Physics and Natural Sciences
The bent straw
Why does a straw in a glass of water look bent at the surface? Which way does it look bent, and why?
- Hint 1What happens to light as it passes from water into air?
- Hint 2Your brain assumes light travels in straight lines. Trace a ray from the tip of the straw to your eye.
- Light from the underwater part of the straw leaves the water at an angle and bends away from the normal as it enters the air, because it speeds up.
- Your eye traces the ray back in a straight line, so the tip appears higher than it is.
- The underwater part therefore looks raised towards the surface, and the straw looks bent upward at the water line.
- The same effect makes pools look shallower than they are.
Where it lands: Refraction at the surface makes the submerged part appear raised, so the straw looks bent at the water line.
The trap: Saying light 'slows down in air'. It speeds up leaving water, which is why it bends away from the normal.
What a tutor might ask next: Roughly how much shallower does a 2 m deep pool look from directly above?
Now do one out loud. In the interview you think aloud with a tutor. Try a Physics problem with hints, follow-ups and Mia's feedback on how you think.