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MCAT Refraction, Snell's Law and Total Internal Reflection Practice Questions

By StudyStore · Published 2026-11-04 · 8 original questions · about 12 min

Eight free MCAT-style physics practice questions on index of refraction, Snell's law, critical angle, total internal reflection, dispersion and double-slit interference, with worked explanations.

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Quick concept primer

The index of refraction is n = c / v, where c = 3.0 x 10^8 m/s. Light slows in a medium, its frequency stays constant, and its wavelength shrinks (lambda = lambda_0 / n). Snell's law is n1 sin(theta1) = n2 sin(theta2), with angles measured from the normal. Light entering a higher-n medium bends toward the normal; entering a lower-n medium it bends away.

Total internal reflection requires light to travel from higher n to lower n at an angle of incidence beyond the critical angle: sin(theta_c) = n2 / n1. Dispersion happens because n depends on wavelength: violet light bends more than red. In Young's double-slit experiment, bright fringes occur when the path difference is a whole number of wavelengths, and fringe spacing is proportional to lambda L / d.

8 practice questions

Try each question before opening the explanation. Click "Show answer and explanation" to check yourself. All questions are original and written for study practice.

Question 1

Light travels through a material with an index of refraction of 1.5. What is its speed in the material? (c = 3.0 x 10^8 m/s)

  1. 4.5 x 10^8 m/s
  2. 3.0 x 10^8 m/s
  3. 2.0 x 10^8 m/s
  4. 1.5 x 10^8 m/s
Show answer and explanation

Answer: C. 2.0 x 10^8 m/s

Speed is v = c/n = 3.0 x 10^8 / 1.5 = 2.0 x 10^8 m/s. Light always travels more slowly in a material than in vacuum, so any answer larger than c can be eliminated immediately.

Question 2

Light passes from air into glass. Which statement is correct?

  1. The frequency decreases
  2. The wavelength decreases and the frequency stays the same
  3. The wavelength increases and the frequency stays the same
  4. The light bends away from the normal
Show answer and explanation

Answer: B. The wavelength decreases and the frequency stays the same

Frequency is set by the source and does not change at a boundary. Because speed drops in glass and v = f lambda, the wavelength decreases. Entering a higher-index medium bends the ray toward the normal.

Question 3

A ray in air (n = 1.0) strikes a transparent material with n = 1.41 at an angle of incidence of 45 degrees. What is the approximate angle of refraction? (sin 45 = 0.707; sin 30 = 0.50)

  1. 45 degrees
  2. 60 degrees
  3. 30 degrees
  4. 15 degrees
Show answer and explanation

Answer: C. 30 degrees

Snell's law gives sin(theta2) = (1.0 x 0.707) / 1.41 = 0.50, so theta2 = 30 degrees. The angle is smaller than the incident angle, which fits a ray bending toward the normal in the denser medium.

Question 4

Total internal reflection can occur only when light

  1. goes from a medium of lower index to a medium of higher index
  2. goes from a medium of higher index to one of lower index at an angle greater than the critical angle
  3. strikes a boundary at normal incidence
  4. goes from a medium of higher index to one of lower index at any angle
Show answer and explanation

Answer: B. goes from a medium of higher index to one of lower index at an angle greater than the critical angle

Two conditions are required: n1 must exceed n2, and the angle of incidence must be larger than the critical angle. If light enters a medium of higher index, a refracted ray always exists. At normal incidence, there is no bending at all.

Question 5

A ray travels inside a medium of index n = 2.0 toward its boundary with air (n = 1.0). Above what angle of incidence does total internal reflection begin?

  1. 60 degrees
  2. 45 degrees
  3. 90 degrees
  4. 30 degrees
Show answer and explanation

Answer: D. 30 degrees

sin(theta_c) = n2 / n1 = 1.0 / 2.0 = 0.50, so theta_c = 30 degrees. For angles of incidence larger than 30 degrees inside the material, all light reflects back. Optical fibers use this to guide light.

Question 6

White light passes through a glass prism. Why is violet light bent more than red light?

  1. Violet light has a lower frequency
  2. Red light is absorbed
  3. Glass has a higher index of refraction for violet light
  4. Violet light travels faster in glass
Show answer and explanation

Answer: C. Glass has a higher index of refraction for violet light

In ordinary glass, the index of refraction is larger for shorter wavelengths, so violet slows more and bends more than red. This wavelength dependence of n is called dispersion. Violet actually has a higher frequency and travels slower in glass.

Question 7

In a double-slit experiment, the wavelength of the light is increased while the slit separation and screen distance stay fixed. What happens to the fringe spacing?

  1. It increases
  2. It decreases
  3. It stays the same
  4. The fringes disappear
Show answer and explanation

Answer: A. It increases

Fringe spacing is proportional to lambda L / d, so a longer wavelength produces wider spacing. Using red light instead of blue light, for example, spreads the pattern. The pattern does not vanish because the slits are still coherent sources.

Question 8

A diver looks up at the surface of a calm pool. Light from under the water strikes the water-air boundary at an angle larger than the critical angle. What is observed?

  1. The light is completely refracted into the air
  2. The light is absorbed by the water
  3. Part of the light continues straight without bending
  4. The light is completely reflected back into the water
Show answer and explanation

Answer: D. The light is completely reflected back into the water

Water (n about 1.33) is denser than air, and beyond the critical angle (about 49 degrees) no refracted ray exists, so all of the light reflects. This is total internal reflection, which is why the underside of the surface can look like a mirror.

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Written by StudyStore from standard scientific facts as a study aid. These are not official exam questions and are not affiliated with or endorsed by any testing organization. Always check values against the reference provided with your exam or course.

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