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PHYSICS, M3 2018 VCE 12

Optical fibres are constructed using transparent materials with different refractive indices.

Figure 14 shows one type of optical fibre that has a cylindrical core and surrounding cladding. Laser light of wavelength 565 nm is shone from air into the optical fibre (\(v=3 \times 10^8\)).
 

  1. Calculate the frequency of the laser light before it enters the optical fibre.  (1 mark)

  2. Calculate the critical angle for the laser light at the cladding-core boundary. Show your working.  (2 marks)

  3. Calculate the speed of the laser light once it enters the core of the optical fibre. Give your answer correct to three significant figures. Show your working.  (2 marks)

Show Answers Only

a.    \(f=5.31 \times 10^{14}\ \text{Hz}\)

b.    \(\theta_c=60.3^{\circ}\)

c.    \(v_{\text{x}}=1.80 \times 10^8\ \text{ms}^{-1}\)

Show Worked Solution

a.     \(f\) \(=\dfrac{v}{\lambda}\)
    \(=\dfrac{3\times 10^8}{565 \times 10^{-9}}\)
    \(=5.31 \times 10^{14}\ \text{Hz}\)

 

b.     \(\theta_c\) \(=\sin^{-1}\Big{(}\dfrac{n_2}{n_1}\Big{)}\)
    \(=\sin^{-1} \Big{(}\dfrac{1.45}{1.67} \Big{)}\)
    \(=60.3^{\circ}\)

 

c.     \(v_{\text{x}}\) \(=\dfrac{c}{n_{\text{x}}}\)
    \(=\dfrac{3 \times 10^8}{1.67}\)
    \(=1.80 \times 10^8\ \text{ms}^{-1}\)
♦ Mean mark 50%.

PHYSICS, M3 2019 VCE 15

A student sets up an experiment involving a source of white light, a glass prism and a screen. The path of a single ray of white light when it travels through the prism and onto the screen is shown in Figure 14.
 

A spectrum of colours is observed by the student on the screen, which is positioned to the right of the prism.

  1. Name and explain the effect observed by the student.  (3 marks)

  2. Points \(\text{X}\) and \(\text{Y}\) on Figure 14 represent either end of the visible spectrum observed by the student.
  3. Identify the two visible colours observed at point \(\text{X}\) and at point \(\text{Y}\).  (1 mark)

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a.    → The observed effect is dispersion.

→ As light enters the glass it slows down as it is entering a denser medium.

→ As the refractive index for different wavelengths of light differs, the angle at which individual wavelengths refract differs slightly.

→ This causes the white light to split up as each wavelength refracts differently through the glass resulting in a rainbow spectrum on the screen.

b.    Point \(X\) is red.

        Point \(Y\) is blue\purple.

Show Worked Solution

a.    → The observed effect is dispersion.

→ As light enters the glass it slows down as it is entering a denser medium.

→ As the refractive index for different wavelengths of light differs, the angle at which individual wavelengths refract differs slightly.

→ This causes the white light to split up as each wavelength refracts at different angles through the glass resulting in a rainbow spectrum on the screen.

♦ Mean mark 53%.

b.    Point \(X\) is red.

        Point \(Y\) is blue\purple.

PHYSICS, M3 2019 VCE 9 MC

A monochromatic light ray passes through three different media, as shown in the diagram below.

Assume that \(v_1\) is the speed of light in Medium 1, \(v_2\) is the speed of light in Medium 2 and \(v_3\) is the speed of light in Medium 3.

Which one of the following would best represent the relative speeds in the media?

  1. \(v_1>v_2>v_3\)
  2. \(v_1>v_3>v_2\)
  3. \(v_3>v_2>v_1\)
  4. \(v_3>v_1>v_2\)
Show Answers Only

\(D\)

Show Worked Solution

→ From Medium 1 to Medium 2, the light bends towards the normal, this means the light is entering a denser medium and so the light will slow down (\(v_1>v_2\)).

→ From Medium 2 to Medium 3, the light bends away from the normal, this means the light is entering a less dense medium and so the light will speed up. Hence (\(v_2<v_3\)).

→ As the angle of the light as it enters medium 3 is greater than the original angle of incidence. Medium three must be less dense than Medium 1, hence the light would travel faster in medium 3, \(v_3>v_1\).

\(\Rightarrow D\)