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

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?

   • 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
   • Foam, since it passes just 5% of the sound out and reflects just 10% back inside
   • Concrete, since it passes just 5% of the sound out, which keeps the music in the room
7. 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 to reflect light toward the pot, and black paper under it to absorb it
   • 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
   • Black paper on the walls and polished metal under the pot, since the metal holds the heat in
   • Polished metal on the walls and under the pot, since it reflects 95% of the light that arrives, the highest share in the table

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

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

9. 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 bends away from the normal
   • It stays at 300 thousand km/s and bends because glass is transparent
   • It slows from 300 to 200 thousand km/s and bends toward the normal
   • It slows from 300 to 200 thousand km/s and goes straight on, since both are clear
10. 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 speeds up from 200 to 225 thousand km/s and bends away from the normal
    • It keeps its speed of 200 thousand km/s and bends because the two materials differ
11. 8.

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

    • 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
    • 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

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?

    • 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
    • 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
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 one reading 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 three readings of each