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?

   • 40%, since each sheet blocks 30% and 100% − 30% − 30% leaves 40% for the light to get through
   • 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

7. Two layers of a lens (example data)

   layer	light transmitted (%)
   Coating	80
   Tinted plastic	50

8. 5.

   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
   • 50%, since the layer that passes less light decides what reaches the eye, whatever the other does
   • 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

9. Three identical glass sheets (example data)

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

10. 6.

    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
    • 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
    • 70%, since each sheet blocks 10% and 100% − 10% − 10% − 10% is 70%

11. 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

12. 7.

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

    • 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
    • Opaque materials absorb all the light, since none of it passes through them, so the rest is taken in

13. Speech sound meeting a wall (example data)

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

14. 8.

    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 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
    • A wave does one thing at a wall, and the table just lists the three things in turn, one per column

15. Sunglass lenses tested (example data)

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

16. 9.

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

    • 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
    • Yes: the dark lens passes less visible light, 15%, so it must pass less ultraviolet light
17. 10.

    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 straw really bends when it gets wet, and the light just shows the new bent shape of the straw
    • 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