Reflection, Refraction and Lenses: Question 6
Syllabus 3.2.2
A diver's underwater torch shines a narrow beam of light upwards through the water towards the flat surface of a swimming pool. The beam strikes the water–air boundary from below at an angle of incidence of to the normal, and some of the light passes out into the air above. What happens to the ray as it crosses from the water into the air, and why?
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Worked solution
Step 1: Compare the optical density of the two materials
Water is optically denser than air, so light travels more slowly in water than it does in air. This means that as the ray crosses from water into air, it is moving into a less dense material, and it speeds up.
Step 2: Apply the rule for bending direction
Refraction bends a ray towards the normal when light slows down entering a denser material, and away from the normal when light speeds up entering a less dense material.
Here, the ray is speeding up as it leaves the water and enters the air, so it bends away from the normal.
Why the other options are wrong
| Option | Claim | Error |
|---|---|---|
| A | Bends towards normal; slower in air | Gets both the direction and the speed comparison backwards |
| B | Bends towards normal; faster in air | Correct about the speed, but the wrong bending direction for speeding up |
| D | Bends away from normal; slower in air | Correct about the direction, but the wrong speed comparison |
Final answers
- The ray bends away from the normal, because light travels faster in air than in water, option C