leaving the water to have a cyan color. 9

10 What is color? The colors of things depend on which colors are scattered or reflected by molecules and also on which colors are absorbed by molecules. The color we see is not in the world around us-the color is in our heads. Electromagnetic waves of different frequencies (or wavelengths) stimulate the sensation of color when the vibrations interact with the cone-shaped receiving antennae in the retina of our eyes. 10

11 Let s do some examples 1) Color depends on what characteristic of light? A) Its frequency. B) Its wavelength. C) Both of these. D) Neither of these. 2) What color of light is transmitted by a piece of blue glass? A) Red. B) White. C) Blue. D) Yellow. E) Orange. 3). The sky is blue because air molecules in the sky act as tiny A) mirrors which reflect only blue light. B) resonators which scatter blue light. C) sources of white light. D) prisms. E) none of these. 4). Red sunsets are due to lower frequencies of light that A) are scattered from larger particles in the air. B) are reflected by clouds and relatively large particles in the air. C) survive being scattered in the air. D) appear reddish orange to the eye. 5) The whiteness of clouds is evidence in the clouds for a variety of A) particle sizes. B) molecules. C) seeds upon which condensation of cloud material forms. D) light intensities. E) water prisms. 11

12 Homework 12 (04/14/04) Chapter 27, P. 530, Exercises: 9, 10, 27,