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, Bo?

   111 m
4. 3.

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

   37 m

5. Two sheets of tinted film (example data)

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

6. 4.

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

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

7. Three identical glass sheets (example data)

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

8. 5.

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

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

9. Opaque materials in a light test (example data)

   material	reflected (%)	absorbed (%)	transmitted (%)
   Black paper	3	97	0
   White paper	80	20	0
   Polished metal	95	5	0

10. 6.

    Milo claims an opaque object absorbs all the light that hits it. What do the data show?

    • Opaque materials absorb all the light, since none of it passes through them, so the rest is taken in
    • Opaque materials transmit none, but each reflects and absorbs light, in different shares
    • Opaque materials reflect all the light, since none of it passes through them, so it all bounces back
    • Only black materials are opaque, since the lighter ones reflect so much of the light that hits them

11. Speech sound meeting a wall (example data)

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

12. 7.

    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
    • 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
    • The wall reflects 75%, so the 20% that transmits must be a measuring error

13. Sunglass lenses tested (example data)

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

14. 8.

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

    • Yes: the dark lens passes less visible light, 15%, so it must pass less ultraviolet light
    • No: ultraviolet passes through any lens equally, so the lens makes no difference
    • No: the clear coated lens passes 92% of visible light but only 1% of the ultraviolet
    • Yes: darker lenses absorb more light, and ultraviolet is light, so the rule holds for lenses
15. 9.

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

    • The straw really bends when it gets wet, and the light just shows the new bent shape of the straw
    • 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
    • Light from the part under water changes speed and bends leaving the water, so the eye traces it to the wrong place
16. 10.

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

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