# PHYS1210 Physics Assignment 3 – Australia.

Subject Code & Title : PHYS1210 Physics
Assessment Type : Assignment 3
Weight of this assignment: 8.33% of final grade
Instructions:
Answers to the assignment can be typed or hand-written and electronically scanned/photographed and submitted. Please make sure that your work can be easily read by the marker. You will need to submit a single pdf file in in portrait orientation, which includes the MQ Cover sheet. The file size must be less than 10 Mb to be successfully uploaded.
PHYS1210 Physics Assignment 3 – Australia.

The Rules :
1.You may consult resources such as textbooks and most websites.
2.You may NOT consult online resources such as Chegg (or websites that provide similar services) to answer the assignment problems.
3.You are NOT permitted to send the assignment questions to Chegg (or websites that provide a similar service).
4.You are not allowed to ask any third party (person or website) to do your assignment for you.
5.You can discuss the assignment problems with your colleagues, however you are not permitted to share your solutions or copy the solutions of others.
6.Copying text from other sources to answer questions is still plagiarism and can lead to an allegation of Academic misconduct. Such allegations are referred to the Faculty or University Discipline Committee for their consideration.

Q1. A medical ultrasound system produces sound waves of frequency 2.5 MHz. Use the information in the table below to answer the following questions.

a) What are the wavelengths of the ultrasound waves in air, blood and bone?
b) What is the reflection coefficient for a sound waves incident on the interface between muscle and bone?
c) A layer of fat is sandwiched between two muscle layers so the reflections at each that the ultra sound produces interference. For a given layer thickness, standing waves are produced.

PHYS1210 Physics Assignment 3 – Australia.

(i) Assuming that the standing wave pattern is of the closed-closed type, draw the patterns for the fundamental and fifth harmonics.
(ii) For what layer thicknesses are standing waves achieved for the fundamental and fifth harmonics?

Q2. Optical fibers are used to transmit light over a flexible line and are used widely in applications such
as in the data communications and keyhole surgery. Light of wavelength in vacuum 625 nm is incident
on the fiber which has a core refractive index of n core = 1.46.

a) What is speed of the light in the core?
b) What is the wavelength in the core?
c) What is the frequency of the light in (i) in air and (ii) in the core?
d) The fiber transmits 150 mW of the red light through it. How many photons per second is this?
e) If the critical angle for reflection at the core boundary is 75°, what is the refractive index of the cladding?

PHYS1210 Physics Assignment 3 – Australia.

Q3. The following figure shows the location of an object with respect to a diverging lens.

a) Copy the diagram and show:
(i) (1 mark) the path of a parallel ray from the top of the object through the lens;
(ii) (1 mark) the path of a ray through the centre of the lens;
(iii) (2 marks) and use these rays to identify the location of the image.
b) Is the image real or virtual? State why.
c) (5 marks) Given that the object is 4.7 m from the lens, and the magnification is 0.25, calculate the focal length of the lens.
d) A person with myopia wears such lenses to correct for their vision of objects in the distance.
The lenses are positioned 15 mm from the eyes. What is the far point for this person?

Q 4. Red light is often chosen for some skin diagnostics as it transmits well through the top layers of the
skin. In this question take the refractive index of skin as n skin = 1.36.
a) (i) Explain what the Brewster angle means for unpolarised incident light.
(ii) In words and a diagram, indicate the direction of the polarization of the light reflected.
b) What is the Brewster angle for the skin?
c) Laser Doppler velocimetry is a technique for measuring blood flow in small blood vessels.Consider the case for a laser at 620 nm and frequency f0. The frequency shifts are very small, but are detectable in practice using inter ferometric techniques. If a red blood cell in the body“sees” a frequency fb that is shifted in frequency due to the Doppler effect by fb – f0 = 25 kHz

Calculate the velocity of the blood cell (speed and direction) with respect to the laser. You can
assume that the cell is moving in a direction parallel to the beam.
[Hint: Avoid evaluating the frequency fb explicitly unless you are prepared to keep careful track
of the correct number of significant figures.]

PHYS1210 Physics Assignment 3 – Australia.

Q5. Boron-12 decays via electron decay (e-) with a half-life of 20.2 ms. An anti-neutrino (ν) is also produced.

a) Complete the following reaction for this type of boron decay.

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