Chapter 13 Light and Reflection Ms. Hanan Anabusi

13-1 Characteristics of Light Objectives: Identify the components of the electromagnetic spectrum. Calculate the frequency or wavelength of the electromagnetic radiation. Recognize that light has a finite speed. Describe how the brightness of a light source is affected by distance.

Vocabulary Electromagnetic wave Spectrum Wavelength Frequency Speed of light Rays Luminous

Nature of Electromagnetic Waves They are Transverse waves without a medium. (They can travel through empty space) They travel as vibrations in electrical and magnetic fields. Have some magnetic and some electrical properties to them. Speed of electromagnetic waves = 300,000,000 meters/second (Takes light 8 minutes to move from the sun to earth {150 million miles} at this speed.)

When an electric field changes, so does the magnetic field. The changing magnetic field causes the electric field to change. When one field vibrates—so does the other. RESULT-An electromagnetic wave.

Waves or Particles? Electromagnetic radiation has properties of waves but also can be thought of as a stream of particles. Example: Light Light as a wave: Light behaves as a transverse wave which we can filter using polarized lenses. Light as particles (photons) When directed at a substance light can knock electrons off of a substance (Photoelectric effect)

Waves of the Electromagnetic Spectrum Electromagnetic Spectrum—name for the range of electromagnetic waves when placed in order of increasing frequency RADIO WAVES MICROWAVES INFRARED RAYS VISIBLE LIGHT ULTRAVIOLET RAYS X-RAYS GAMMA RAYS

Examples include: (textbook page 447 – Table 13-1) Picture source:

The Electromagnetic Spectrum Picture from the New York Physical Setting/ Physics reference tables

The Electromagnetic Spectrum More than meets the eye!

Examples from Space!

Wavelength The distance from one wave crest to the next Radio waves have longest wavelength and Gamma rays have shortest!

Listed below are the approximate wavelength, and frequency limits of the various regions of the electromagnetic spectrum. Wavelength (λ)Frequency (f) Radio wavesλ > 30 cmf < 1.0 x 10 9 Hz Microwaves30 cm > λ > 1 mm1.0 x 10 9 Hz < f < 3.0 x Hz Infrared (IR) waves1 mm > λ > 700 nm3.0 x Hz < f < 4.3 x Hz Visible light700 nm (red) > λ > 400 nm (violet)4.3 x Hz < f < 7.5 x Hz Ultraviolet (UV) light400 nm > λ > 60 nm7.5 x Hz < f < 5.0 x Hz X-rays60 nm > λ > nm5.0 x Hz < f < 3.0 x Hz Gamma-rays0.1 nm > λ > nm3.0 x Hz < f < 3.0 x Hz

Questions Which visible light has the shortest wavelength?___________________ Which visible light has the longest wavelength? __________________ Which electromagnetic spectrum item has the smallest frequency? ______________ Which electromagnetic spectrum item has the shortest wavelength? _____________ Violet Red Radio Waves Gamma Rays

All electromagnetic waves move at the speed of light. In vacuum light travels at x 10 8 m/s. In air light travels at x 10 8 m/s, slightly slower. For our purposes we will use 3.0 x 10 8 m/s.

Sample Problem The AM radio band extends from 5.4 x 10 5 Hz to 1.7 x10 6 Hz. What are the longest and shortest wavelengths in this frequency range? Given: Unknown:

Use the wave speed equation:

Pop Question!!! Using Table 1 page 447 in your text book answer the following question: Which of the following electromagnetic waves has the highest frequency? a)Radio b)Ultraviolet radiation c)Blue light d)Infrared radiation

Huygens’ Principle Huygens’ principle: Every point on a wave front acts as a point source; the wave front as it develops is tangent to their envelope. Huygens’ principle is used to derive the properties of any wave that interacts with matter. The straight line perpendicular to the wave front is called a ray. This simplification is called ray approximation.

Illuminance or Brightness Intensity of light depends on: Amount of light energy emitted (watts) Distance from the source (m) Light bulbs are rated by their input measured in watts (W) and their light output. The rate at which light is emitted from a source is called the luminous flux and is measured in lumens (lm) Luminous flux is a measure of power output, but is weighted to take into account the response of the human eye to light. Illuminance is the luminous flux divided by the area of the surface and measured in lm/m2

Light from a source spreads out in space. The further from the source the less light per unit area there will be (the source is not as bright). The brightness drops off as one over the distance squared. This is called the inverse square law.

In words: Brightness of a source is inversely proportional to the square of its distance from you. Inverse Square Law of Brightness Expressed mathematically:

The Inverse Square Law Scientists have calculated a theoretical relationship between brightness and distance. This predicted relationship between brightness and distance is called the inverse square law. It says that when the distance from a light doubles, its brightness should decrease by a factor of four. The equation for the brightness, written as B in the equation below, and the distance from the light, written as d, is B = C/d 2 In this equation, C is a constant that depends on how luminous the light is (in other words, what "wattage" the light bulb is). The equation for C is C = B d 2. You do not need to understand this equation in detail. The important point is that the brightness depends on distance, and that when the distance doubles, the brightness goes down by a factor of four. Brightness and Distance

Assignments Class-work: Practice A, page 449, even questions. Homework: Section review on page 450 odd questions. Additional practice A, odd questions.