1. Phys141 Principles of Physical Science Chapter 7 Wave Effects and Optics Instructor: Li Ma Office: NBC 126 Phone: (713) 313-7028 Email: malx@tsu.edumalx@tsu.edu Webpage: http://itscience.tsu.edu/ma Department of Computer Science & Physics Texas Southern University, Houston Oct. 4, 2004

2. Reflection Refraction Diffraction Interference Skip §7.3, §7.5 & §7.6 Topics To Be Discussed

3. The effects of waves are all around us: –Mirrors – reflection –Lenses – refraction –Sound waves “bend” around the corner; we can be heard in the next room, but not seen – diffraction –Colorful soap bubble or rainbow – interference These phenomena can be described or explained through the effects and interactions of sound waves or light waves The Effects of Waves

4. Light waves travel through space in a straight line Light waves can be diverted from its original direction by an obstacle Its direction will change if the light strikes and rebounds from a surface or the boundary between two media –A change in direction by this method is called reflection Reflection: light is “bouncing off” a surface Reflection

5. A ray is a straight line that represents the path of light A beam of light may be a group of parallel rays When an incident light ray is reflected from a surface, –The angle of reflection θ r is equal to the angle of incidence θ i –The reflected and incident rays are in the same plane as the normal direction Reflection (cont)

6. Law of Reflection θiθi θrθr Normal Reflected ray Incident ray Reflecting surface θ r = θ i

7. A smooth surface produces regular (or specular) reflection A rough surface produces irregular (or diffuse) reflection –On a rough surface, the normal direction of the different points may be different. So parallel incident rays result in diffused reflection rays Both reflections obey the law of reflection Here we ignore the wave nature of light: absorption and emission Reflection (cont)

8. A ray diagram is a diagram that indicates the relationship between incident and reflected rays –For a plane mirror, the image is located behind the mirror at the same distance as the object is in front of the mirror – Figure 7.3 on page 138 –To see your complete figure in a plane mirror, the height of the mirror must be at least one-half of your height – Figure 7.4 on page 138 It is the reflection of light that enables us to see things Reflection (cont)

9. When light strikes a transparent medium, some light is reflected and some is transmitted The transmitted light has changed direction in going from one medium to another –Phenomenon example: a spoon in a glass of water appears to be displaced –This effect is called refraction The deviation of light from its original path arises because of a change in light speed Refraction

10. Index of refraction n: –The ratio of the speeds in different media –n is a pure number, Table 7.1 on page 139 Refraction (cont) index of refraction = speed of light in vacuum speed of light in medium n = ccmccm or

11. Refraction (cont) θ1θ1 Normal Reflected rayIncident ray boundary surface θ2θ2 c m1, n m1 c m2, n m2 θ 2 < θ 1 c m2 < c m1 n m2 > n m1 Refracted ray

12. Refraction (cont) θ1θ1 Normal Reflected rayIncident ray boundary surface θ2θ2 c m1, n m1 c m2, n m2 Refracted ray θ 2 > θ 1 c m2 > c m1 n m2 < n m1

13. Example to explain how light is “bent” (or refracted) when it pass into another medium from air: –Band marching cross a field and entering a wet, muddy region obliquely –Still same frequency, but slow down due to slipping –Wave frequency remains the same, but wave speed is reduced, so is the wavelength –Wavelength – the distance covered in each step Refraction (cont)

14. All waves – sound, light, and so on – are bent, as they go through relatively small slits or pass by the corners of objects. The deviation of waves in such cases is referred as diffraction Diffraction enables waves overcome the obstacle The degree of diffraction depends on the wavelength of the wave and the size of the opening or object Diffraction

15. In general, the larger the wavelength compared to the size of the opening or object, the greater the diffraction The diffraction readily occurs for sound because the wavelength of sound are larger than or about the same size as objects For example, you can be heard around the corner, but you can not be seen (the wavelength of visible light is much shorter) Diffraction (cont)

16. Other examples: –Shadows: much shorter wavelength of visible light –Radio waves: very long wavelength, easily diffracted around the building => making radio reception quite efficient –AM reception is better than FM reception: the wavelength of AM bands range from 180 m to 570 m, whereas the wavelength of FM bands is about 3 m Diffraction (cont)

17. Two or more waves meet, they interfere with each other Principle of superposition –At any time, the combined waveform of two or more interfering waves is given by the sum of the displacements of the individual waves at each point in the medium After interfering, the waves pass on with their original forms Interference

18. Constructive interference –Waves reinforce one another while interfering, causing the amplitude of the combined waveform to be greater than that of either pulse Destructive interference –Two waves cancel each other while interfering, causing the amplitude of the combined waveform to be smaller than that of either pulse Figure 7.20 on page 152 Example: the colorful displays seen in soap bubbles Interference (cont)