SAT · ACT · MCATGoal-based prepLive 1:1 + GoalGap AI
MCAT · Chemical and Physical Foundations · Physics
Waves, sound and optics is part of Physics in the MCAT Chemical and Physical Foundations section. The questions below are written in the style of the real test, each with the answer and a worked explanation.
Question types you will see here include doppler shift for moving source or observer, standing waves and harmonics, intensity, distance and decibels and refraction and total internal reflection.
Common traps: reversing the sign of the shift; saying the wave speed changes with source motion; confusing frequency with wavelength.
Question 1
A ventilator alarm is turned up from 60 dB to 80 dB at the nurse's position. By what factor does the sound intensity increase?
1.3 times
4.0 times
20 times
100 times
Answer: D. 100 times
Each 10 dB is a factor of 10 in intensity, so 20 dB is 10 × 10 = 100 times (D). 1.3 times (A) takes the ratio of the decibel values; 4.0 times (B) assumes each 10 dB doubles the intensity; 20 times (C) treats the 20 dB difference as the factor itself.
Question 2
Technetium-99m, used in bone scans, has a half-life of 6.0 hours. What percentage of a dose remains undecayed 24 hours after it is prepared?
0%
6.25%
12.5%
25%
Answer: B. 6.25%
Twenty-four hours is four half-lives, so (½)⁴ = 1/16 of the dose remains: 6.25% (B). 0% (A) subtracts 25% per half-life (or treats four half-lives as the full lifetime); 12.5% (C) counts only three half-lives; 25% (D) divides by the number of half-lives instead of halving four times.
Question 3
An object is placed 30 cm in front of a converging lens with a focal length of 10 cm. How far from the lens does the image form?
1.5 cm
7.5 cm
10 cm
15 cm
Answer: D. 15 cm
Thin-lens equation: 1/f = 1/d_o + 1/d_i, so 1/d_i = 1/10 − 1/30 and d_i = 15 cm (positive: a real image on the far side). Adding the reciprocals instead of subtracting gives 7.5 cm.
Question 4
Ultraviolet light with a wavelength of 248 nm falls on a metal with a work function of 2.3 eV. What is the maximum kinetic energy of the ejected electrons? (hc ≈ 1240 eV·nm)
2.3 eV
2.7 eV
5.0 eV
7.3 eV
Answer: B. 2.7 eV
Photon energy is hc/λ = 1240/248 = 5.0 eV; the most energetic electron leaves with 5.0 − 2.3 = 2.7 eV (B). 2.3 eV (A) is the work function; 5.0 eV (C) is the photon energy before the work function is subtracted; 7.3 eV (D) adds the work function instead of subtracting it.
Question 5
A dentist holds a concave mirror, focal length 4.0 cm, at a distance of 2.0 cm from a tooth. What image does the mirror form?
Virtual, upright, half the size of the tooth
Real, inverted, twice the size of the tooth
Real, upright, twice the size of the tooth
Virtual, upright, twice the size of the tooth
Answer: D. Virtual, upright, twice the size of the tooth
1/d_i = 1/4.0 − 1/2.0 = −1/4.0, so d_i = −4.0 cm: a virtual image behind the mirror, and m = −d_i/d_o = +2, upright and twice as large (D). An object inside the focal point never gives a real image (B); (C) pairs a real image with upright orientation, which a single mirror cannot do; (A) inverts the magnification.
A few questions show the style. The free scan shows which MCAT skills cost you the most points, then builds a plan around them.