Focal length by the parallax method — NEET & JEE 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 L2 · understanding

In a parallax-based optical measurement, two points are judged to lie in the same plane when

• A. their relative position does not shift as the observer moves sideways
• B. their images are equally bright
• C. both are at the optical centre
• D. the lens is removed

Question 6 · 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 7 · 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 8 · difficulty L3 · application

A convex lens forms a sharp real image with object distance 30 cm and image distance 60 cm. Using magnitudes, what is its focal length?

• A. 15 cm
• B. 20 cm
• C. 30 cm
• D. 45 cm

Question 9 · difficulty L3 · application

A convex lens has focal length 15 cm and an object is placed 30 cm from it. Where is the real image formed?

• A. 15 cm from the lens
• B. 20 cm from the lens
• C. 30 cm from the lens
• D. 60 cm from the lens

Question 10 · 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