The problems are on the how light travels worksheet. Try them on your own first, then compare with the answers below.
Quick answer: Light is a form of energy that travels in straight lines called rays. These worked answers correct the 20 problems of the grade 6 science worksheet on how light travels, from two kinds of source to laser rangefinder: each solution shows the formula, the substitution and the final answer with its unit.
Use these answers to check your work. For calculations, write the formula, substitute with units and give the result in a sensible unit. For explanations, check that you named the source of light and the path it takes to the eye.
Level: Grade 6 science (Year 7 · Class 6) · Curriculum: MS-PS4-2
Problem 1 Two kinds of source
1. It makes its own light.2. It reflects light from a source.
3. Example: the Sun (luminous), a book (non-luminous).
Problem 2 Sort the sources
Primary: Sun, candle flame, firefly, laptop screen, glow stick. Secondary: bicycle reflector, Venus.Problem 3 Light and materials
Transparent: window glass, clear plastic bottle. Translucent: tracing paper, frosted window. Opaque: brick, aluminium foil.Problem 4 True or false
1. False: the Moon reflects light from the Sun.2. True.
3. True.
4. False: it lets some light through but scatters it.
Problem 5 Lamp, book and eye
1. A.2. B.
3. The eye does not send out light. Light must travel from the book into the eye, so C points the wrong way.
Problem 6 Speed of light units
1. 3.0 × 10⁸ m/s.2. 3.0 × 10⁸ ÷ 1000 = 3.0 × 10⁵ km/s.
3. 1.5 × 10⁸ × 1000 = 1.5 × 10¹¹ m.
Problem 7 Street lamp
1. t = d ÷ v.2. 150 ÷ 3.0 × 10⁸ = 5.0 × 10⁻⁷ s.
Problem 8 Sunlight delay
1. 1.5 × 10¹¹ ÷ 3.0 × 10⁸ = 500 s.2. 500 ÷ 60 = 8.3 minutes.
3. We see the Sun as it was about 8 minutes ago.
Problem 9 Lighthouse beam
1. 24 × 1000 = 2.4 × 10⁴ m.2. 2.4 × 10⁴ ÷ 3.0 × 10⁸ = 8.0 × 10⁻⁵ s.
3. 8.0 × 10⁻⁵ s = 80 μs.
Problem 10 Cards in a row
1. No.2. The middle hole is not on the straight line between the other two, and light travels only in straight lines.
3. Lower the middle card by 2 cm so that all three holes line up.
Problem 11 Four materials
1. Glass: transparent; curtain and frosted glass: translucent; plywood: opaque.2. 91 − 28 = 63 percentage points.
3. Glass, because it lets the most light through.
Problem 12 Dark room
1. She can see nothing.2. With no source, there is no light to reflect off objects, so no light reaches her eyes.
Problem 13 Torch through a glass
1. Clear water: transparent; cloudy water: translucent; black card: opaque.2. 84 ÷ 90 × 100 = 93 %.
3. 36 ÷ 90 × 100 = 40 %.
Problem 14 One minute of light
1. 3.0 × 10⁸ × 60 = 1.8 × 10¹⁰ m.2. 1.8 × 10¹⁰ ÷ 1000 = 1.8 × 10⁷ km.
Problem 15 Choosing materials
(a) Transparent: students need a clear view and daylight. (b) Translucent: light enters but nobody can see in. (c) Opaque: it blocks the view. (d) Translucent: it spreads soft light without glare.Problem 16 Pinhole camera
1. Rays from the top of the flame cross at the hole and reach the bottom of the screen, so the image is inverted.2. Image height = 6.0 × 10 ÷ 30 = 2.0 cm.
3. The image becomes larger.
Problem 17 Signal from Mars
1. 2.25 × 10¹¹ ÷ 3.0 × 10⁸ = 750 s.2. 750 ÷ 60 = 12.5 minutes.
3. 2 × 12.5 = 25 minutes.
Problem 18 Fair test for materials
1. A: (82 + 80 + 84) ÷ 3 = 82 lux; B: (31 + 29 + 30) ÷ 3 = 30 lux; C: 0 lux.2. 82 %, 30 % and 0 %.
3. For example the torch distance and the torch brightness, and the position of the meter.
4. A transparent, B translucent, C opaque.
Problem 19 Headlights in fog
1. Water droplets scatter some of the light sideways into his eyes, so the path of light shows up.2. The droplets scatter and weaken the light from distant objects.
3. Translucent: some light passes, but it is scattered.
Problem 20 Laser rangefinder
1. 3.0 × 10⁸ × 2.0 × 10⁻⁷ = 60 m.2. 60 ÷ 2 = 30 m.
3. The light went to the wall and back, so it covered the distance twice.
