There are times when it's a good idea to turn off Pokemon Go's augmented reality feature. Turning off AR switches off the camera and gives you a plain background. It also allows you to catch a Pokemon without aiming your phone's camera at it.
Turning off AR mode is helpful when Pokemon appear in people's houses. You really don't want strangers to call the cops because you're standing in front of their house pointing your phone at them. Unfortunately, you can only turn AR on or off during a battle, so you might find yourself stuck with the wrong settings.
So, to turn on AR you'll need to find another Pokemon. Tap on the Pokemon to engage it and the AR toggle will pop up in the upper right-hand side of the screen.
Make sure to toggle the AR switch before you capture the Pokemon or you will be stuck without AR mode until you find another Pokemon to battle.