When waves are reflected, absorbed or passed through

Answer Key - Grade 8 Science Worksheet

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

1. 1.

   Ari claps in front of a cliff and hears the echo 0.5 s later. Sound travels about 340 m/s in this air. How far away is the cliff?

   85 m
2. 2.

   Esme rides toward a long tunnel wall and hears a horn echo after 1.2 s. Sound travels about 340 m/s in this air. How far is the wall?

   204 m

3. Read this first

   Window films tested (example data)

   film	visible light passed (%)	ultraviolet passed (%)
   A	70	90
   B	70	2
   C	15	1

4. 3.

   Jun wants a window film that keeps the room bright but blocks most ultraviolet light. Which film is best?

   • Film C, since it passes the least ultraviolet light, 1%, and that settles the choice
   • Film A, since it passes the most visible light and the most ultraviolet light, 90%
   • Film C, since a darker film protects the eyes better than a clearer film
   • Film B, since it passes 70% of the visible light but just 2% of the ultraviolet

5. Wall layers tested (example data)

   layer	reflected (%)	absorbed (%)	transmitted (%)
   Concrete	90	5	5
   Foam	10	85	5
   Thin curtain	20	30	50

6. 4.

   Kira wants a wall layer that keeps music inside a practice room and also cuts echoes. Which layer should be used?

   • Foam, since it passes just 5% of the sound out and reflects just 10% back inside
   • Concrete and the thin curtain together, since two layers beat one layer of foam
   • Thin curtain, since 50% transmitted is the largest share, so it must do the most work
   • Concrete, since it passes just 5% of the sound out, which keeps the music in the room

7. Speed of light in materials (reference data, rounded)

   material	speed of light (thousand km/s)
   Air	300
   Water	225
   Glass	200
   Diamond	124

8. 5.

   A ray of light goes from air into glass at an angle. What happens to its speed and path, Milo?

   • It slows from 300 to 200 thousand km/s and bends toward the normal
   • It slows from 300 to 200 thousand km/s and bends away from the normal
   • It slows from 300 to 200 thousand km/s and goes straight on, since both are clear
   • It stays at 300 thousand km/s and bends because glass is transparent
9. 6.

   A ray of light goes from glass into water at an angle. What happens to its speed and path, Noah?

   • It speeds up from 200 to 225 thousand km/s and bends toward the normal
   • It keeps its speed of 200 thousand km/s and bends because the two materials differ
   • It speeds up from 200 to 225 thousand km/s and bends away from the normal
   • It slows down to 225 thousand km/s and bends toward the normal
10. 7.

    A ray of light goes from air straight into diamond, along the normal. What happens to it, Pia?

    • It slows from 300 to 124 thousand km/s but keeps going straight, with no bend
    • It slows from 300 to 124 thousand km/s and bends toward the normal
    • It keeps its speed and goes straight on, since the angle to the normal is 0°
    • It speeds up to 124 thousand km/s and goes straight on, since diamond is thicker

11. Light blocked by samples (example data)

    sample	thickness (mm)	light reaching the meter (lux)
    Paper	1	300
    Cardboard	3	40
    Foil	0.1	0

12. 8.

    Ren says cardboard blocks light better than paper. Which change would make the test fair?

    • Cut all the samples to one thickness and keep the lamp, distance and meter the same
    • Use a brighter lamp for the cardboard so the meter can read its smaller amount of light more clearly
    • Take the cardboard reading three times and keep the single paper reading as it is
    • Move the meter closer to the paper than to the cardboard when comparing readings

13. Sound level behind each sample, three trials (example data)

    trial	foam (dB)	plywood (dB)	cork (dB)
    1	52	49	63
    2	54	61	66
    3	50	64	64

14. 9.

    Tala tests which sample lets the least sound through. Which conclusion do the data support?

    • Plywood, since its first reading, 49 dB, is the lowest of the nine readings in the table
    • Cork, since a higher reading means that more of the sound was blocked
    • Foam and plywood tie, since each gave a reading below 55 dB at least once
    • Foam, since its readings average 52 dB, the lowest average of the three materials
15. 10.

    Zee wants a fair test of how foam thickness changes the sound that gets through. Which setup is best?

    • Use 2, 4 and 6 cm of the same foam with one speaker, distance and meter, and take one reading of each
    • Use 2 cm of foam, 4 cm of plywood and 6 cm of cork, so each thicker sample is also a new material to test
    • Use 2, 4 and 6 cm of the same foam, but turn the speaker up for each thicker sample so the meter can read it
    • Use 2, 4 and 6 cm of the same foam with one speaker, distance and meter, and take three readings of each