Light tells me what stars are made from!

And I'm not even joking - it's called line spectra.
You see, different elemants emit light at different wavelegnths of the visible spectrum. By passing the light through a diffraction grating, (a series of tiny slits which splits up the light,) one can see the different colours of light seperated in a spectrum.
By identifying the different wavelegnths (what defines the colour of the light,) of the light emmitted from the star, (or any source of light,)  a scientist can identify the presence, and abundance of the elements.

 

The line spectra of hydrogen - if a light source gives off this exact combination of lines of the spectrum, it must be made of hydrogen. Each element emits a different set of lines. Of course, it is more difficult to identify the makeup of a star, which has many elements present, which means that the line spectra has many more lines.

Falling trees in secluded areas and stuff like that... Sound.

As in the sound we hear. Not the common slang expression to express your acceptance.

Sound waves are longitudinal (waves in which particles oscillate along lines in the direction in which the wave travels,) waves produced by variations in air pressure. The wave is a mechanical wave, as it requires a medium in which to occur. Thus, in a vacuum where there is nothing, sound can not travel.

Sound is made by a vibrating source which moves the molecules in air, creating areas of compression and rarefaction. When a molecule moves, it collides with the next one and thus also makes it move. The energy of a sound wave travels away from the source through the series of collisions parallel to the direction of the wave.

Rarefaction is the reduction of a medium's density, or the opposite of compression. Half of a sound wave is made up of the compression of the medium, and the other half is the decompression or rarefaction of the medium.

Sound travels faster through denser materials because a higher density leads to more elasticity in the medium and increases the ease by which compression and rarefaction can take place.

Why does the sound of emergency sirens change as you pass them by?

This effect is called Doppler Shift. What does that mean, you ask. Well...

When the source of the waves, in this case the siren, is moving toward the observer, each successive wave is emitted from closer to the observer than the previous wave. Each wave takes less time to reach the observer than the previous wave and thus the time between the arrivals of successive wave crests are reduced, causing an increase in the frequency.

If the source of waves is moving away from the observer, each wave is emitted farther from the observer than the previous wave, so the arrival time between each wave is increased, reducing the frequency.

This is also the case when a car passes you by; you get a higher pitch on the approach, followed by a lower pitch noise as it has passed you by.

It sounds like (taking a deep breath for childish noise time...) NEEEEEEEVVVOOOOOOOOOOMMMM

Light can be bent... (Sort of.)

Diffraction is the bending of waves around an object in its path. This is due to how it spreads out from the source, not in straight lines, but in curved waves. Light is a wave, so in effect it does bend.

When waves reach a narrow slit, the wave in the slit vibrates like a point source. The waves thus sent out from secondary sources along the slit are nearly in-phase when arriving any point in the forward direction. The diffracted wave resembles a circularwave with centre at the slit.

The amount of diffraction depends on the wavelength and the size of the object that the wave hits. Waves with a larger wavelength diffract more, and smaller wavelegnths diffract less.

An example of this is your shadow. Shadows aren't sharp images; they are fuzzy around the edges, which is due to diffraction. As the rays hit you, the waves bend around you and then continue on, but they have been changed, and so the image projected on the ground is no longer sharp.

Sound also travels around corners because of it. Lower frequency sound waves diffract better.Light does not diffract well, as it has a very small wavelength, and is thus why we cannot see around corners. (It still can, just only a bit.)