Light Part II (and review) Lecture 8 2/11/2014

Transcription

1 Light Part II (and review) Lecture 8 2/11/2014

2 Announcements Celebration of Knowledge (aka Exam 1) will be February 13, will include all information covered including today closed note bring a calculator (you cannot use your phone) Review Sessions (all in KSS 330): Sky: Today, February 11 Catherine: Tomorrow, February 12 BRING QUESTIONS!!!

3 Review: Electromagnetic Radiation Visible light is one part of the electromagnetic spectrum Other parts of the electromagnetic spectrum include X-rays, gammarays, radio waves, etc.

4 Review: Electromagnetic Spectrum The electromagnetic spectrum is described by: c = λf The energy of a waves is proportional to its frequency E = hf = hc λ

5 The Electromagnetic Spectrum Blue Shorter Wavelength Red Longer Wavelength λ f

6 The Speed of Light The speed of light is constant (same always, all the time, all kinds of light) The speed of light is finite i.e. not infinite... light takes time to travel from place to place!! Just like you can only travel so fast in a car, on foot, etc. 6

7 The Speed of Light Say Venus explodes right now. Assuming Venus is million miles from Earth. How long will it take before we (on Earth) see the explosion? a) we will see it instantaneously b) 1 minute c) 2 minutes d) 10 minutes Speed of Light = 670 million mph = 3 x cm/s

8 Luminosity is the total energy (light) emitted by an object in each second. Stefan-Boltzmann law Luminosity depends on an surface area (A), and its temperature (T 4 ) Luminosity = 5.67x10 8 (Α)T 4 Big and Hot objects have greater luminosity than small cool objects

9 Finish Lecture Tutorial Luminosity, Temperature and Size (pg 55) Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask another group.

10 If you pass white light through a prism, it separates into its component colors. long wavelengths R O Y G B I V short wavelengths spectrum

11 Which object is hotter, an object that is emitting mainly red light or mainly blue light? increasing temperature

12 Which object is hotter, an object that is emitting mainly red light or mainly blue light? increasing temperature

13 blue 460 nm green 530 nm yellow 580 nm orange 610 nm red 660 nm Filter Detector UV IR Blackbody Curve - a graph of an object s energy output versus wavelength. The PEAK of this curve is related to the object s temperature.

14 Blackbody Curve - a graph of an object s energy output versus wavelength. The WAVELENGTH that the PEAK of this curve occurs at tells us about the object s TEMPERATURE and COLOR. Energy Output UV IR Wavelength

15 Hot objects emit light that PEAKS at short wavelengths (blue). Cool objects emit light that PEAKS at long wavelengths (red) increasing temperature

16 Wien s law Relates the temperature of an object to the wavelength of the peak in the black body curve. peak = T The higher the object s temperature, the shorter the wavelength of the peak for the light emitted by the object.

17 What is the wavelength of the PEAK of this Blackbody curve A)Purple B)Red C)Green D)Yellow

18 What color is our 5800K Sun? The Sun emits all wavelengths of electromagnetic radiation (light); however, the wavelengths of light it emits most intensely are in the green/yellow part of the spectrum.

19 What color does the Sun appear? A star, like the Sun, which peaks in the middle of the visible part of the spectrum (green/ yellow light) will appear WHITE to the human eye because it is giving off nearly equal amounts of all the visible colors of light. WHITE!!

20 Our Sun What if the Sun became hotter?

21 Our Sun What if the Sun became hotter? What if the Sun became cooler?

22 Our Sun What if the Sun became hotter? What if the Sun became cooler?

23 The graph at the right shows the blackbody curve for three different stars. Which of the stars is at the highest temperature? A) Star A B) Star B C) Star C Energy Output per second Star A Star B Wavelength Star C 23

24 Tutorial: Blackbody Radiation (pg 59) Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask another group.

25 The Debrief