In Science Education, rote learning fails to add much to the mental development of the individuals causing the information to reproduce irregularly and to be forgotten after a certain period (Karaer and Kösterelioğlu, 2005). It has been found that even students who are very successful in standard tests have difficulties in organizing their learning in class or applying these learning skills to daily life outside of school (Arslan, 2007). A teaching strategy should be adopted for the student to think, discover, understand the connection between the scientific knowledge he/she learned and develops his/her questioning ability to prevent this situation arising (Tezel and Bıyık, 2018). The Turkish Ministry of Education (MED, 2018) determined that students should be able to express their ideas comfortably in activities, support their thoughts for different reasons and be provided with the right environments to improve their knowledge with their peers. The inquiry-based learning process begins with questions on the analysis of data obtained through experimentation or observation. The questions should be of the type that draws the student's attention to scientific and critical thinking. Later, students should be given the opportunity to express their thoughts with scientific knowledge, along with their reasons and share these thoughts in the classroom. In this way, all students in the class take the initiative to critically question these thoughts and to construct explanations based on evidence (Tezel and Bıyık, 2018). The purpose of this research is to examine the effect of the teaching of an eight-stage inquiry-based teaching activity on students' learning achievement on “How is the Light Spread?”, “What is Reflection?” and “The Laws of Reflection” was taught in the fifth-grade science course. The current study findings make an important contribution to educators and researchers by showing the effectiveness of this activity plays in the success of student learning. In the research, formative assessment probes and open-ended questions were used to determine the students' prior knowledge and learning success about the light diffusion unit. In this research, the experimental design was used to determine the effectiveness of the inquiry-based teaching effectiveness on the success of fifth-grade students learning the light emission unit. Lessons in the experimental groups were taught with an eight-step inquiry-based teaching activity from "Save Eylül from the labyrinth" developed by Tezel and Bıyık (2018). In the control group, the current program was implemented without any new activity. Pretest post-test was applied to two experiments and a control group to determine the effectiveness of the application. The study group consisted of students (n = 87) studying in three-fifth grades of a secondary school affiliated to Beyoğlu District National Education Directorate of Istanbul Province in 2018-2019 academic year. Two of the three classes in which the researcher conducts the science course were determined as two experiments and one as a control group. The control group (n = 26) and experimental 1 group (n = 27) consisted of a classroom with moderately successful students. The experimental 2 group (n = 34) consisted of students who received top grades in the qualifying exam consisting of Mathematics, Science, Turkish, English and Social Studies courses at the beginning of the year at a high school level. The formative assessment probes questions created by Keeley (2005, 2007, 2008, 2009) was translated into Turkish before some changes made by Ayvacı and Candaş (2018) were used to assess and measure students' knowledge on the reflection of light. In addition, open-ended questions were added by taking the opinions of two science teachers and a faculty member. All questions were used to Sorgulama Temelli Öğretim Etkinliğinin 5. Sınıf | 231 identify the students' preliminary information on the subject and measure the gains for the light diffusion unit in pretest and post-test in both groups. A research-inquiry course was planned to provide the second acquisition of the "Light Spread" unit (which explains the relationship between the incoming light reflected beam and surface normal in the reflection of the light). "Save Eylül from the labyrinth! " was developed to investigate the experimental groups by discovering how the light reflects with the help of plane mirrors by Tezel and Bıyık (2018). An eight-step inquiry-based activity was implemented. The implementation steps of the event are as follows: 1. Initial situation and initial inquiries; 2. Clarification/writing of the problem; 3. Making predictions or establishing a hypothesis; 4. Selection method; 5. Testing forecasts; 6. Comparison of predictions and results; 7. Synthesis; 8. Evaluation. An answer key consisting of 43 points, summing up the formative probe questions and the correct answers to the open-ended questions, was prepared. Correct are awarded 1 point for each subquestions in the formative probe question titles, and 0 points for each incorrect and unanswered questions. In open-ended questions, answers containing key concepts and correct drawings were awarded 1 point, and the incorrect ones were awarded 0 points. The statistical analyses of the obtained data were analysed with SPSS 21.00 package program. In the analysis of the data; to determine the differences between the groups, normal distribution, t-test for unrelated (independent) samples, t-test for related (dependent) samples were carried out. The research findings revealed no significant statistic differences between the pretest posttest mean scores of the control group. However, significant statistic differences are found between the pretest posttest mean scores of the experimental groups in favour of the post-test. Accordingly, the coursebased activity-supported processing of the course; “How Light Spreads?”, “What is Reflection?” and “The Laws of Reflection” has a positive effect on learning achievements on students. The findings obtained from formative polling and open-ended questions applied at the beginning of the study concluded that there is a lack of information regarding the reflection of light, incoming beam, reflected beam and surface normal, and the phenomenon of vision before the application of control and experimental groups. However, this changed positively after the application for experimental groups where the courses were taught with inquiry-based teaching activity. Following the application of inquiry-based activity the experimental groups were actively involved in the activity were able to express their thoughts comfortably, and that the concepts in the unit were questioned and discussed leads to a better understanding of the subject. The current study and other research results highlight the importance for teachers to design and deliver lessons with activities that help students to structure information, generate evidence-based thoughts by questioning through experimentation or observation (Akkuş, Günel and Hand, 2007; Çelik, 2012; Duban, 2008; Duran, 2015; Erdoğan, 2005; Keçeci and Kırbağ-Zengin, 2016; Taşkoyan, 2008; Tatar and Kuru, 2006; Wu and Krajcik, 2006). According to the results of the research, the following suggestions can be made; • Specialist in this field can contribute to the use of science courses by developing inquiry-based activities on various topics. • In their lessons, teachers should focus on the kind of activity supported applications that will develop students' inquiry, analysis, and reasoning skills. • In order to enable teachers to use inquiry-based teaching up-to-date methods and approaches in their lessons. Teachers should take part in continuous professional training and receive support in pre-service and in-service teacher training.