The diagram shows an optical fibre carrying light from a sensor in a chemical factory control room. The fibre has a glass core with refractive index 1.50 an...
Question 1 Report
The diagram shows an optical fibre carrying light from a sensor in a chemical factory control room. The fibre has a glass core with refractive index 1.50 and a plastic outer layer with refractive index 1.40. A ray reaches the boundary inside the fibre at an angle of incidence of 75 degrees.
(a) State the name of the process that keeps the light ray inside the glass core. [1]
(b) Calculate the critical angle for light travelling from the glass to the plastic. Use sin c = nplastic / nglass. [3]
(c) Explain why total internal reflection occurs for the ray shown. [3]
(d) Describe one advantage of using an optical fibre rather than a metal cable to carry information. [2]
(e) State one use of optical fibres in medicine. [1]
(f) Explain why the outer layer must have a lower refractive index than the core. [2]
Answer Details
(a) The process is total internal reflection. [1]
(b) \[\sin c=\frac{n_{\text{plastic}}}{n_{\text{glass}}}=\frac{1.40}{1.50}=0.933\] \[c=\sin^{-1}(0.933)=69^\circ\] [3]
(c) The light travels from glass, with higher refractive index \(1.50\), to plastic, with lower refractive index \(1.40\). Its angle of incidence is \(75^\circ\), greater than the critical angle of \(69^\circ\). Therefore total internal reflection occurs. [3]
(d) An optical fibre has less signal loss and is not affected by electromagnetic interference. It can also carry more information and is lighter or thinner than a metal cable. Any two are valid. [2]
(e) Optical fibres are used in an endoscope to view inside the body. [1]
(f) The outer layer must have a lower refractive index than the core so the boundary is from higher to lower refractive index. This allows rays in the core to meet the condition for total internal reflection. [2]
Download The App On Google Playstore
Everything you need to excel in your exams
