Waves, measurement and refraction
Đã giới thiệu| English | Ý nghĩa |
|---|---|
| frequency/ˈfriːkwənsi/ | Tần số |
| wavelength/ˈweɪvleŋθ/ | B sóng |
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?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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.
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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?
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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.
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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 4.8 metres per second
- Frequency is 12 Hz and wavelength is 0.40 m.
- Find the wave speed. If the speed becomes 3.6 metres per second at a boundary, find the new wavelength. Frequency stays the same.
Wave frequency always changes when a wave enters a new medium.
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
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.
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
wavelength means Distance between successive points in phase.
Frequency is 12 Hz and wavelength is 0.40 m. What is the wave speed, and what is the wavelength if the speed becomes 3.6 metres per second in a new medium?
Wave speed is frequency times wavelength. A new medium can change speed and wavelength while the source frequency stays fixed.
What is the wave speed in the first medium?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
12 times 0.40 is 4.8 metres per second. Other options — 2 metres per second (the lesson wave at 8 Hz and 0.25 m); 0.30 m (the wavelength after the boundary).