Light, color, and frequency pg. 71

Objectives Physics terms Describe a light wave. light Understand qualitatively the relationship between color, frequency, and wavelength for light. Use the equation for the speed of light to calculate frequency and wavelength. light electromagnetic wave medium vacuum speed of light (c) scattering

This lesson focuses on two properties: speed and color.

Light waves Light is an electromagnetic wave: The electric and magnetic fields are directly related, and travel at right angles to each other. Light is a transverse wave: The electric and magnetic fields oscillate at right angles to the velocity of the wave.

Light waves Light waves can travel through many mediums: gases liquids Solids Light is shown here traveling through air, glass and water. Can you name some other light media? 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.

Test your knowledge What is the frequency for visible light traveling through a vacuum with a wavelength of 500 nm? Givens: Unknown: Equation: Substitution: Solution: λ = 500 nm 500 x 10-9 m, 3 x 108 m/s frequency, f 𝒇= 𝒄 𝝀 𝒄=𝒇𝝀  𝒇= 𝟑 𝒙 𝟏𝟎 𝟖 𝟓𝟎𝟎 𝒙 𝟏𝟎 −𝟗 f = 6 x 1014 Hz

Math Practice 1. What is the frequency of light that has a wavelength of 2.0 x 10-10 m? 2. What is the frequency of light that has a wavelength of 4.0 μm? 3. What is the wavelength of light with a frequency of 1010 Hz?

Investigation How is the color of light related to its wavelength and frequency? In investigation 22A you will explore this connection. Get logged in & select the “interactives” tile, the one that looks like a computer mouse Go down to ch.22 & select 22A: Color, frequency & Wavelength of light

Investigation Part 1 Part 1: Relating the color of light to its frequency and wavelength You can change the light in any of three different ways: Move the slider from left to right. Enter a value in the box for frequency. (6.0e14 = 6.0 x 1014 Hz) Enter a value in the box for wavelength (in nm).

Investigation Part 1 Q & A What is the wavelength of blue light? Cyan? Green? What color of light corresponds to a wavelength of λ = 580 nm? What color of light corresponds to a frequency of f = 6.9×1014 Hz? What color(s) have the highest frequencies? Longest wavelengths?

Investigation Part 1 Q & A Put these colors in order of increasing frequency: blue; green; indigo; orange; red; violet; yellow. How are frequency and wavelength related to each other?

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.

Homework Light is made up of a combination of what two oscillating fields? electric and magnetic magnetic and gravitational electric and gravitational soccer and baseball 2. What light has higher frequency, violet or red? What light has longer wavelengths, violet or red?

Homework 3. How are electromagnetic waves similar to sound waves? How are they different? What is the frequency of visible light that has a wavelength of 550 nm in a vacuum?

Homework What is the wavelength of an x-ray with a frequency of 3 × 1018 Hz? 10−26 m 10−10 m 1010 m 1026 m