Lenses and ray diagramsEdexcel A-Level Physics: Subtopic test

1. 1
   A student wants to find the focal length of a thin converging lens. She holds the lens so that it forms a sharp image of a distant window on a white screen and measures the distance from the lens to the screen. The distance is 0.125 m.

   (a)

   Which statement correctly describes the principal focus of a converging lens?

   [1 mark]

   • AThe point at the centre of the lens where rays pass through undeviated
   • BThe point on the object side from which rays appear to diverge after refraction
   • CThe point on the axis to which rays parallel to the axis converge after refraction
   • DThe point where the image of an object at twice the focal length is formed

   (b)

   What is the power of the lens?

   [1 mark]

   • A0.125 D
   • B8.0 D
   • C0.80 D
   • D80 D

   (c)

   Explain why the distance from the lens to the screen is equal to the focal length of the lens.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A diverging lens of focal length 0.40 m is used to view a small illuminated object placed on its principal axis. A student plans to locate the image by drawing a scale ray diagram of the arrangement.

   (a)

   Which statement describes the image of a real object formed by a diverging lens?

   [1 mark]

   • AVirtual, upright and smaller than the object
   • BReal, inverted and smaller than the object
   • CVirtual, upright and larger than the object
   • DReal, upright and the same size as the object

   (b)

   A ray parallel to the principal axis strikes the diverging lens. After refraction the ray

   [1 mark]

   • Apasses through the principal focus on the far side of the lens
   • Bpasses through the optical centre without deviation
   • Cis brought to a focus 0.40 m beyond the lens
   • Dappears to come from the principal focus on the object side of the lens

   (c)

   Describe how the student should use two rays from the top of the object to locate the top of the image.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An optician combines two thin lenses in contact on a common principal axis. The first is a converging lens of focal length 0.20 m. The second is a diverging lens of focal length 0.50 m (magnitude). A parallel beam of light is sent through the combination along the axis.

   (a)

   Calculate the power of each lens and the power of the combination.

   [3 marks]

   (b)

   Determine the focal length of the combination. Explain why the parallel beam is brought to a focus further from the lenses than it would be by the converging lens alone.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student has a thin converging lens of focal length 0.15 m, a lit candle and a white screen on an optical bench. She places the candle on the principal axis at distances of 0.45 m, 0.20 m and 0.10 m from the lens in turn, and draws a scale ray diagram for each position.

   (a)

   Describe how the student should construct the ray diagram for the candle at 0.45 m and state the nature and position of the image. Explain how the nature of the image changes when the candle is moved to 0.20 m and then to 0.10 m.

   [6 marks]

   (b)

   The student now places a second thin converging lens, of power 10.0 D, in contact with the first lens. Calculate the power and focal length of the combination. For a distant object, explain where the image is now formed compared with the first lens alone, and why.

   [6 marks]

   Total for question 4: 12 marks

End of questions