Light travels more slowly in water than in air. When a ray leaves the water at an angle, this change in speed makes it bend away from its original path, an effect called refraction.
The light coming from the underwater part of the straw is refracted as it exits the surface. But your brain assumes light travels in straight lines, so it traces the rays straight back to a point higher than where the straw really is. The submerged part therefore looks lifted and displaced, and the straw seems to break at the water line.
The same bending makes a swimming pool look shallower than it is, and a coin in a bowl of water appear to rise. It is why spearfishing needs practice, since the fish is not quite where it appears.
In SPM you should explain refraction by the change of speed between media and use ray diagrams to show the apparent shift.
Common misconceptions
- The straw physically bends in water -> The straw is straight; only the light path bends, so it looks bent.
- Light bends because water is heavier than air -> It bends because its speed changes between the two media, not because of weight.
- The pool is really shallower than it looks -> Refraction makes it look shallower; the real depth is greater.
The physics behind it
Refraction is the change in direction of light when it passes between two media in which it travels at different speeds. The refractive index n compares these speeds, n = c/v, where c is the speed of light in a vacuum, about 3.0 × 10⁸ m s⁻¹, and v is its speed in the medium.
Water has n ≈ 1.33.
From this, the speed of light in water is v = c/n = 3.0 × 10⁸ m s⁻¹ ÷ 1.33 ≈ 2.26 × 10⁸ m s⁻¹, noticeably slower than in air. When a ray crosses the surface at an angle, this speed change bends it, following n = sin i / sin r, where i and r are the angles of incidence and refraction.
The same index controls apparent depth. An object under water appears at a depth given by apparent depth = real depth / n.
A fish truly 40 cm below the surface appears at 40 cm ÷ 1.33 ≈ 30 cm. Your eye, assuming straight-line light, places the underwater part of a straw shallower and shifted, so the straw looks broken at the water line.
See it in daily life
At a fish shop or a restaurant's live tank, the fish never sit quite where they seem. A fish resting near the bottom looks closer to the glass and higher up than it truly is, because light from it bends away from the normal as it leaves the water, and your brain traces those rays back along straight lines to a shallower point.
Reach in to grab the fish where you see it and your hand arrives too high and too near, which is exactly the same trap refraction sets. The bending is greater for rays leaving at a steep angle, so a fish viewed from the side of the tank appears more displaced than one seen from directly above.
The same illusion makes a clear river or a flooded drain look shallower than it really is, which is a genuine safety warning: the water is deeper than your eyes suggest. Anyone judging depth by sight alone can badly underestimate it because of refraction at the surface.
How this comes up in SPM
In Paper 2 this idea is examined with command words such as explain, describe, relate and define. You may be asked to define refractive index, to explain the bending using the change of speed, or to relate real depth and apparent depth in a calculation.
Within the same Light and Optics chapter, refraction leads directly into total internal reflection, where a ray inside a denser medium is reflected instead of refracted once it exceeds the critical angle. It also underpins how lenses form images, since a lens works by refracting light at its curved surfaces.
A frequent task is to describe a ray passing from air into a glass block and to calculate n from measured angles using n = sin i / sin r, or to find an apparent depth. Draw the normal at the surface, mark the angles of incidence and refraction clearly, and carry units through any speed or depth calculation so the working stays consistent.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)