Experiments you can run yourself.
EM waves (radio→micro→IR→visible→UV→X→γ) differ in wavelength. c=λf, E=hf — the shorter λ, the higher the energy. In a vacuum they all travel at speed c.
In an EM wave the electric field E and magnetic field B are mutually perpendicular and perpendicular to the direction of motion (a transverse wave), oscillate in phase, and E/B = c.
In an LC circuit energy sloshes between the inductor and the capacitor at frequency f=1/(2π√(LC)). A connected antenna emits an EM wave. Smaller L·C → higher frequency.
E=hf=hc/λ; a shorter wave carries more energy.
v=c/n; light in a medium travels slower than in vacuum.
X-rays pass through soft tissue but are absorbed by dense bone and metal (Nobel Prize in Physics 1901).