Light and optics

A narrow ray meets a plane mirror at 35° to the normal. What is the angle between the reflected ray and the normal?

1. A

   20°

2. B

   35°

3. C

   55°

4. D

   70°

1. a

   Select and explain Select the correct angle and explain the rule you used.

   2
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   Correct choiceB

   1. 1

      Selects option B: the reflected ray makes an angle of 35° with the normal.

   2. 2

      States that the angle of reflection equals the angle of incidence when both are measured from the normal.

A student directs light from air into the same transparent block and measures each angle from the normal.

1. a

   Describe Describe the pattern in the data.

   2
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   1. 1

      States that the angle of refraction increases as the angle of incidence increases.

   2. 2

      States that the refracted angle remains smaller than the incident angle, so the ray bends towards the normal.

2. b

   Calculate For the 30° reading, calculate sin(i) ÷ sin(r). Use sin(30°) = 0.500 and sin(19°) = 0.326.

   2
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   1. 1

      Substitutes 0.500 ÷ 0.326.

   2. 2

      Obtains 1.53, to three significant figures or a consistent precision.

3. c

   Evaluate Suggest two changes that would make the conclusion more reliable.

   2
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   1. 1

      Suggests repeating each measurement and calculating a mean, or taking more angle pairs across the range.

   2. 2

      Suggests a second valid improvement such as using a narrower ray, avoiding parallax, or drawing a more precise normal.

A youth centre wants to project a phone screen onto a wall using a converging lens and a cardboard box. The design must be inexpensive, safe, and usable in a room that cannot become completely dark.

1. a

   Explain Explain where the phone should be placed relative to the lens focal length to form a real enlarged image on the wall.

   2
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   1. 1

      Places the phone beyond one focal length from the converging lens so that a real image can form.

   2. 2

      Places it between one and two focal lengths for an enlarged real image, with the image inverted.

2. b

   Propose Propose two practical design features that would make the projected image easier to use.

   2
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   1. 1

      Proposes a focusing adjustment by sliding the phone, lens, or box sections while keeping them aligned.

   2. 2

      Proposes another defensible feature such as rotating the phone display, shielding stray light, using a matte white screen, or ventilating the phone.

3. c

   Evaluate Evaluate whether this projector is a suitable replacement for a commercial classroom projector in the stated room.

   3
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   1. 1

      Uses a benefit such as low cost, repairability, or accessible construction.

   2. 2

      Uses a physics-based limitation such as low brightness, reduced contrast in ambient light, optical distortion, or limited focus.

   3. 3

      Gives a justified conclusion linked to the youth centre's conditions rather than a simple yes or no.