1.1 Background of the Study

Vocabulary knowledge plays an excellent role for learners in acquiring a language（Cameron，2001）.Second language learners and foreign language teachers are aware that learning a second or foreign language involves the learning of a large number of words.L2 learners rely heavily on vocabulary knowledge；insufficient vocabulary knowledge has become their main obstacle to overcome（Beheydt，1987）.

Vocabulary knowledge is essential as Schmitt（2010）stated that nothing can be conveyed without words，but something can still be expressed without grammar.When learners travel，they do not carry grammar books；they carry dictionaries（Lewis，1993）.Sufficient vocabulary ensures fluency as well as other proficiency in language learning（Nation，2001）.Better achievement can be made in listening，speaking，reading and writing with an extensive vocabulary.The importance of vocabulary learning is now commonly acknowledged by all of us，as it is the stepping stone leading to language learning success.

However，from beginning to advanced learners，almost everyone has the experience struggling with vocabulary learning.According to Nation（2001），vocabulary knowledge has been classified into three main dimensions：form，meaning and use.Knowing a word involves not only recognizing the word，knowing its meaning and matching it with its native language's（L1）counterpart but also being able to use the word in a specific context with the correct spoken form or written form.Most English language L2 learners are confronted with the difficulty of memorizing vocabulary（spelling and meaning），let alone using it（Templeton，2003）.For Chinese L2 learners，there is no exception.

There are morphological differences between Chinese and English.Chinese is an isolated language with different characteristics in vocabulary systems.Although inflectional and derivational morphology can be found in Chinese morphology（Packard，2000），Chinese language exhibits a fairly limited system of inflectional morphemes.For instance，modern Chinese does not mark aspects on verbs，such as：marking tenses，marking numeral classifier on nouns，or even marking third person singular“-s”morphologically.The Chinese language displays an even more limited system of derivational processes.For example，in Chinese，we add a nominalizing“de”particle to change the verb or verb-object compound into a noun phrase，as in“chang（sing）ge（song）de，”which means“singer” （a person who sings songs）.Chinese adds the relevant morphemes onto the original morpheme through an agglutinative process rather than fusing the markings into a single lexical item.Chinese focuses on a highly productive lexical compounding in morphology when creating new words especially for nouns.It can be seen that Chinese differs from English in inflectional and derivational processes in word formation，which explains why Chinese students face great difficulty in learning new vocabulary.

Given the difficulties of vocabulary learning in a second language，helping learners to expand their vocabulary knowledge and improve their vocabulary development is extremely important.Vocabulary instruction has also been put on the top agenda for language teachers.With the assistance of language teachers in classroom，words can be learned intentionally. Intentional learning refers to“the presence of an explicit instruction to learn”（Hulstijin，2003，p.354）.In the field of L2 learning，researchers have identified that intentional learning is responsible for most second language vocabulary learning（Laufer，2001； Rodgers，2013； Webb，2008）. Intentional learning has its clear advantages in vocabulary learning；however，it seems impossible for all vocabulary to be learned within the classroom（Webb＆Nation，2017）.Only a tiny proportion of vocabulary can be learned through explicit classroom instruction within the limited time of class.More words need to be learned independently and incidentally out of class.

Besides，it is interesting to find that at school，there are courses on reading，speaking，listening，grammar，etc.，but there are not specific courses for vocabulary.In the view of Oxford and Crookall（1990），in most language classes，vocabulary is not explicitly taught by teachers but incidentally“pick-up”by students on their own.Even though there is vocabulary instruction，the so-called explicit instruction merely includes providing students with word lists to memorize or opportunities to practice due to the limited class time and onerous teaching tasks.

Hence L2 learners should pay attention to the role of incidental vocabulary learning.Incidental learning means picking up words incidentally as a by-product of being exposed to large amounts of the second language input（Hulstijin，2001）.Incidental learning has been acknowledged as a complementary way to learn certain language.According to Rodgers（2013），explicit vocabulary teaching is efficient and effective for the first 3，000 most common words of English，for these words have been systematically listed in L2 textbooks and could be taught regularly in L2 classes.However，when it comes to the mid-frequency vocabulary（frequency levels between 3，000 and9，000），explicit teaching vocabulary becomes problematic in choosing which word to teach.Therefore，it is essential to employ authentic materials to increase the exposure of mid-frequency vocabulary for learners，thus promoting possible incidental learning of this vocabulary.

Vocabulary knowledge is growing slowly.Students need multiple exposures over time to add their growing network of knowledge about the word.Most of a student's vocabulary is learned through multiple exposures to words in multiple contexts beyond the classroom boundary（Nagy＆ Scott，2000）.Therefore，multiple out-of-class exposures to English language input have been encouraged（Peters et al.，2019），especially for L2 learners who are not exposed to English speaking environment.Simply put large input and exposure to target language is a prerequisite for incidental learning.Audio-visual input materials should be fully used to provide opportunities for incidental vocabulary uptake in out-of-class time.

Recent years have seen an increased research interest in the number of studies on incidental vocabulary learning from audio-visual input（Ahrabi Fakhr et al.，2021；Çekiç，2022；Feng＆Webb，2020；Gorjian，2014；Peters＆Webb，2018；Rodgers＆Webb，2017；Vidal，2011）.These studies have shown that L2 learners are able to pick up new lexical knowledge incidentally from audio-visual input materials.In terms of incidental vocabulary learning through audio-visual input，the following types of videos were used as the input materials in previous studies：documentaries，academic lectures，short clips of TV series，full-length episode of TV programs（see Table 1-1）.However，there has been little research on incidental vocabulary learning through TEDtalks.

With the development and expansion of technology，multimedia has penetrated every ordinary person's life.As a popular form of learning，online talks have won a reputation for providing viewers with eye-catching and authentic materials.TED-talks are the most well-known online talks posted for free distribution under the slogan“ideas worth spreading”（Nurmukhamedov，2017，p.769）.

“TED”stands for the concepts of Technology，Education and Design（Nurmukhamedov，2017）.On the TED website（www.ted.com），people can quickly access more than 3，500 TED-talks on various topics in more than 110 languages.These TED-talks are delivered in a sophisticated studio with stylish backdrops，following a format that focuses learners on the presenter and visual aids.Short but mighty as they are，TED-talks have intellectually inspired people from all over the world.Some presentations have been viewed more than 16 million times due to the platform's effectiveness in stimulating learners'desire to click，view and learn（Romanelli，Cain＆McNamara，2014）.Now，the TED-talks website has already become the fourth most popular technology website and the most popular conference and events website in the world.

As the platform continues to grow and evolve，the academic role ofTED deserves to be explored.TED-talks could be used as introductory materials to spark students interest related to particular topics in flipped classroom settings（Sohrabi＆ Iraj，2016）.Besides，they could be used as an effective pedagogical tool in public speaking course to promote learners'oral presentation skills（Kusuma，2017）.Most importantly，as authentic language materials，TED-talks could be used to develop learners'overall language skills（Takaesu，2013）.Exposure to real-life TED-talks may present a good opportunity for incidental uptake of vocabulary（Nguyen＆Boers，2019）.The current study attempts to consider TED-talks as a potential source of L2 audio-visual input for incidental vocabulary learning from the following perspectives：

First，it is well-known that language input is vital for language learning and this input should be authentic（Nunan，2002）.Most of the TED-talks were delivered by native speakers and due to this authentic nature，TED-talks meet the requirement of offering suitable and qualified input for L2 learning.These real-life talks and speeches present in front of the L2 learners how language is actually used in the real English world.Shakibaei，Namaziandost，and Shahamat（2019） investigated the effect of authentic input materials on English as a foreign language（EFL）learners'incidental vocabulary learning.52 Iranian female students participated in the research and were put in two groups.One is the experimental group in which students read authentic texts selected from world news，the other is the control group，in which students do not read authentic texts but traditionally structured materials.After a month，a posttest was administered to see the possible effects of the treatment on participants'vocabulary development.The results showed that authentic input could motivate students'interest in language learning，thus enhancing more extraordinary vocabulary achievement.The results collaborated with the findings from Mousavi and Iravani（2012），who explored the effect of authentic listening materials on the listening comprehension of Iranian EFL learners.The results revealed that authentic radio tapes facilitated higher listening comprehension scores，indicating that compared to non-authentic materials，authentic materials are more motivating，engaging and relevant to students'lives（Floris，2008）.It is suggested that authentic materials should be employed in students'courses，which could generate greater interest among learners.

Second，the input material should be enjoyable and could hold learners'attention（Nation，2007；Rodgers，2013）.Previous research showed that L2 learners are more interested in learning through videos than other input modes（Gruba，2006；Wang，2012）.The online talks，like TED-talks，contain a wide range of valuable and entertaining content for learners who can always find topics of interest，from science to business to global issues.Nowadays，more and more Chinese L2 learners enjoy viewing TED-talks and choose to view them regularly in their spare time.They claimed they had benefited greatly from the art of presentation and authentic language used in the stunning speeches.The regular viewing of TED-talks provided opportunities for repeated encounters with low and mid-frequency words，thus leading to sizeable incidental vocabulary gains，which is in line with Webb and Rodgers'（2009）finding.According to Rodgers and Webb（2011），video viewing could increase the possibility of midfrequency words（especially beyond 4，000 most frequently used words）to be learned incidentally.

In addition，TED-talks can be accessed conveniently for free without the limitation of time or space as long as one is connected to the Internet.Hence TED-talks are appropriate materials for extensive viewing，which was proved to be an effective method for enlarging learners'vocabulary size（Webb，2015）.For China，a country lacking English learning contexts，TED-talks“fill the need for greater L2 input”（Webb，2015，p.159）.Learners can view TED-talks in English class or do them as an out-ofclass activity.Learners'engagement in a large amount of out-of-class exposure is beneficial for language learning，especially for vocabulary development（Peters，2018）.Due to the lack of ample research to be done on incidental vocabulary learning through online talks，this study fills this gap and makes use of TED-talks as a potentially effective source of audio-visual input for incidental vocabulary learning.

To the best of our knowledge，reading is a powerful tool for vocabulary development and has been most widely researched with regard to incidental vocabulary learning.The previous research results showed that reading could significantly facilitate the incidental uptake of vocabulary knowledge at the level of written form，meaning，grammar，and collocation（e.g.，Feng＆Webb，2020；Paribakht＆Wesche，1999；Ponniah，2011；Vidal，2011）.However，among the different types of input that L2 learners may encounter outside of the classroom，viewing TV programs and listening to songs are welcomed by most L2 learners（Pavia，Webb＆Faez，2019；Peters，2018），meaning that L2 learners prefer audio-visual input for language learning.In Peters's（2018） study，40% of the learners in Belgium were reported to watch TV series and movies several times a week in their spare time，while only 1% of the learners read L2 books to the same extent.

For learners，compared to written input，exposure to TV and songs can be easy and fun，and at the same time these input sources contribute to L2 vocabulary learning（Pavia，Webb＆Faez，2019；Rodgers＆Webb，2017）.Besides，although incidental vocabulary learning from reading is possible，the process is complex and incremental（Schmitt，2010）.Hence，it is necessary to conduct a comparative study between learning words through viewing and reading.Dual-coding theory suggests that demonstrating information in verbal and pictorial forms together can improve learning，revealing that learning through audio-visual input might be superior to written input.However，it is difficult to know their relative values for L2 vocabulary learning without any empirical studies comparing the two input modes.The current study attempted to compare audio-visual input with written-text-based reading input to determine whether audio-visual input exerted the same effect on learners'vocabulary learning performance as reading did or whether audio-visual input could outperform reading in some aspects.Although written input is not the initial interest of this study，it may provide comparable results for investigating the effect of audio-visual input.