When waves are reflected, absorbed or passed through

Answer Key - Grade 8 Science Worksheet - Building Confidence

Circle the best answer, or write your answer on the line.

1. Read this first

   Light on three materials (example data)

   material	reflected (%)	absorbed (%)
   Sunglass lens	4	76
   Window film	12	40
   Cardboard	20	80

   Light that is not reflected or absorbed is transmitted.

2. 1.

   Bo shines a lamp on three materials. What percent of the light does the window film transmit?

   48 %
3. 2.

   A lamp shines on three materials. What percent of the light does the lampshade transmit, Esme?

   45 %

4. Panels tested with the same sound (example data)

   panel	reflected (%)	absorbed (%)	transmitted (%)
   Foam	10	85	5
   Plaster	80	5	15
   Plywood	55	10	35

5. 3.

   Jun wants to cut echoes in a music room. Which panel should cover the walls?

   • Plaster, since it reflects 80% of the sound, and reflecting is how a wall takes sound away
   • Plywood, since 35% of the sound passes through it, which is the most of any panel here
   • Plaster, since it is the hardest panel, and hard walls soak up the most sound
   • Foam, since it reflects just 10% of the sound and absorbs 85% of it

6. Sunglass lenses tested with a lamp (example data)

   lens	reflected (%)	absorbed (%)	transmitted (%)
   A	5	15	80
   B	6	74	20
   C	60	10	30

7. 4.

   Kira wants sunglasses that let the least light reach the eyes. Which lens should be chosen?

   • Lens A, since it absorbs the least light, so it keeps the light from heating the eyes
   • Lens A, since it lets 80% of the light through, the largest share of the three lenses
   • Lens B, since just 20% of the light passes through it to the eyes
   • Lens C, since it reflects the most light, 60%, and reflected light does not reach the eyes

8. Mirror test (example data)

   angle of incidence from the normal (°)	angle of reflection from the normal (°)
   20	20
   35	35
   50	50

9. 5.

   A ray hits a mirror at 60° from the normal. What is the angle of reflection from the normal, Milo?

   • 30°, the angle between the reflected ray and the mirror surface
   • 55°, since the mirror absorbs a little light in the bounce
   • 120°, the angle between the incoming ray and the reflected ray
   • 60° from the normal, equal to the angle of incidence
10. 6.

    A ray leaves a mirror at 25° from the normal. What was its angle of incidence from the normal, Noah?

    • 25° from the normal, equal to the angle of reflection
    • 65°, the angle between the incoming ray and the mirror surface
    • 50°, the angle between the incoming ray and the reflected ray
    • 20°, since the light lost some energy on the way into the mirror
11. 7.

    A ray strikes a mirror at 30° to the mirror's surface. What is its angle of reflection from the normal, Pia?

    • 30°, the angle the question gives, which is already the angle of reflection
    • 120°, the angle between the incoming ray and the reflected ray
    • 90°, since the ray turns a right angle at a flat mirror
    • 60° from the normal, since the ray is 90° − 30° from it

12. Cloth in sunlight for 10 minutes (example data)

    cloth	temperature rise (°C)
    Black	14
    White	5

13. 8.

    A black cloth warmed more than a white cloth in the sun. Which explanation fits the data, Ren?

    • The white cloth absorbed the light, and the absorbed energy vanished, so it stayed cool
    • The black cloth reflected the most light, and reflected light is what heats a cloth
    • The black cloth absorbed more light, and absorbed light energy becomes thermal energy
    • The black cloth transmitted more light, which warmed the air trapped under it
14. 9.

    Foam on a studio wall absorbs sound. What happens to the sound energy it absorbs, Tala?

    • It becomes a very small amount of thermal energy in the foam
    • It is stored in the foam as sound and released the next day as an echo
    • It bounces back out of the foam as a quieter echo, and that counts as absorbing it
    • It disappears, since energy is used up when a wave is absorbed

15. Echo length in a gym (example data)

    room	echo lasts (s)
    Bare gym	2
    Gym with mats on the walls	0.6

16. 10.

    Mats on the walls shortened a gym's echo. Which explanation fits the data, Zee?

    • The mats absorbed some of the sound, so less was reflected back and the echo faded sooner
    • The mats reflected the sound up toward the ceiling, and reflecting quiets a room as absorbing does
    • The mats made the gym smaller, and a smaller room has a shorter echo whatever is on its walls
    • The mats made the air in the gym colder, so the sound traveled slower and faded away