How would life be different today if the earth was 50% larger?

I presume you mean 50% more massive, but still much the same ocean and land composition? So gravity would increase. The atmosphere would be more compressed. Atmospheric absorption of solar radiation would occur in a thinner lower layer of the atmosphere, leading to much higher air temperatures, higher evaporation, higher humidity, and total cloud cover over most of the planet. Most photosynthesis would no longer be able to function except in polar regions, where a wet and sunny climate would prevail. The rate of climate change would accelerate to ‘off the scale', rapidly melting all ice, and raising sea levels by more than 60 metres, thereby inundating all coastal cities on the planet - which wouldn't matter because they would all be uninhabitable anyway.

There are several global warming feedback mechanisms which are currently considered unimportant, but which might become important in this new scenario. These include radically different atmospheric and ocean circulation patterns, and methane clathrate- tundra release which could tip the balance into a runaway greenhouse scenario. If that were to happen then everyone and everything dies.

PS Then there is the question of what happens to the moon-earth equilibrium dynamics? Maybe the extra gravity would spin the moon closer, in which case sea tide levels would dramatically increase. I leave you to speculate how corals and inshore marine ecologies would cop[e with that issue.

  Assuming everything stays the  same exactly. The moon is still equally distant. that seems like the second biggest factor to the Earths new gravity from mass. Third would be a slight increase in temperature from the sun. I would guess that everything living would be about a third bigger.

What changes when light travels from one medium to another?

If you are referring to properties, then it's the speed of light and wavelength which changes from one medium to another.Frequency is an inherent property and doesn't change across the medium.Also, light always bends towards the normal while moving from lighter to denser medium and vice versa when moving from denser to lighted medium.

What would have happened if Hitler had been killed after the Nazis came to power?

Here's a little thought for you, if Hitler had died before Sept 39 and the invasion of Poland, when he had taken Austria, Czechoslovakia, the Rhineland, etc. he would probably have went down in history as the greatest German leader who ever lived.