I am confused about a few things.... As I understand it, a ray of light is merely a stream of photons. Every photon that falls onto the detector will be detected and hence, a frequency will be registered. The frequency of light registered by a detector is actually the photon's frequency. Photon is a wavepacket. Questions: 1.) How is it that in classical theories regarding light, we can picture successfully (to a certain extent), light in terms of a sinusoidal wave (infinite sine wave), when, light is only a stream of photons (particles) ? 2.) In the classical theory of light, how did they detect the crest and trough of the infinite sinusoidal wave? In QM, we talk about photons. So, how does the concept of crest and trough in classical theory related to photon/QM? 3) Photon is created when an electron drops down from one level to a lower level of energy, in an atom. If this is so, we should get a photon with a definite energy and frequency. Why is it that a photon is a wavepacket?