Diffraction and lensesIB MYP Sciences: Subtopic test

1. 1
   Waves in a harbour at Mombasa pass through a narrow gap in a sea wall into a sheltered bay. The gap is about 5 m wide, which is similar to the wavelength of the waves.

   (a)

   What happens to the waves as they pass through the gap?

   [1 mark]

   • AThey are all reflected back out to sea
   • BThey travel straight on with no change
   • CThey are completely absorbed by the sea wall
   • DThey spread out into the bay

   (b)

   The harbour authority widens the gap from 5 m to 50 m. How does the spreading of the waves change?

   [1 mark]

   • AThere is less spreading and the waves mostly continue straight on
   • BThere is more spreading than before
   • CThe amount of spreading stays the same
   • DThe waves stop completely

   (c)

   Describe the shape of the waves after they pass through the 5 m gap and state what happens to their wavelength.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student uses a converging lens as a magnifying glass to read the small print on a medicine label. The lens has a focal length of 10 cm.

   (a)

   What does a converging lens do to parallel rays of light that pass through it?

   [1 mark]

   • AIt spreads them apart
   • BIt brings them together at a focus
   • CIt reflects them back
   • DIt leaves them unchanged

   (b)

   For the lens to work as a magnifying glass, where must the label be?

   [1 mark]

   • AMore than 20 cm from the lens
   • BBetween 10 cm and 20 cm from the lens
   • CCloser than 10 cm to the lens
   • DExactly 10 cm from the lens

   (c)

   Describe the image that the student sees when the label is in the correct position.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A group of students tests the idea that the more curved a converging lens is, the shorter its focal length. They use two lenses made of the same glass and with the same diameter: lens A is strongly curved and lens B is gently curved. Each lens is used to focus the image of a distant window onto a white screen, and the distance from the lens to the screen is measured with a metre rule. Four readings are taken for each lens. For lens A the readings are 9.8 cm, 10.2 cm, 10.0 cm and 10.0 cm. For lens B they are 19.6 cm, 20.4 cm, 20.0 cm and 20.0 cm.

   (a)

   Calculate the mean focal length of each lens and state whether the results support the students' idea.

   [3 marks]

   (b)

   Evaluate the students' method and suggest two improvements.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Many people in low-income regions cannot see clearly because they have never had their eyes tested or cannot afford spectacles. A long-sighted person needs a converging lens and a short-sighted person needs a diverging lens. Some charities now supply low-cost spectacles with plastic lenses, but trained staff must test each person's eyes and plastic lenses scratch easily. Studies show that children given spectacles do better at school.

   (a)

   Explain how the lens of the human eye produces a clear image, and why a converging lens in spectacles helps a long-sighted person.

   [6 marks]

   (b)

   Discuss the benefits and drawbacks of giving low-cost spectacles to people in low-income regions, and evaluate whether governments and charities should fund such programmes.

   [6 marks]

   Total for question 4: 12 marks

End of questions