Focal length by the parallax method — practice questions

Question 1 · difficulty L1 · recall

The no-parallax condition between two images occurs when they appear to

• A. lie in the same plane
• B. have different colours
• C. move oppositely under every eye movement
• D. have equal brightness only

Question 2 · difficulty L1 · understanding

Parallax is observed when two viewed points at different depths

• A. remain exactly superposed for all viewpoints
• B. shift relative to each other as the eye moves sideways
• C. have the same brightness
• D. are both at infinity only

Question 3 · difficulty L2 · understanding

For a convex lens, one parallax method can locate an image position by

• A. measuring only lens mass
• B. closing one eye without moving viewpoint and ignoring alignment
• C. matching the image with a movable pin until no relative shift is seen
• D. using a magnetic field

Question 4 · difficulty L2 · application

Once object distance u and image distance v are measured for a thin convex lens, focal length can be found from

• A. f=u+v always
• B. f=uv
• C. 1/f=1/u-1/v regardless of convention
• D. 1/f=1/v-1/u with Cartesian signs

Question 5 · difficulty L3 · application

For a real object and real image using positive magnitudes U and V, focal length can be written

• A. f=UV/(U+V)
• B. f=U+V
• C. f=UV/(U-V) always
• D. f=(U+V)/UV

Question 6 · difficulty L3 · application

Why should the eye be moved sideways when checking parallax?

• A. Sideways motion changes focal length
• B. Relative motion with viewpoint is what reveals depth mismatch
• C. It eliminates refraction
• D. It makes lens power zero

Question 7 · difficulty L4 · analysis

An object is 30 cm from a convex lens and a sharp real image is found 60 cm on the other side. The focal length is

• A. 30 cm
• B. 60 cm
• C. 20 cm
• D. 90 cm

Question 8 · difficulty L4 · analysis

If the image pin is closer to the observer than the optical image, moving the eye sideways produces a relative parallax whose direction can be used to

• A. calculate lens mass directly
• B. remove the need for repeated readings
• C. prove the image is virtual regardless of setup
• D. decide which reference must be moved to reach the image plane

Question 9 · difficulty L5 · analysis

Why are several object-image configurations useful when estimating focal length?

• A. They allow averaging and reveal inconsistent readings
• B. Focal length changes randomly for every object distance
• C. Only one reading can never form an image
• D. The lens must be heated between trials

Question 10 · difficulty L5 · analysis

A student records U and V from the lens's metal stand edge instead of its optical centre, introducing a constant distance offset. This is mainly

• A. a purely random error
• B. a systematic error
• C. no error because U+V cancels all offsets
• D. an error in time measurement