# Tag Info

Accepted

### What exactly is "dense" in Optical Density?

In this particular context, "optical density" refers to the index of refraction of the material. From my memory, this is not a common usage in college and beyond. So, beware using 'optical ...
• 6,628

### Are laser diffuse reflections really so dangerous?

imagine...I shine it on a white wall. How small is the spot? With a lens to focus it, you can focus all of that energy onto an incredibly tiny spot. Let's now say that I am at 1m distance from the ...
• 9,297

### Units make speed of light dimensionless..?

It's a bit ill-posed but this is defining natural units (maybe look it up on wikipedia for more info), we just set the unit of speed as c (speed of light), so as the speed is unit 1 it means that ($[]$...

### What is the clear difference between interference and diffraction?

I didn't know this Feynman's quote, and to be honest it worries me a bit, because I didn't feel that there was a problem here, but Feynman usually was right! Here's, however, how I usually explain ...
• 682

### Are laser diffuse reflections really so dangerous?

Your calculation is fine, but it makes some assumptions that would be an unwise foundation on which to rest your eye safety. You assume that you are 1 meter away. This might be true most of the time, ...
• 10.5k
1 vote

### Tangential rainbows?

It is kind of hard to tell what is what from the photos - I'm sure it was stunning in person though. I reckon what you're seeing is a form of halo though - an optical phenomenon produced as light ...
• 11
1 vote

### What is the significance of back focal length in a lens?

Never mind, found the answer on photography stack exchange: https://photo.stackexchange.com/questions/61728/what-is-the-importance-of-the-back-focal-length-of-a-lens Basically, it's just a mechanical ...
1 vote
Accepted

### How does a rainbow show all of its colours?

The diagram produced in the question has a couple of errors and almost answers the question. To a very good approximation the rays coming from the Sun are approximately parallel and the reflection ...
• 77.1k
1 vote

### What is the clear difference between interference and diffraction?

Feynman was likely referring specifically to the DSE/light and not waves in general. Diffraction and "interference" are clearly distinct. Water waves going thru a single slit DO diffract (...
• 2,089
1 vote

### What lenses are generally used to focus a beam of light into a horizontal line?

If you look at their documentation, you will read this: Laser scanners – often referred to as profile sensors – use the laser triangulation principle for two-dimensional profile detection on ...
• 10.3k
1 vote

### Can we tell on which side of a lens an image appears?

Yes, as shown below. Assuming the object is on the other side of the lens compared with the eye a virtual image is formed on the same side as the object whereas a real image is formed on the the ...
• 77.1k
1 vote

### Is the dimming of a phone screen under bright sunlight an example of destructive interference of visible light?

No. This is just because difference between white pixels and black pixels is overwhelmed by the sunlight that affects all pixels equally, and your eye can't tell the difference between white pixels ...
• 863

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