Add "Lights and Shadows" by Bartosz Ciechanowski

README.md

@@ -36,6 +36,7 @@ Contributions are welcome. Please read the [contributing guideline](CONTRIBUTING
 - [Guide to Focal Reducers for Astronomy](https://agenaastro.com/articles/focal-reducers-guide.html) - How focal reducers work, what types are there, and how to use them.
 - [Guide to Preprocessing of Raw Data With PixInsight](https://pixinsight.com/forum/index.php?threads/guide-to-preprocessing-of-raw-data-with-pixinsight.11547/) - Mistakes that happen during the pre-processing stages cannot be corrected anymore in post-processing. This guide from Bernd Landmann acquaints fellow astrophotographers with PixInsight's tools needed for recognising and avoiding such mistakes.
 - [How to Set the Correct Back Focus](https://optcorp.com/blogs/deep-sky-imaging/how-to-set-the-correct-back-focus) - An easy to follow guide on what back focus is and how to set it.
+- [Lights and Shadows](https://ciechanow.ski/lights-and-shadows/) - This interactive guide to light by Bartosz Ciechanowski, teaches about travel vectors, irradiance, radiance, angles, luminance, and more.
 - [Mastering Flat Frames: Gain matching](https://www.astroworldcreations.com/blog/understanding-flats-part-1-gain-matching) - This blog series by Willem Jan Drijfhout will take you on a deep dive into the topic of flat frames ([Part 2: Flat Panels compared](https://www.astroworldcreations.com/blog/understanding-flats-part-2-flat-panels-compared), and [Part 3: Bias, Flat-Darks and Conclusions](https://www.astroworldcreations.com/blog/understanding-flats-part3-conclusions)).
 - [Optical Glass Types](https://www.stellarvue.com/optical-glass-types/) - What makes a top-tier telescope? This article will clear up some common misconceptions.
 - [Selecting a Guide Scope and Autoguiding Camera for Astrophotography](https://agenaastro.com/articles/guides/selecting-a-guide-scope-and-autoguiding-camera-for-astrophotography.html) - Choosing the right autoguiding equipment can be challenging. This guide will help with making the right decision.