Questions OBJECTIVE - II and Answer (Optical Instruments) HC Verma Part 1

 Q#1

When we see an object, the image formed on the retina is
(a) real
(b) virtual
(c) erect
(d) inverted.

Answer: (a), (d).
The retina is like a screen on which only a real image can be projected. Also, the eye lens is a convex lens in which real images are formed inverted. Hence the options (a) and (d).

Q#2
Which of the following the final image is erect?
(a) Simple microscope
(b) Compound microscope
(c) Astronomical telescope
(d) Galilean telescope.

Answer: (a), (d).
In a simple microscope, the object is placed just inside the focal length of the convex lens, so the virtual and erect image is formed.

In a Galilean telescope, the rays coming from the converging objective is intercepted by a diverging eyepiece, the diverging eyepiece diverges the rays to form a virtual and erect final image.

Q#3
The maximum focal length of the eye lens of a person is greater than its distance from the retina. The eye is
(a) always strained in looking at an object
(b) strained for objects at large distances only
(c) strained for objects at short distances only
(d) unstrained for all distances.

Answer: (a).
The maximum focal length of an eye lens should be equal to the distance between the retina and the eye lens. If it is greater than this distance, then the place of the images formed goes behind the retina for focusing an object at different distances. Hence his eyes will always be strained in looking at an object. Hence option (a).

Q#4
Mark the correct options.
(a) If the far point goes ahead, the power of the divergent lens should be reduced.
(b) If the near point goes ahead, the power of the convergent lens should be reduced.
(c) If the far point is 1 m away from the eye, a divergent lens should be used.
(d) If the near point is 1 m away from the eye, a divergent lens should be used.

Answer: (a), (c).
Let the far point of a person be at x. If he is to see distant objects clearly, the virtual image should be formed at X. For a focal length f and u = ∞

1/f =1/(-x) – 1/(-∞) = -1/x
P = -1/x

Hence if x increases, P decreases. Option (a) is true.
Since the far point is 1 m away, it is nearsightedness or Myopia. In this case, a divergent lens should be used because the negative value of P says that it is a divergent lens. Option (c) is true.

If the near point is at 1 m, it is more than normal "least distance of clear vision, D". It is a case of farsightedness and a convergent lens should be used. Option (d) is not true.

If the near point goes ahead, it means the problem of farsightedness has increased. Hence the power of the convergent lens should be increased. Option (b) is not true.

Q#5
The focal length of the objective of a compound microscope is f₀ and its distance from the eyepiece is L. The object is placed at a distance u from the objective. For proper working of the instrument,
(a) L < u
(b) L > u
(c) f₀ < L < 2f₀
(d) L > 2f₀

Answer: (b), (d).
Practically the focal length of the objective of a compound microscope is very small. The object distance for the proper working of the instrument u is kept more than f₀ such that the magnified, real and inverted image is formed near the eyepiece (within the focal length of the eyepiece).
Since this image is magnified, it means u < 2f₀ and the image is at more than 2f₀ inside the microscope. Hence L > 2f₀ > u. Hence options (b) and (d).