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PSL300: Optics of the Eye (Tweed) | 80-Question Master Exam Prep
Section 1: The Physics of the Reduced Eye
1. What is the total refractive power of the human "reduced eye" in its relaxed
state?
A) 40 D
B) 60 D
C) 20 D
D) 15 D
Elaboration: The total power is the sum of the cornea (~40D) and the lens (~20D). In the
simplified "Reduced Eye" model, we treat the eye as a single refracting surface with a
power of 60D.
2. A lens has a focal length of 0.25 meters. Calculate its power in Diopters.
A) 2.5 D
B) 4.0 D
C) 0.4 D
D) 40 D
Elaboration: Power ($P$) is the reciprocal of focal length ($f$) in meters ($P = 1/f$). $1 /
0.25 = 4.0$ Diopters.
3. Where does the greatest change in refractive index occur as light enters the eye?
A) Between the lens and the vitreous humor
B) Between the aqueous humor and the lens
C) Between the air and the cornea
D) At the retina
Elaboration: Refraction depends on the difference in refractive index ($n$). The jump
from air ($n=1.0$) to the cornea ($n=1.376$) is the largest transition, which is why the
cornea provides the majority of the eye's focusing power.
4. What is the axial length of an emmetropic eye with 60D of power?
A) 17.5 mm
, B) 22.22 mm
C) 25.0 mm
D) 15.0 mm
Elaboration: $f = 1/P$. $1/60 = 0.0166$ meters of focal length in air. However, inside
the eye ($n=1.33$), the distance is $n/P$, which is $1.333/60 = 22.22$ mm.
5. In the reduced eye, the "nodal point" is the point through which:
A) Light rays are refracted most severely
B) Light rays are reflected
C) Light rays pass without being refracted
D) Images are inverted
Elaboration: The nodal point is the optical center; any ray passing through it does not
change direction. This is crucial for calculating the size of the retinal image.
6. How does the refractive power of the eye change when submerged in water
($n=1.33$)?
A) It increases.
B) It decreases significantly.
C) It stays the same.
D) It doubles.
Elaboration: The difference between water ($n=1.33$) and the cornea ($n=1.38$) is
very small, so the cornea loses its refractive power, leading to extreme hypermetropia.
7. Which structure is responsible for "Accommodation"?
A) The cornea
B) The crystalline lens
C) The iris
D) The sclera
Elaboration: While the cornea is static, the lens can change its shape to increase
refractive power for near objects.
8. What is the refractive index of the aqueous humor?
A) 1.00
B) 1.33
, C) 1.40
D) 1.52
Elaboration: The aqueous and vitreous humors have a refractive index very similar to
water (1.33).
9. The "Principal Point" of the eye is located:
A) On the retina
B) At the front surface of the cornea
C) Inside the lens
D) At the fovea
Elaboration: The principal point is the theoretical location where the single refracting
surface of the reduced eye exists.
10. If the eyeball is 25mm long but the power is 60D, the image focuses:
A) In front of the retina
B) Behind the retina
C) On the retina
D) In the lens
Elaboration: Since the focal length (22.22mm) is shorter than the physical eye (25mm),
the light converges too early.
Section 2: Refractive Errors (Ametropia)
11. A student can only see objects clearly up to 50 cm. What is their "Far Point"?
A) Infinity
B) 50 cm
C) 25 cm
D) 100 cm
Elaboration: The Far Point is the furthest distance an object can be seen clearly. For
myopes, this distance is finite.
12. To correct the student in Q11, what power lens is needed?
A) +2.0 D
B) -2.0 D
PSL300: Optics of the Eye (Tweed) | 80-Question Master Exam Prep
Section 1: The Physics of the Reduced Eye
1. What is the total refractive power of the human "reduced eye" in its relaxed
state?
A) 40 D
B) 60 D
C) 20 D
D) 15 D
Elaboration: The total power is the sum of the cornea (~40D) and the lens (~20D). In the
simplified "Reduced Eye" model, we treat the eye as a single refracting surface with a
power of 60D.
2. A lens has a focal length of 0.25 meters. Calculate its power in Diopters.
A) 2.5 D
B) 4.0 D
C) 0.4 D
D) 40 D
Elaboration: Power ($P$) is the reciprocal of focal length ($f$) in meters ($P = 1/f$). $1 /
0.25 = 4.0$ Diopters.
3. Where does the greatest change in refractive index occur as light enters the eye?
A) Between the lens and the vitreous humor
B) Between the aqueous humor and the lens
C) Between the air and the cornea
D) At the retina
Elaboration: Refraction depends on the difference in refractive index ($n$). The jump
from air ($n=1.0$) to the cornea ($n=1.376$) is the largest transition, which is why the
cornea provides the majority of the eye's focusing power.
4. What is the axial length of an emmetropic eye with 60D of power?
A) 17.5 mm
, B) 22.22 mm
C) 25.0 mm
D) 15.0 mm
Elaboration: $f = 1/P$. $1/60 = 0.0166$ meters of focal length in air. However, inside
the eye ($n=1.33$), the distance is $n/P$, which is $1.333/60 = 22.22$ mm.
5. In the reduced eye, the "nodal point" is the point through which:
A) Light rays are refracted most severely
B) Light rays are reflected
C) Light rays pass without being refracted
D) Images are inverted
Elaboration: The nodal point is the optical center; any ray passing through it does not
change direction. This is crucial for calculating the size of the retinal image.
6. How does the refractive power of the eye change when submerged in water
($n=1.33$)?
A) It increases.
B) It decreases significantly.
C) It stays the same.
D) It doubles.
Elaboration: The difference between water ($n=1.33$) and the cornea ($n=1.38$) is
very small, so the cornea loses its refractive power, leading to extreme hypermetropia.
7. Which structure is responsible for "Accommodation"?
A) The cornea
B) The crystalline lens
C) The iris
D) The sclera
Elaboration: While the cornea is static, the lens can change its shape to increase
refractive power for near objects.
8. What is the refractive index of the aqueous humor?
A) 1.00
B) 1.33
, C) 1.40
D) 1.52
Elaboration: The aqueous and vitreous humors have a refractive index very similar to
water (1.33).
9. The "Principal Point" of the eye is located:
A) On the retina
B) At the front surface of the cornea
C) Inside the lens
D) At the fovea
Elaboration: The principal point is the theoretical location where the single refracting
surface of the reduced eye exists.
10. If the eyeball is 25mm long but the power is 60D, the image focuses:
A) In front of the retina
B) Behind the retina
C) On the retina
D) In the lens
Elaboration: Since the focal length (22.22mm) is shorter than the physical eye (25mm),
the light converges too early.
Section 2: Refractive Errors (Ametropia)
11. A student can only see objects clearly up to 50 cm. What is their "Far Point"?
A) Infinity
B) 50 cm
C) 25 cm
D) 100 cm
Elaboration: The Far Point is the furthest distance an object can be seen clearly. For
myopes, this distance is finite.
12. To correct the student in Q11, what power lens is needed?
A) +2.0 D
B) -2.0 D
