1. 1
   A student places a drinking straw into a glass of water. Looking at the glass from the side, the straw appears to bend sharply at the point where it enters the water's surface.

   (a)

   What physical process best explains why the straw appears to bend at the water's surface?

   [1 mark]

   • ARefraction of light as it changes speed crossing from air into water
   • BReflection of light off the surface of the water
   • CDiffraction of light around the edge of the straw
   • DTotal internal reflection of light within the water

   (b)

   Light from the submerged part of the straw travels from the water (a denser medium) into the air (a less dense medium) before reaching the student's eyes. As it crosses the boundary at an angle, what happens to its direction?

   [1 mark]

   • AIt bends towards the normal
   • BIt bends away from the normal
   • CIt continues in exactly the same direction with no bending
   • DIt reverses completely back into the water

   (c)

   Explain what is meant by refraction, in terms of the properties of a wave, and state the one circumstance in which a wave crossing a boundary between two media would not change direction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In the 17th century, a scientist directed a narrow beam of white sunlight through a triangular glass prism in a darkened room and observed the light emerging from the far side spread out into a continuous band of colours, from red through to violet.

   (a)

   What name is given to the effect where white light is split into a spectrum of colours by a prism?

   [1 mark]

   • AReflection
   • BDiffraction
   • CDispersion
   • DPolarisation

   (b)

   White light contains many different wavelengths of visible light. Why does the prism split white light into separate colours rather than letting it pass through as a single beam?

   [1 mark]

   • ABecause the prism absorbs all colours except one
   • BBecause only white light can travel through glass
   • CBecause the prism reflects each colour at a different angle
   • DBecause different wavelengths of light are refracted by different amounts by the glass

   (c)

   The scientist observed that violet light was refracted (bent) by a greater amount than red light as it passed through the prism. Explain why this happens, and describe how this leads to the colours forming an ordered band from red to violet.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An optician tests a rectangular block of glass by directing a ray of light into it from air at an angle of incidence of 40°. The ray is measured to refract to an angle of refraction of 25° inside the glass.

   (a)

   Calculate the refractive index of the glass.

   [3 marks]

   (b)

   Using the refractive index you calculated, calculate the critical angle for this glass, and explain what happens to a ray of light travelling inside the glass block that strikes the glass–air boundary at an angle greater than the critical angle.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A surgeon uses a medical endoscope, a thin flexible tube containing bundles of thin glass optical fibres, to view inside a patient's body during keyhole surgery without needing to make a large incision. Separately, a telecommunications company relies on optical fibres running along the seabed to carry internet data between continents as pulses of light.

   (a)

   Explain how total internal reflection allows light to travel along a thin optical fibre inside the endoscope, from one end to the other, even when the fibre is bent around corners inside the body.

   [6 marks]

   (b)

   Discuss why total internal reflection in optical fibres is equally important for the telecommunications company's undersea data cables, and explain one advantage that transmitting data as light along optical fibres has over sending electrical signals along traditional copper cables.

   [6 marks]

   Total for question 4: 12 marks

End of questions