1. Huygens’s eyepiece is also known as __________
a) Spherical Eyepiece
b) Positive Eyepiece
c) Negative Eyepiece
d) Double Eyepiece
Explanation: In Huygens’s eyepiece, the real inverted image formed by the objective of the microscope lies behind the field lens and that image is virtual. Due to this, Huygens’s eyepiece is also known as Negative Eyepiece. This eyepiece cannot be used to examine a real image formed by the objective.
2. Which of the following is not a part of an Electron Microscope?
a) Electron Gun
b) Objective
c) Magnetic lens
d) Fluorescent Screen
Explanation: The electron microscope works on the principle that a beam of electrons exhibit wave nature and they can be focused by suitable electric and magnetic fields. Thus, electron microscope has an electron gun for producing electrons, a Magnetic lens to focus the electron beam and the fluorescent screen to obtain the image.
3. The condition in which lines in one direction appear to be well focused while those in perpendicular direction appear distorted is known as ___________
a) Presbyopia
b) Myopia
c) Hypermetropia
d) Astigmatism
Explanation: Astigmatism occurs when the cornea is not spherical. Due to this, lines in one direction seems to be focused while in the perpendicular direction looks distorted. It can be corrected by the use of cylindrical lenses.
4. A thin converging lens and a thin diverging lens of each focal length 10 cm are placed coaxially 5cm apart. What will be the focal length of the combination?
a) +10cm
b) -10cm
c) +20cm
d) -20cm
Explanation: We know, f1=10 cm, f2 = -10 cm, d = 5 cm
\(\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}-\frac{d}{f_1f_2}\)
= 1/10 – 1/10 + 5/100
f = + 20 cm.
5. Two lenses of focal length 8 cm and 6 cm are placed a certain distance apart. If they form an achromatic combination, the separation between them is ___________
a) 5cm
b) 6cm
c) 7cm
d) 8cm
Explanation: For an achromatic combination, the separation x is given by,
x = (f1 + f2)/2
As we know, f1 = 8cm and f2 = 6cm
x = 14/2
x = 7 cm.
6. The effective focal length of Ramsden’s eyepiece is 3cm. The focal length of a single lens is ___________
a) 3cm
b) 4cm
c) 5cm
d) 6cm
Explanation: Here, F = 3 cm, f =? d =?
In Ramsden’s eyepiece, F = f and d = 2f/3
Therefore, applying the formula
\(\frac{1}{F}=\frac{1}{F}+\frac{1}{f}-\frac{d}{Ff}\)
F = 3f/4
f = 4F/3
f = 4 X 3 /3
f = 4 cm.
7. The far point of a myopic person is 40 cm. What should be the power of the lens that he must use to see clearly?
a) -0.4 D
b) +0.4 D
c) -2.5 D
d) +2.5 D
Explanation: Myopia is the defect when a person can’t see distant object clearly. To see clearly, the person should use a concave lens of focal length -40 cm.
Hence, the power of the lens = 100/-40
= -2.5 Diopters.
8. If the magnification of a lens is dependent on the distance from the principal axis, the aberration that arises is called __________
a) Coma
b) Astigmatism
c) Curvature
d) Distortion
Explanation: Distortion is the defect that arises when the images of equal parts of an object will not be of the same length. In such a case, the magnification of the lens is dependent on the distance from the principal axis
9. The refractive index of a Plano-convex lens is 1.6 for violet and 1.5 for red light. The radius of curvature is 0.20 cm. The separation between violet and red foci of the lens is ____________
a) 0.05 cm
b) 0.06 cm
c) 0.07 cm
d) 0.08 cm
Explanation: Here, μv = 1.6, μr = 1.5, R1 = 0.20 m, R2 = ∞
Using lens maker’s formula: \(\frac{1}{F_v}=(μ_v-1)(\frac{1}{R_1}+\frac{1}{R_2})\)
Fv = 0.33 m
Similarly solving for red color: \(\frac{1}{F_r}=(μ_r-1)(\frac{1}{R_1}+\frac{1}{R_2})\)
Fr = 0.40 m
Separation between red and violet, Fr–Fv = 0.40 – 0.33
= 0.07 m.
10. The component in an optical instrument used to increase the angular object field and to minimize aberrations is called as ___________
a) Objective lens
b) Eye lens
c) Field Lens
d) Plano-concave lens
Explanation: Field lens is placed between the objective and the eye lens. It increases the field of view and brightness of the image. It helps in reducing the aberrations in the lens. It brings the center of the exit pupil near the eye lens. Together, the field lens and the eye lens constitute an eyepiece or ocular