Batch 3 - Class 238 - Mirrors (Physics)

• (Note: With class on zoom, one can just use mirror mode on zoom to illustrate all of these)
• Demonstration: When you stand in front of a mirror and wave my right hand, the person in the mirror seems to wave his left hand. But the same flip doesn't seem to happen top to down. How does the mirror know left-right versus top-down?
• Write the word "FOOD" on a piece of paper, and show it to mirror - it appears laterally inverted. But it is right side up. Why?
• Let students think about it and come up with explanations
• Look at turning FOOD again, and point out that while turning it towards the mirror, I had in fact turned in laterally. So its not the mirror doing lateral inversion, but me doing it in order to show it to the mirror. If I had turned the card vertically around, I would in fact see top-to-down inversion (and no lateral inversion, which you can tell because the "D" reads the right way)
• So if I look at the word food written and can read it right, it will also appear right on the mirror. Its because I flipped it that it appeared flipped. Demonstrate this by taking a transparent slide to write on.
• Similarly of you take a pen and point it to the right/left/top/down (note that I dont have to turn it around now), the mirror is pointing in that same direction. But if I point ti towards the mirror, the mirror points it back to me. The mirror is flipping the image, but front-to-back! When we look at our reflection in mirror, its like having walked around to the other side of mirror. If two of us stand in front of each other, one person's right hand will correspond to another person's left hand - because, we had flipped front-to-back. 
• Why do we perceive this front-to-back flip as left-to-right? Most of the world around us is symmetric around vertical axis, including ourselves. Hence we perceive it as left-to-right flip - our face is symmetric around vertical axis. However, if I look at my hair parting, which is not symmetric, it will become clear that this is a front-to-back flip - as if I was wearing a mask that has been inverted now.
• Have you seen top-down inversion?
• Yes, if you are standing on a mirror. Remember, the mirror doesn't know. Its doing the same "front-to-back" reflection.

• Which "you" do you prefer? The one you are used to seeing every morning in mirror, or the you that people around you see? How are those different? Try to use your zoom mirror settings to flip and check out your two versions. In which version, would you find it more comfortable to comb your hair? The two versions for Abraham Lincoln are shown below:
• Photo version: 

  
• Mirror version: 

  
• How big is the size of image of an object on the surface of the mirror?
• Let kids answer, and think about it. They may conclude that it depends on distance to the image. 
• https://www.youtube.com/watch?v=zRP82omMX0g at 5:20 - its always the same size and about half the size of the object

• Draw ray diagrams
• Checking out where the image of a person would be and drawing a straight line from observer
• Tabulate all results
• What is the relationship between "seeing" relationship
• It is "symmetric" - just like "="
• Is it "reflexive"? Can a person always see themselves?
• Is it "transitive"? If A can see B, and B can see C, then can A see C?
• So "see" function doesn't quite behave like "=", but has its own properties. Different functions can have different properties. For eg ">=" is reflexive and transitive, but not symmetric. 
• Since our question is "who all can Allan see", we can actually use the symmetric property to check who all can see Allen, and hence just take reflection of Allan to answer the question

• Let students describe what is going on
• Why does it work at night, but not in day?
• How can you test your thesis?

• Suppose you were inside a giant spherical mirror - what would it look like...
• First, since mirrors reflect light, if we switched a light on and then turned it off, would the light keep bouncing and keep the spherical room illuminated? Or if we kept the light on, would the room keep getting brighter and brighter? Why/ why not?
• If you started really close to the mirror and moved towards the center and beyond, how would you see your image?
• Lets look at the Optic bench  https://www.physicsclassroom.com/Physics-Interactives/Reflection-and-Mirrors/Optics-Bench (Use in mirror mode and move the object from very close to the mirror to all the way 
• We saw the Abraham Lincoln images above, where lateral inversion occurs and hence the left and right sides seem flipped. What would happen if you took a flexible mirror and started to bend it into a cylinder?
• See  https://www.youtube.com/watch?v=zRP82omMX0g at 4:06
• You can get the same effect by using two plane mirrors at an angle to each other

• If you have two mirrors placed at an angle, how many images do they form
• One mirror?
• Two mirrors at 90 degrees (produce 3 images)
• Two mirrors at 60 degrees (> 3 images)

• Two mirrors parallel to each other (infinite images)
• Can you now replace a point in above diagram with a word, to determine which of the images would be laterally inverted or not?

• Why is angle of incidence the same as angle of reflection?
• Light is not really particulate - in quantum theory, "who can see whom"?