Lecture 5 Chapter 1 & 2. Sources of light

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Madeline Paul
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1 We are here Lecture 5 Chapter 1 & 2 Some history of technology How vision works What is light Wavelength and Frequency: c = f λ Scientific notation and metric units Electromagnetic spectrum Transmission and opacity Speed of light Amplitude Direction: Rays and wavefronts Polarization Phase difference Resonance Sources of light Start Chapter 2 1 Sources of light Flame (atomic emission) Incandescent Gas discharge (neon or mercury atomic emission) Fluorescent (mercury gas discharge + phosphor) Plasma TV (phosphors have color) Light emitting diode (LED) Laser All sources involve moving electrons! Demo: laser pointer 2

2 Flame and gas discharge have excited atoms that emit Emission of light Atomic electron starts at high energy level Absorption of light Atomic electron at a lower energy level Atomic electron ends at a lower energy level Extra energy is carried off by light (photon) Light (a photon) hits an atom Atomic electron at high energy level (atom gains energy from light) Demo: gas discharge lamp (lec. 4) 3 Hot materials emit blackbody radiation (incandescence) Object temperature emission: People 300 K infrared Charcoal fire 2000 K dull red Tungsten filament 3000 K reddish white Sun 6000 K orange white Brighest star 10,000 K bluish white Note that hotter objects emit bluer light (shorter wavelength). All wavelengths are emitted (a whole spectrum), some wavelengths are emitted more. Demo: bulb on dimmer gets redder 4

Higher temperatures have peak emission at shorter wavelengths 5 Temperatures can be in F, K or C F is Fahrenheit, water freezing is 32 F C is Centigrade,

3 Higher temperatures have peak emission at shorter wavelengths 5 Temperatures can be in F, K or C F is Fahrenheit, water freezing is 32 F C is Centigrade, freezing is 0 C K is similar to C but the zero is shifted, water freezing is 273 K 0 K is the coldest possible temperature A change of 1 C is a change of 1.8 F. 6

4 Chapter 2 Geometrical Optics We are here 1. Shadows 2. Reflection 3. Refraction 4. Dispersion 7 1. Shadows a. Point source or diffuse source b. Umbra and penumbra c. How tall is my shadow? d. Pinhole camera 8

Size of a source is measured by angle to decide.

5 Point source or diffuse source? Point-like source of light is small in size. Diffuse source of light has a large area. Size of a source is measured by angle to decide. Sun looks smaller than a television 9 Nothing here, so we have a plain white background for shadows in the classroom! Demo: shadows (red and blue), big bulb, small bulb casting shadows 10

6 Umbra and penumbra In the umbra, all light from the source is blocked In the penumbra, some light is blocked. Penumbra Umbra Sun Earth What you see located there: 11 Light from blue and yellow sources combine to make white. Can you understand the shadows cast on the screen? Two lights Screen Demo this 12

7 Larger source casts fuzzier shadow Some light no light Demo with bulbs 13 Umbra and penumbra In the umbra, all light from the source is blocked In the penumbra, some light is blocked. Penumbra Umbra Sun Earth What you see located there: 14

8 Light from blue and yellow sources combine to make white. Can you understand the shadows cast on the screen? Two lights Helpful hints: Blue + Yellow looks white White yellow looks blue White blue looks yellow Screen Demo this 15 This black slide to kill projector

9 How tall is my shadow? Hint: Use similar triangles = Closer to source = bigger shadow 40 ft. 6 ft. 100 ft. 17 What is a pinhole camera? A box, a pinhole, a waxed paper or ground glass that scatters light toward the eye. No ground glass = no light toward eye 18

10 Chapter 2 Geometrical Optics We are here 1. Shadows 2. Reflection a) Specular or diffuse b) Equal angle rule c) Mirror images, ray tracing 3. Refraction 4. Dispersion a. Point source or diffuse source b. Umbra and penumbra c. How tall is my shadow? d. Pinhole camera 19 Specular or diffuse? Diffuse reflection (paper) Specular reflection (mirror) Diffuse transmission (wax paper) Demo wax paper 20

11 Equal angle rule θ i = angle of incidence θ r = angle of reflection Good news! You don t actually need to use this for ray tracings. θ i = θ r is specular reflection θ i θ r Normal Mirror A normal is a line perpendicular to the surface. 21 Image in a mirror 1. If a point on the object is distance X in front of the mirror, the same point in the image appears to be distance X in back of the mirror, or X object = X image. 2. The image point is on the normal (extended) from the object to the mirror. Mirror normal extended X object X image 22

12 Ray tracing: Draw the image first, then the rays First: draw rays from image to eyes Viewed from the side. Mirror X object X image 23 Ray tracing: Draw the image, then the rays First: draw rays from image to eyes Second: draw rays from mirror to object Mirror X object X image θ i = θ r happens automatically using this method. Demo on board 24

13 Right side up image? The top ray goes to the top of the bottle. It is right side up. Mirror X object X image θ i = θ r happens automatically using this method. 25 Right side up image? The top ray goes to the bottom of the bottle. It is upside down. X object Extension Mirror (to do this drawing, the mirror must be extended) X image θ i = θ r happens automatically using this method. 26

14 Periscope? You see a reflection of a reflection mirror mirror This is NOT a ray tracing. 27 Periscope? Image 1 Extension of mirror 1 This is the start of a ray tracing. 28

15 Periscope? Image 1 Extension of mirror 2 Image 2 29 Having filled in the images, you can draw the rays On homework, you should show all these lines for full credit! This is the image you see 30

16 Bottle on its side Viewed from the side. Mirror (to do this drawing, the mirror must be extended) θ i = θ r happens automatically using this method. 31 Why is writing reversed? (Viewed from above) AR ЯA 32

17 Right side up image? 33 Right side up image? 34

18 Right side up image? No, not right side up 35

Physics 1230: Light and Color

Physics 1230: Light and Color Chuck Rogers, Charles.Rogers@colorado.edu Ryan Henley, Valyria McFarland, Peter Siegfried physicscourses.colorado.edu/phys1230 Congratulations on completing Exam 1. Great