Rainbows in one minute

No glue, no tools, no permission. One clean window and some direct sun.

1
Find the window the sun actually hits

East glass gives you morning rainbows, west glass late afternoon ones.
2
Wipe the glass, then peel and place

Smooth it outward from the middle. Crooked is fixable, it does not mind.
3
Wait for the sun to come round

That is the whole job. The rest happens without you.

It needs direct sun on the glass. No sunbeam, no rainbow.

Do

Clean, dry glass
A window that gets direct sun
Smooth outward from the centre

You do not need to

Buy soapy water or a spray
Use glue, tape or a hook
Drill anything, or ask your landlord

About a minute of work, then this

Before you ask

If something goes wrong

There are bubbles under it

Lift the edge, peel it back to the bubble and smooth outward again from the middle. It is static cling, so it will go back down as many times as you need.

Can I move it to another window?

Yes. Peel it off, rinse it under the tap if it has picked up dust, let it dry and put it up somewhere else.

My window gets no direct sun

Then you will not get rainbows there, and no sticker from anyone will change that. Try a window that catches the sun in the morning or the late afternoon.