Waves, measurement and refraction
הוצג| English | משמעות |
|---|---|
| frequency/ˈfriːkwənsi/ | תדר |
| wavelength/ˈweɪvleŋθ/ | אורך גל |
What would explain this observation?
- A wave can carry energy across water while a floating marker mainly oscillates. Energy transfer and bulk transfer of matter are different.
Build the model
- Frequency 频率 is oscillations per time; wavelength 波长 is distance between successive points in phase.
- Wave speed equals frequency multiplied by wavelength.
- Transverse oscillations are perpendicular to propagation.
- wavelength: Distance between successive points in phase.
- frequency: Number of oscillations per unit time.
When a wave enters a slower medium, what stays constant?
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
Frequency is set by the source and cannot change at the boundary - the waves arriving must equal the waves leaving. It is the speed that changes in a slower medium, and the wavelength adjusts because v = f x wavelength.
Match each technical term to its precise meaning.
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- At a boundary, frequency stays fixed by the source.
- A change of speed changes wavelength.
- Refraction follows from speed differences; angles are measured from the normal.
In the lab: Measure several wavelengths and divide to reduce fractional reading uncertainty. Use a ray box with a normal drawn at the boundary. Keep the beam away from eyes and record incident and refracted angles clearly.
Which two habits make the investigation or model in this case more defensible?
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- Known: frequency 5 Hz and wavelength 0.40 m.
- Use v = fλ. v = 5×0.40 = 2.0 metres per second.
- If speed falls to 1.5 metres per second at the same frequency, wavelength = v/f = 1.5/5 = 0.30 m.
Your turn: Frequency is 8 Hz and wavelength is 0.25 m. Find wave speed.
Frequency is 8 Hz and wavelength is 0.25 m. Find wave speed.
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
The result is 2 m/s. Known: f = 8 Hz; wavelength = 0.25 m. Use v = f x wavelength. v = 8 x 0.25 = 2 metres per second. If this wave entered a slower medium, f would stay 8 Hz and the wavelength would shrink.
Check the conclusion and its limits
- The wave frequency does not change simply because the medium changes.
- A longitudinal wave has oscillations parallel to propagation.
Not true: Wave frequency always changes when a wave enters a new medium. Frequency is fixed by the source, so it stays the same across a boundary. What changes is speed, and therefore wavelength through v = f x wavelength.
- Frequency is oscillations per time; wavelength is distance between successive points in phase.
- At a boundary, frequency stays fixed by the source.
- Watch for: the wave frequency does not change simply because the medium changes.
Write from a wave speed of 3.0 metres per second
- Frequency is 4 Hz and wavelength is 0.75 m. The speed then falls to 2.0 metres per second at the same frequency.
- Find the first speed and the new wavelength. Frequency does not change at a boundary.
Wave frequency always changes when a wave enters a new medium.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
Frequency is fixed by the source, so it stays the same across a boundary. What changes is speed, and therefore wavelength through v = f x wavelength.
Distance between successive points in phase: write the technical term.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
wavelength means Distance between successive points in phase.
Frequency is 4 Hz and wavelength is 0.75 m. The speed then falls to 2.0 metres per second at the same frequency. What were the first speed and the new wavelength, and what stays fixed at a boundary?
Wave speed is frequency times wavelength. The new wavelength is the new speed divided by the unchanged frequency.
What is the first wave speed?
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
4 times 0.75 is 3.0 metres per second. The 0.50 m figure is the wavelength after the speed falls. Other options — 0.50 m (the new wavelength); 4 Hz (the frequency).