A student places a pencil in a glass of water at an angle and looks at it from the side. The pencil appears to bend sharply right at the water's surface, even though it is perfectly straight and hasn't moved. What is actually happening to make the pencil look bent?
- The water magnifies the submerged part of the pencil like a lens, stretching it sideways so it looks offset
- Light reflects off the bottom of the glass and overlaps with the real pencil to create a shifted double image
- The pencil is very slightly rotating in the water, which the eye reads as a permanent bend rather than movement
- Light rays from the submerged part of the pencil bend as they pass from water into air, and the brain assumes light travels in straight lines, so it places the underwater part in a shifted position
Why D? And why not the others?
Correct answer: D. Light rays from the submerged part of the pencil bend as they pass from water into air, and the brain assumes light travels in straight lines, so it places the underwater part in a shifted position
Light travels more slowly in water than in air, so when light rays leaving the submerged part of the pencil cross the water-air boundary, they change speed and bend away from their original path - this is refraction. Because the brain always assumes light has travelled in a perfectly straight line to reach the eye, it reconstructs the underwater part of the pencil as if it were coming from a shifted position, producing the illusion of a bend exactly at the surface. The option describing magnification is wrong because refraction changes the apparent position and angle of the light, not the pencil's size, and no lens-like enlargement is actually occurring here. The option describing a reflected double image is wrong because a genuine reflection off the glass bottom would appear below the pencil, not as a break in the pencil itself at the waterline. The option describing rotation is wrong because the pencil is physically stationary throughout the observation; the apparent bend is a property of how light bends at the boundary, not of any movement of the object being viewed.
Source: BBC Bitesize GCSE Physics: Light -- refraction