The angle subtended at the eye by an object is equal to the angle subtended at the eye by the virtual image produced by a magnifying glass. In what sense then does a magnifying glass provide angular magnification?
Think about how a magnifying glass allows you to bring objects closer than the least distance of distinct vision, and how this affects the angular size.
Explanation:
Correct Answer: B - A magnifying glass allows the object to be placed closer to the eye than the least distance of distinct vision (typically 25 cm). When an object is closer, it subtends a larger angle at the eye, making it appear larger. This is the true meaning of angular magnification.
Why others are wrong: A) Magnifying glasses don't change the actual size of objects. C) They don't alter the wavelength of light. D) While the image distance is important, the key factor is the ability to bring the object closer than normal viewing distance.
Question 2
In viewing through a magnifying glass, one usually positions one's eyes very close to the lens. Does angular magnification change if the eye is moved back?
Consider how the angle subtended at the eye changes as you move your eye position relative to the lens.
Explanation:
Correct Answer: C - When you move your eye back from the magnifying glass, the angular magnification decreases slightly because the angle subtended at the eye becomes slightly less than the angle at the lens. However, this change is usually small for typical viewing distances.
Why others are wrong: A) Angular magnification does depend on eye position. B) It decreases, not increases, when moving back. D) The change is predictable and follows optical principles.
Question 3
Magnifying power of a simple microscope is inversely proportional to the focal length of the lens. What then stops us from using a convex lens of smaller and smaller focal length and achieving greater and greater magnifying power?
Consider practical limitations in lens manufacturing and optical aberrations that become significant with very short focal lengths.
Explanation:
Correct Answer: B - As focal length decreases, spherical aberration (blurring due to lens shape) and chromatic aberration (color fringing) become increasingly severe, limiting practical magnification to about 20× for a single lens.
Why others are wrong: A) Lens weight isn't the limiting factor. C) Cost isn't the primary limitation. D) Image inversion occurs regardless of focal length in simple microscopes.
Question 4
Why must both the objective and the eyepiece of a compound microscope have short focal lengths?
Consider the magnification formulas for both the objective and eyepiece and how focal length affects them.
Explanation:
Correct Answer: B - The objective's magnification is large when its focal length (f₀) is small compared to the object distance. The eyepiece's angular magnification is inversely proportional to its focal length (fₑ). Thus, short focal lengths maximize both magnifications.
Why others are wrong: A) Size reduction is a side benefit, not the primary reason. C) Short focal lengths actually decrease working distance. D) Light loss isn't significantly affected by focal length.
Question 5
When viewing through a compound microscope, our eyes should be positioned not on the eyepiece but a short distance away from it for best viewing. Why?
Think about how eye position affects the field of view and light collection through the eyepiece.
Explanation:
Correct Answer: B - Positioning the eye slightly back from the eyepiece (at the "eye-ring") allows collection of more refracted light and maintains a wider field of view. Too close, and the field of view shrinks dramatically, blurring the image.
Why others are wrong: A) Eye strain isn't the primary concern. C) While this might be hygienic, it's not the optical reason. D) Chromatic aberration isn't affected by eye position.
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