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.

   Bo's shout echoes back from a far wall after 0.8 s. Sound travels about 340 m/s in this air. How far away is the wall?

   136 m
3. 3.

   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

4. Read this first

   Window films tested (example data)

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

5. 4.

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

   • Film B, since it passes 70% of the visible light but just 2% of the ultraviolet
   • 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 C, since it passes the least ultraviolet light, 1%, and that settles the choice
6. 5.

   None of them transmits light. Luz builds a solar cooker. What should line the walls, and what should line the bottom under the pot?

   • Polished metal on the walls and under the pot, since it reflects 95% of the light that arrives, the highest share in the table
   • Polished metal on the walls to reflect light toward the pot, and black paper under it to absorb it
   • Black paper on the walls and polished metal under the pot, since the metal holds the heat in
   • Black paper on the walls and under the pot, since it absorbs 95% of the light that arrives, the highest absorbed share in the table

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. 6.

   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 goes straight on, since both are clear
   • It stays at 300 thousand km/s and bends because glass is transparent
   • 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 bends toward the normal
9. 7.

   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 slows down 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

10. Light blocked by samples (example data)

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

11. 8.

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

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

12. 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

13. 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
    • Foam, since its readings average 52 dB, the lowest average of the three materials
    • Foam and plywood tie, since each gave a reading below 55 dB at least once
    • Cork, since a higher reading means that more of the sound was blocked
14. 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 three readings of each
    • 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 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 with one speaker, distance and meter, and take one reading of each