Question

Why does the pencil look bent in the water?

1. It's because light travels slower through water than through air.

2. It's because the light travels slower through the water than through the air.

3. It's because light goes slower through water than through air.

4. It's because light moves slower through water than through air.

5. It's because light passes slower through water than through air.
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Which ones are grammatical? Do we have to use 'the' or not? What about verbs? Which verbs can we use?

Answer 1

There's no need for "the" before "light."

Instead of "slower" (an adjective), you need to use "more slowly" (an adverb) in all these sentences.

I'd say that the verbs <i>travels, moves, and passes</i> are the best and most accurate.

Answer 2

Thank you for your help.
What about 'in water' instead of 'through water'?

Answer 3

Either preposition will work, but "through" is more precise, I think. It's also more accurate when using the verbs that indicate action rather than simply the state of being.

Answer 4

Thank you.

What about the verb 'goes'? Is it possible, too?

3. It's because light goes slower through water than through air.

Answer 5

... goes more slowly ...

I guess that works, but if you want me to check with a science tutor to be sure, let me know.