/i:/ is the vowel that we find in the word FLEECE. I put that word in capitals because that is how that vowel is often referred to by linguists: the fleece vowel - or FLEECE for short. (This is not random, the word was specifically chosen for a number of specific reasons.) It is the vowel sound at the end of the word guarantee. In transcriptions of British English it has a colon [ : ] in the symbol to describe the length.
/ɪ/ is the vowel in the word KIT. It is known as the kit vowel - or KIT for short. It is the vowel we find in prefixes and suffixes, the bits we stick onto the beginnings and ends of words. So, for example it is the vowel we hear in --ing verb endings.
The vowel represented by /i/ at the ends of words in dictionaries is usually referred to as the happy vowel - HAPPY. This vowel may sound like either FLEECE or KIT, but is always short in duration.
If you say the < y > sound that we find in the word yes, and then say the < e> we find in the word end, the kit vowel is somewhere between the two sounds. This is the first vowel in the word infinitely. This word would sound very odd to a native speaker if it was said with a fleece vowel, /i:/! It would sound like a made-up word: eenfinitely.
The Original Poster asks if there is a big difference between these vowels. If we are talking about the physical difference between the sounds, the answer is: no. In fact, it is very unusual to have two vowels that are so similar in one language. They are very close together. In most languages these would count as one vowel. However, if we are talking about the meaning, or the effect on a listener, the answer is: yes! There is a big difference. There are very, very, very many words that we can be confused about if you say the wrong vowel. For example, the words peace and piss. Nobody wants to say Piss man!, when they mean Peace man!.
If you want to type IPA script, this website is very useful
Hope this helps,
Peace!
North America has what we call the father-bother merger, where /ɑ/ and /ɒ/ often end up as the same sound, oftentimes /ɑ/.
So thus /ɑ/ would be a better choice in North America.
Merriam-Webster's system is a bit unique. I don't like to use it in a linguistic discussion. /ä/ = IPA /ɑ/.
IPA is the more universal option for phonetics as a science, even though several symbols will trick English speakers (e.g. /a/, /e/, /o/, /y/ don't exist alone in major English dialects, and /j/ is not the English J, it is the Y consonant, and people love to eschew the standard rhotic sign in English for the trill symbol /r/, which does not exist in most English speech).
Across the pond, Received Pronunciation British and other dialects don't merge "father" and "bother" vowels like that, thus you get /ɒ/ to accommodate (by the way, /ɒ/ is the rounded version of /ɑ/.) Which transcription is "correct" would depend on the dialect of focus, in this case American English.
Best Answer
No, there is no difference. See, sea and C are pronounced the same in English, at least the English that I speak. Let's have a little fun:
I would suggest that spelling in English is more of an art form than a science, and it isn't going to help you to look for subtle differences in pronunciation in words that are spelled differently. Doing that assumes -- incorrectly -- that those differences are there.
There are indeed some rules in English spelling (you can google those, so I won't get into them), but there are exceptions to nearly all of them, and there are some spellings that simply don't make much sense at all.
Here are four words that rhyme:
Here are four words that are pronounced differently:
Here are four words that are pronounced exactly the same as those four:
(Bow has two pronunciations; the one that is pronounced the same as bough is the bending-at-the-waist version, not the ribbon-in-the-hair version. Also, for a full list of different ways to pronounce -ough-, see this.)
So, spelling in English is pretty much a rote learning concept. You can get clues, from the etymology of a word and such, but in the end you just have to memorize the spellings of words. You may find this article about "spelling bees" (spelling contests) interesting and enlightening.
I will leave you with this amusing little poem: Eye Halve a Spelling Chequer.