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Waves, sound and optics
MDCAT · Physics · 8 questions
Name: ____________________ Date: ____________
A wave has a frequency of 50 Hz and a wavelength of 4 m. What is its speed?
- A 54 m/s
- B 400 m/s
- C 12.5 m/s
- D 200 m/s
Sound cannot travel through:
- A Water
- B Air
- C Steel
- D A vacuum
Light travels at 3 x 10⁸ m/s in vacuum and 2 x 10⁸ m/s in glass. What is the refractive index of the glass?
- A 2.0
- B 1.5
- C 5.0
- D 0.67
Total internal reflection occurs when light goes from:
- A A denser to a rarer medium at an angle greater than the critical angle
- B A denser to a rarer medium at an angle less than the critical angle
- C A rarer to a denser medium at any angle
- D Air into water at 90 degrees
What is the power of a lens whose focal length is 0.5 m?
- A 1 D
- B 0.5 D
- C 2 D
- D 5 D
When a wave passes from one medium to another, which quantity stays the same?
- A Speed
- B Frequency
- C Wavelength
- D Amplitude
A convex lens can form a real image when the object is placed:
- A At the lens itself
- B Closer than the focal length
- C Only at infinity
- D Beyond the focal length
As a source of sound moves towards a stationary listener, the listener hears:
- A No change in frequency
- B A higher frequency
- C A lower speed of sound
- D A lower frequency
Answer key
1. D, 2. D, 3. B, 4. A, 5. C, 6. B, 7. D, 8. B
v = f x wavelength = 50 x 4 = 200 m/s.
Sound is a mechanical wave and needs a medium. In a vacuum there are no particles to carry it.
n = c / v = 3 x 10⁸ / 2 x 10⁸ = 1.5.
Light must travel from a denser into a rarer medium, and the angle of incidence must exceed the critical angle.
Power = 1 / f in metres = 1 / 0.5 = 2 dioptres.
The frequency is set by the source. Speed and wavelength change when the medium changes.
If the object is beyond the focal point, the rays converge on the far side and form a real image. Inside the focal length the image is virtual.
This is the Doppler effect. Waves in front of a moving source are squeezed closer together, so the frequency is higher.