In this article, we show the easy way to compare the readability of two physics school texts written in the same language. We show that the readability of scholar texts depends on the frequency of terms. We compare the readability of texts from two physics textbooks written in Slovak. In these comparisons, we used the frequency of words and terms with a given length (probability distribution) instead of using a readability formula. The concept of analysing the frequency of words in a text agrees with our observations made by eye tracking systems. We verified our results by cloze tests. The method comparing the probability distribution of words and terms of two texts (written in the same language) is very simple, intuitive as well as language independent (in contrary to readability formulae).