1. 1
   A child places a straight drinking straw into a glass of water at an angle and observes that the straw appears to bend sharply at the point where it enters the water's surface, even though the straw itself is perfectly straight.

   (a)

   What causes this apparent bending effect?

   [1 mark]

   • ADiffraction of light around the edge of the straw
   • BReflection of light off the surface of the glass
   • CRefraction of light as it passes from water into air
   • DThe straw physically bending due to the pressure of the water

   (b)

   Which statement correctly compares the speed of light in water with the speed of light in air?

   [1 mark]

   • ALight cannot travel through water at all
   • BLight travels more quickly in water than in air
   • CLight travels at exactly the same speed in water and in air
   • DLight travels more slowly in water than in air

   (c)

   Using the terms normal, angle of incidence and angle of refraction, explain why light bends as it passes from the water into the air on its way to the child's eye, causing the apparent bending of the straw.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A jeweller examines a rectangular glass block by shining a ray of light into one flat side and tracing its path through the block and out the other side. The ray of light enters the glass at an angle to the normal and is observed to bend towards the normal as it enters the glass.

   (a)

   What does this bending towards the normal indicate about the speed of light in glass compared with in air?

   [1 mark]

   • ALight travels at the same speed in glass and air
   • BLight travels more quickly in glass than in air
   • CLight travels more slowly in glass than in air
   • DLight does not refract when entering glass

   (b)

   As the ray of light leaves the glass block through the second, parallel face and re-enters the air, what happens to its direction?

   [1 mark]

   • AIt is completely reflected back into the glass and does not exit at all
   • BIt bends further towards the normal, continuing to curve in the same sense as it did when entering the glass
   • CIt travels in a completely random new direction, unrelated to the original ray
   • DIt bends away from the normal, becoming parallel to its original direction before entering the glass, but laterally displaced

   (c)

   The jeweller measures the angle of incidence in air as 40 degrees and the angle of refraction in the glass as 25 degrees. Use the equation n = sin i / sin r to calculate the refractive index of the glass, giving your answer to two decimal places.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A telecommunications engineer explains that signals are sent along thin glass optical fibres by relying on light bouncing repeatedly along the inside of the fibre without escaping through its curved sides.

   (a)

   (a) State what is meant by the critical angle, and describe what happens to a ray of light travelling inside the glass fibre if it strikes the boundary with the outside air at an angle greater than the critical angle.

   [3 marks]

   (b)

   The glass used for the optical fibre has a refractive index of 1.5. Using the equation n = 1 / sin c, calculate the critical angle for this glass, and explain why the engineer must ensure that light entering the fibre always strikes its curved sides at an angle greater than this critical angle for the fibre to work correctly for long-distance communication.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A diver shining a torch upwards from underwater towards the surface of a swimming pool notices that when the torch beam is angled steeply upwards (close to vertical), the beam passes out through the surface into the air above, but when the beam is angled closer to the horizontal (closer to the water's surface), the beam appears to reflect entirely back down into the water instead of escaping.

   (a)

   Explain fully, using the concepts of refractive index, critical angle and total internal reflection, why the behaviour of the torch beam depends so strongly on the angle at which it strikes the surface of the water.

   [6 marks]

   (b)

   A second diver, exploring a much deeper part of the pool where a layer of slightly saltier, denser water lies below the fresher water above (with a slightly higher refractive index than the fresher water), shines the same torch upwards at a fixed angle from within this saltier layer towards the boundary with the fresher water above (not the surface of the pool). Explain fully, using refractive index and critical angle, why the critical angle for light travelling from the saltier water into the fresher water is likely to differ from the critical angle for light travelling from the fresher water directly into the air, and explain the general relationship between the size of the difference in refractive index across a boundary and the size of the critical angle at that boundary.

   [6 marks]

   Total for question 4: 12 marks

End of questions