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. 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?

   • 49%, since the second sheet passes 70% of the 70% the first one let through
   • 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
   • 35%, since two sheets of film cut the light by half, and half of 70% is 35%

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?

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

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?

   • 70%, since each sheet blocks 10% and 100% − 10% − 10% − 10% is 70%
   • 27%, since three sheets with 9 tenths each leave 3 times 9 percent
   • 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

10. 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

11. 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 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
    • Opaque materials transmit none, but each reflects and absorbs light, in different shares

12. Speech sound meeting a wall (example data)

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

13. 7.

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

    • 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
    • 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

14. Sunglass lenses tested (example data)

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

15. 8.

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

    • Yes: darker lenses absorb more light, and ultraviolet is light, so the rule holds for lenses
    • 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: the dark lens passes less visible light, 15%, so it must pass less ultraviolet light
16. 9.

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

    • The curtains destroy the sound energy that reaches them, so no energy is left in the room to make echoes
    • Sound energy absorbed in the soft fibers becomes a little thermal energy, so less sound is reflected back
    • The curtains reflect the sound upward, and sound that goes upward cannot come back down into the room again
    • The curtains transmit the sound into the wall, and the wall stores it as sound until it is needed
17. 10.

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

    • 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
    • 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