When waves are reflected, absorbed or passed through

Answer Key - Grade 8 Science Worksheet - Challenge Me

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

1. Read this first

   Depth finder echoes on a fishing boat (example data)

   echo from	time after the pulse (s)
   Fish school	0.05
   Lake bottom	0.15

2. 1.

   Sound travels about 1480 m/s in water. How far above the lake bottom is the school of fish, Ari?

   74 m
3. 2.

   Sound travels about 1480 m/s in water. How far above the lake bottom is the school of fish, Esme?

   37 m

4. Two sheets of tinted film (example data)

   sheet	light transmitted (%)
   Sheet 1	70
   Sheet 2	70

5. 3.

   Light passes through both sheets in a row. What share of the light gets through them, Jun?

   • 35%, since two sheets of film cut the light by half, and half of 70% is 35%
   • 40%, since each sheet blocks 30% and 100% − 30% − 30% leaves 40% for the light to get through
   • 70%, since the sheets are identical, so the second sheet changes nothing about the light
   • 49%, since the second sheet passes 70% of the 70% the first one let through

6. Two layers of a lens (example data)

   layer	light transmitted (%)
   Coating	80
   Tinted plastic	50

7. 4.

   Light passes through both layers in a row. What share of the light gets through them, Kira?

   • 65%, since the average of 80% and 50% is what reaches the eye, so the layers share the work
   • 30%, since the layers block 20% and 50%, and 100% − 20% − 50% leaves 30% for the eye
   • 40%, since the tinted plastic passes 50% of the 80% the coating let through
   • 50%, since the layer that passes less light decides what reaches the eye, whatever the other does

8. Three identical glass sheets (example data)

   sheet	light transmitted (%)
   Sheet 1	90
   Sheet 2	90
   Sheet 3	90

9. 5.

   Light passes through three glass sheets in a row. What share of the light gets through them, Luz?

   • 90%, since the three sheets are identical and the extra sheets add nothing
   • About 73%, since each sheet passes 90% of the light that reaches it
   • 27%, since three sheets with 9 tenths each leave 3 times 9 percent
   • 70%, since each sheet blocks 10% and 100% − 10% − 10% − 10% is 70%

10. Speech sound meeting a wall (example data)

    wave	reflected (%)	absorbed (%)	transmitted (%)
    Speech	75	5	20

11. 6.

    Noah says a wave reflected by a wall cannot also pass through it. What does the table show?

    • The same wall reflects 75% of the sound and transmits 20%, so one wave splits into shares
    • The wall reflects 75%, so the 20% that transmits must be a measuring error
    • A wave does one thing at a wall, and the table just lists the three things in turn, one per column
    • The reflected sound equals the transmitted sound, since the wall passes both of them equally

12. Sunglass lenses tested (example data)

    lens	visible light passed (%)	ultraviolet passed (%)
    Clear lens with UV coating	92	1
    Dark lens, no coating	15	60

13. 7.

    Pia says darker lenses always block more ultraviolet light. Do the data support that?

    • No: the clear coated lens passes 92% of visible light but only 1% of the ultraviolet
    • No: ultraviolet passes through any lens equally, so the lens makes no difference
    • Yes: darker lenses absorb more light, and ultraviolet is light, so the rule holds for lenses
    • Yes: the dark lens passes less visible light, 15%, so it must pass less ultraviolet light
14. 8.

    A straw in a glass of water looks broken at the water line. Which explanation fits the wave model, Ren?

    • The water absorbs the light from the part under water, and that energy vanishes, so the part looks shifted
    • The water surface reflects the light from the straw sideways, so the part under water seems shifted from where it is
    • The straw really bends when it gets wet, and the light just shows the new bent shape of the straw
    • Light from the part under water changes speed and bends leaving the water, so the eye traces it to the wrong place
15. 9.

    Thick curtains hang on the walls of a rehearsal room to cut echoes. Which explanation fits the wave model, Tala?

    • Sound energy absorbed in the soft fibers becomes a little thermal energy, so less sound is reflected back
    • The curtains destroy the sound energy that reaches them, so no energy is left in the room to make echoes
    • The curtains transmit the sound into the wall, and the wall stores it as sound until it is needed
    • The curtains reflect the sound upward, and sound that goes upward cannot come back down into the room again
16. 10.

    White light passes through a glass prism and spreads into a rainbow. Which explanation fits the wave model, Zee?

    • Different colors travel at slightly different speeds in glass, so each color bends a different amount
    • The glass absorbs the white light and releases colors afterward as brand-new waves
    • The prism reflects each color off its back surface at a different angle, which spreads the colors apart
    • The glass adds color to the white light, tinting different parts of the beam differently