This study examines the relationship between instructional clarity in physics lessons and students' motivation and self-confidence to learn physics. We conducted a secondary analysis of data from the Trends in International Mathematics and Science Study (TIMSS) 2019. Using data from six countries, whose students' science achievement scores were higher than the TIMSS 2019 scale centrepoint, this study explores the relevance of instructional clarity in physics lessons to students' motivation and self-confidence to learn physics by using confirmatory factor analysis and latent multi-group structural equation modelling. We identify a statistically significant positive effect of instructional clarity on students' motivation to learn physics. The findings corroborate expectancy value theory by establishing the mediating role of self-confidence in learning physics in the relationship between instructional clarity in learning physics and motivation to learn physics. Our findings support cognitive load theory by revealing the statistically significant effect of giving explanations in physics lessons on instructional clarity in learning physics. Physics teachers can reduce the heavy intrinsic load associated with the physics subject and increase instructional clarity in physics lessons by giving explanations and using signalling and redundancy.