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Chapter 5 - Physics
#1 Posted : Friday, June 11, 2021 2:42:26 AM
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Hi, I had some physics questions from chapter 5:

1)For question 93: How do you know it is referring to intensities and not the dB? How is one supposed to add the dB, (150+150) is not the answer?

2)Question 100: I understand that a real image forms behind the lens, but for this question it is asking in relation to the eye? If the image was behind the eye and not in front how would one be able to see it?

3)Is intensity = frequency squared?

4)For Question 101 in the lecture 5 question set can you please explain why D is correct? Will a higher frequency not produce a greater amplitude?

5)What is attenuation? When light moves from a lower index of refraction to a higher index of refraction is the reflected light less dim? (Less intense)

6)Why does light of all colours contain the same velocity? I thought violet light would be faster than red light? For chromatic dispersion EK says that blue light travels more slowly than red light and therefore it bends more?

7)Why can virtual images not be projected onto a screen?
#2 Posted : Tuesday, June 15, 2021 8:13:00 PM
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1. The main issue is that noise levels are measured on a logarithmic scale, just like pH. If you want to calculate the total noise levels of something, YOU CANNOT DIRECTLY ADD DECIBELS because they are measured in a log scale.

The same logic applies to pH.

For example: if you have two samples of water that have a pH of 7 and you mix them, does the mixture now have a pH of 14? Of course not! Two neutral solutions mixed don't make a basic solution. 😉

So, we need to realize that we have numbers that are in "log units", and convert BACK to "normal numbers" to be able to add the values together.

Based on the passage, we are told that a handgun has a sound level of 150 dB. Then,

dB = 10log(I/10^-12)

Solve for I.

150 dB = 10 log (I / 10^-12)
15 = log(I / 10^-12)
10^15 = I/10^-12
I = 10^3

We are told in the problem statement that there are TWO identical handguns, so the intensity will be DOUBLED, so

I = 2x10^3

Note: this is equivalent to ADDING the two intensities together (i.e. 10^3 + 10^3), which we can do now because we're no longer in "log units".

Now we need to convert back to dB because the answers are in dB.

dB = 10log(I/10^-12)
dB = 10log(2 x 10^3 / 10^-12)
dB = 10log(2 x 10^15)
dB = 10(15 + 2/10)
dB = 10(15.2)
dB = 152

The closest answer is A.

2. When the question talks about "the eye", they're referring to the surface of the eye that you can touch with your finger. The image actually forms on the retina, as shown here:


which is technically "behind the eye".

If the image doesn't form perfectly on the retina, that's when you need glasses or contacts (🤓).

3. As far as I understand, they are proportional.

4. Surprisingly, there is no relationship between amplitude and frequency.

5. Attenuation is the loss of intensity when a wave travels through a medium. Example include sunglasses, which attenuate light waves, or earplugs, which attenuate sound waves.

When light moves from a medium with a low index of refraction to a high index of refraction, the REFRACTED (not reflected) light does not change in intensity.

6. All electromagnetic radiation (which includes visible light) travels with the same speed (approx 3 x 10^8 m/s) in a vacuum. The same is basically true for radiation travelling through air. The only reason we can "see" colour is because of how light interacts with different media. The classic example is a prism:


When white light is incident on a prism, the different wavelengths of light are BENT at different angles according to their relative wavelengths. Red has the longest wavelength, so it's bent the LEAST. Purple has the shortest wavelength, so it's bent the MOST. (the short ones are EASIER to bend)

Now we can see all the pretty colours 😍😍😍😍.

Because of the INTERACTION of light with a different medium, NOW the speed of the different components of light are different (chromatic dispersion, like what you said!).

But if you leave light alone in its natural habitat and don't touch it, it'll keep going along at 3 x 10^8 m/s and remain as white, blended, undisturbed light. 😉

7. Virtual images are like "imaginary friends". Only your mind can see them, but they're not actually there! So, even if you put up a screen and want to show all of your family your virtual images... they don't exist. 😫😫😫
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