Physics 10. Lecture 29A. "There are two ways of spreading light: to be the candle or the mirror that reflects it." --Edith Wharton

Transcription

1 Physics 10 Lecture 29A "There are two ways of spreading light: to be the candle or the mirror that reflects it." --Edith Wharton

2 Converging Lenses What if we wanted to use refraction to converge parallel light rays to a single focal point? What type of shape should we use? Recall our prism example: A parallel light ray was pushed upward. What would happen when if flipped the prism? A parallel light ray was pushed downward.

3 Converging Lens This is the basis behind constructing different types of lenses for distorting light. We can construct the following lens: This is known as a converging lens. It is a lens consisting of plastic or glass that refracts light. It focuses parallel light rays at a single point known as the focal point.

4 Converging Lens Converging lenses are thick in the middle and thin near the edges. They have positive focal lengths. A thin lens has two focal points, corresponding to parallel light rays from the left or from the right.

5 Diverging Lens We can also construct the following lens: This is known as a diverging lens. It is also a lens consisting of plastic or glass that refracts light. It defocuses parallel light rays to make it appear that it came from a single point known as the focal point.

6 Diverging Lens Diverging lenses are thick on the edges and thin in the middle. They have negative focal lengths. A thin lens has two focal points, corresponding to parallel light rays from the left or from the right.

7 Interference Treating light as a particle helped us to understand how images are formed by lenses and mirrors by constructing ray diagrams. But we can also understand different phenomena by treating light as a wave. For example, light waves will interfere with each other just like the sound waves and waves on a string that we dealt with earlier. This means we can get areas of constructive interference and destructive interference just like we had when we produced standing waves.

8 Double Slit Experiment Two narrow slits, S 1 and S 2, can act as sources of waves. The waves emerging from the slits originate from the same wavefront and therefore are always in phase (coherence). The light from the two slits form a visible pattern on a screen. The pattern consists of a series of bright and dark parallel bands called fringes.

9 Double Slit Experiment Constructive Interference occurs at the center point. There is no path length difference between these two waves. Therefore, they arrive in phase with each other. This will result in a bright area on the screen. This bright spot is called the central maximum (zeroth order maximum).

10 Double Slit Experiment If we moved to a spot, Q, below the center spot, P, we find that the upper wave, S 1, to travel farther than the lower wave, S 2. If the upper wave travels one wavelength farther than the lower wave; the waves will still arrive in phase. A bright spot will occur. This bright spot is called the first order maximum.

11 Double Slit Experiment The fringe pattern formed by a Young s Double Slit Experiment would look like the picture to the right. Alternating bright and dark fringes are created. Constructive interference occurs where a bright fringe appears. Destructive interference results in a dark fringe.

12 Single Slit Experiment A single slit placed between a distant light source and a screen produces a diffraction pattern (similar to a double slit experiment). It will have a broad, intense central band. The central band will be flanked by a series of narrower, less intense secondary bands (secondary maxima).

13 The interference in thin films is cau Interference in Thin Films Have you ever looked a soap bubble and observed patterns of different colors? Light wave interference can be observed in thin films (such as an oil film on water or soap bubbles). With this phenomenon some colors are being enhanced. This comes about due to the wave nature of light.

14 The interference in thin films is cau Interference in Thin Films When light hits a semi-transparent region it has two choices: 1) to be reflected or 2) to go through. If two separate light rays travel the different paths, then they are no longer equivalent. One light ray had to travel further than the other light ray.

15 The interference in thin films is cau Interference in Thin Films This path length difference can lead to light rays now having crests meet up with troughs. This can lead to destructive interference (if the path length difference is just right). So, some colors will not be seen, making the other colors appear more vibrant.

16 Thin Film Interference Thin film interference is used by Morpho butterflies to intensify the colors reflected. The Morpho butterfly has ridge stacks on its wings that causes blue to be emphasized and all other colors to be de-emphasized.

17 Polarization of Light Light from the sun is produced by the vibrations of multitude of atoms located there. Each atom produces a wave with its own orientation of the electric field. All directions of the electric field vector are equally possible and are in a plane perpendicular to the direction of propagation. This type of wave is known as an unpolarized wave.

18 Polarization of Light A wave is said to be linearly polarized if the resultant electric field vibrates in the same direction at all times at a particular point. It is possible to polarize an unpolarized beam. The most common technique for polarizing light is called polarization by selective absorption.

19 Polarization of Light In this technique, you use a material that transmits waves whose electric field vectors in that plane are parallel to a certain direction. This material also absorbs waves whose electric field vectors are perpendicular to that direction. This device is known as a polarizer. The material is known as a Polaroid.

20 Polarization of Light Light can also be polarized by scattering it off of particles. The electrons in the medium can absorb and reradiate part of the light. Sunlight that reaches an observer on earth becomes polarized as it scatters off of air molecules.

21 Clicker Question 29A-1 Suppose the viewing screen from our in-class double-slit demonstration is moved closer to the double slit apparatus. What happens to the interference fringes? A) They get brighter but otherwise do not change. B) They get brighter and closer together. C) They get brighter and farther apart. D) No change will occur. E) They fade out and disappear.

22 For Next Time (FNT) Start reading Chapter 30. Start the homework for Chapter 29. Finish the homework for Chapter 28.